CAB International 2001.

Fungi as Biocontrol Agents

(eds T.M. Butt, C. Jackson and N. Magan) 347
13 Safety of Fungal Biocontrol Agents
Mark S. Goettel,
1
Ann E. Hajek,
2
Joel P. Siegel
3
and
Harry C. Evans
4
1
Lethbridge Research Centre, Agriculture and Agri-Food Canada, PO Box
3000, Lethbridge, Alberta T1J 4B1, Canada;
2
Department of Entomology,
Cornell University, Comstock Hall, Ithaca, NY 14853-0901, USA;
3
Horticultural Crops Research Laboratory, USDA/ARS, 2021 South Peach
Avenue, Fresno, CA 93727, USA;
4
CABI Bioscience, Silwood Park, Ascot,
Berkshire SL7 7TA, UK
Introduction
Fungi contain a diverse array of taxa with a great diversity of properties. They are rel-
atively common and are important in regulating pest populations. There has been con-
siderable interest in the use of fungi as microbial control agents of pest insects,
nematodes, weeds and plant pathogens and some have been developed as commercial
biocontrol products. However, fungi also contain many species that are pests in them-
selves, causing untold losses to crops, forests, stored products and buildings and also
affecting animal and human health. Consequently the development and use of fungi
as biocontrol agents requires an assessment of unintended effects associated with their
use.
In this chapter we provide an overview of the potential hazards and safety con-
cerns associated with biocontrol fungi. We restrict our discussions to fungi with bio-
control potential for plants, plant diseases and arthropods. We provide examples of
how fungi can be detrimental as well as how they have been used safely as biocontrol
agents. Finally, we shall review how regulations and registration requirements attempt
to address and mitigate potential safety issues. Previous reviews on the safety of bio-
control fungi are those of Austwick (1980), Goettel et al. (1990), Prior (1990), Goettel
and Johnson (1992) and Evans (1998, 2000). Reviews on the safety of microbial con-
trol agents in general are those of Flexner et al. (1986), Laird et al. (1990), Cook et
al. (1996) and Goettel and Jaronski (1997). Guidelines for testing the pathogenicity
and infectivity of entomopathogens to mammals have been reviewed by Siegel (1997)
and guidelines for evaluating effects of entomopathogens on invertebrate non-target
organisms have been reviewed by Hajek and Goettel (2000). Several authors have also
addressed the protocols and guidelines that should be followed in order to introduce
exotic agents for weed biocontrol (Wapshere, 1974, 1975, 1989; Klingman and
Coulson, 1982).
Some Examples of the Detrimental Effects of Fungi
In addressing the potential detrimental effects of fungi used for biological control, it
is useful to reect on the detrimental effects of fungi in general. This is especially so
because, to date, the detrimental effects of fungi used specically as biological control
agents are either non-existent, have been minimal or have generally gone unnoticed.
We have therefore included several examples of the detrimental effects of various fun-
gal pathogens, both endemic and introduced, in order to illustrate the potential detri-
mental effects of a biological control fungus in what we would consider a worst-case
scenario. We once again stress that these are examples of detrimental effects that have
not arisen from attempts to use fungi as biocontrol agents.
Beauveriosis in the silkworm industry
Prior to the early 20th century, silk production throughout the world suffered because
of the devastating effects of several diseases in the silkworm, Bombyx mori (Steinhaus,
1975). In France and Italy especially, annual losses due to muscardine were tremen-
dous. For instance, silk production in Italy suffered losses of approximately 5 million
kg year
1
until c. 1925 (Bell, 1974). However, initially, the role of pathogens in dis-
ease was not understood. Agostino Bassi (1835, as cited by Bell, 1974, and Steinhaus,
1975) rst demonstrated the germ theory of disease using mycosis in the silkworm.
He showed that the muscardine disease was caused by a vegetable parasite, which
grows and develops within the silkworm, eventually killing its host. He demonstrated
how the seeds produced on the surface of the cadaver were responsible for disease in
new individuals and how these seeds could be destroyed by chemical and physical
means. He recommended disinfection practices using lye, wine and brandy, boiling
water, burning and exposure to sunlight. Thus he was also one of the founders of dis-
infection. The muscardine in question was a fungus, which was later named Beauveria
bassiana, in his honour.
With time, strict hygienic practices and legislation were adopted and muscardine
disease in silkworm insectaries was brought under control. For instance, in China and
Japan, the application of B. bassiana and other microbial control agents was restricted
to areas where no silkworms are bred, to avoid contamination of the mulberry leaves
used to feed the silkworms (Hussey and Tinsley, 1981; Goettel et al., 1990). And,
through careful strain selection, it may even be possible to use B. bassiana to control
a pest in the immediate vicinity of a silkworm rearing facility; in a pilot test in a
Chinese 110 ha pine plantation, good control of pine caterpillars (Dendrolimus spp.)
was achieved through six applications of 2100 kg of B. bassiana, with no signicant
effect on the silkworms within a rearing shed built within the plantation (Anon., 1981a,
as cited by Goettel et al., 1990). The strains of B. bassiana used against the pine cater-
pillars were 100 times less virulent to silkworms than strains isolated from silkworms
themselves.
This example illustrates the potential that entomopathogenic fungi have in dev-
astating insect colonies in insectaries. It also illustrates the importance of differences
in the specicity of different fungal strains or isolates. Through modern disinfection
and hygienic practices, diseases in insectaries can, for the most part, be kept under
control.
348 Safety of Fungal Biocontrol Agents
Chalkbrood in honey-bees
Reports of diseases in the honey-bee date as early as 700 BC (Steinhaus, 1975). The
most important fungal disease of the honey-bee is chalkbrood, caused by the ascomycete
Ascosphaera apis (Gilliam and Vandenberg, 1990). Although the disease is not gener-
ally considered serious, infections of certain colonies can be persistent and damaging.
Chalkbrood is present on all continents where honey-bees are found (Bradbear, 1988;
Anderson and Gibson, 1998). Although known in Europe since the early 20th cen-
tury, the fungus has only been documented more recently in honey-bee colonies in
North America and elsewhere.
The origins of the more recent ndings remain a mystery and several possibilities
exist: (i) the fungus was recently introduced into North America, possibly on imported
pollen; (ii) the fungus has gone unnoticed until recently; or (iii) the fungus has occurred
in feral and solitary bees and only recently invaded honey-bee colonies. Nevertheless,
the fungus is spreading to honey-bee colonies throughout the world and is causing
serious problems in some locations in some years. The disease is little understood and
could be triggered by stress factors, such as chilling.
There is no chemotherapeutic agent registered for its control. However, Gilliam
et al. (1988) demonstrated that genetically inherited hygienic behaviour is an impor-
tant component of resistance of bee colonies to the disease. Colonies in which work-
ers were able to detect and remove infected or dead bees were less susceptible to the
disease.
Chalkbrood in honey-bees provides a rare example of a behavioural mechanism
of disease resistance. It also demonstrates the difculties of preventing the spread of a
detrimental fungal pathogen. For instance, despite strict hygiene and importation
restrictions of bees, bee equipment and bee products, chalkbrood has recently been
detected in Australia and has rapidly spread over a wide geographical area within that
country (Anderson and Gibson, 1998).
Plant pathogens
Plague or pestilence affecting cultivated crops has been recorded since biblical times,
and fungal plant pathogens have been implicated in most of these catastrophes (Large,
1940; Agrios, 1997). However, the rst scientically documented and certainly one of
the most dramatic examples of the detrimental effects of plant pathogens, or indeed
of any pest, concerns the potato late blight in Europe during the 19th century. The
disease or murrain was rst reported in 1845, but was initially ascribed to low tem-
peratures, and it was not until several years later, after disastrous crop losses through-
out Europe, that it was identied as being caused by a fungus, Phytophthora infestans,
by the leading mycologist of the time, M.J. Berkeley, who further speculated that the
pathogen had arrived from the New World (Large, 1940). This biotic invasion has
been described by the latter author as a historic determinant of human affairs, since,
in Ireland alone, at least 1,000,000 people died between 1846 and 1860 as a conse-
quence of the potato famine, and more than 1.5 million emigrated. It also had imme-
diate political implications in the UK because it led to the repeal of the Corn Laws
and the fall of the government (Ramsbottom, 1953). To quote Disraeli: The myste-
rious malady of a single tuber changed the political history of the world. As correctly
deduced by Berkeley, the fungus had co-evolved with potato and its relatives in the
M.S. Goettel et al. 349
neotropics the specic locale is now considered to be Mexico (Fry et al., 1992)
and had arrived in Europe almost certainly through the introduction of infected germ-
plasm.
Unfortunately, the story does not end with this initial invasion. The disease had
a major resurgence in Europe in the 1980s. Prior to this recent event, it was deter-
mined that the European pathogen populations comprised only a single mating type
and thus they were relatively uniform, often dominated by a single genotype. However,
a second mating type was identied and its source was eventually traced to a second
invasion from Mexico, probably in the 1970s (Fry et al., 1992). Indeed, the threat
from invasive plant pathogens should not be underestimated, because many of the
worlds major food crops are genetically vulnerable (Kingsolver et al., 1983). In addi-
tion, fungal pathogens also pose an actual and potential threat to natural ecosystems,
as well as to amenity trees and forestry. The catastrophic impact of successive inva-
sions of Dutch elm disease, caused by Ceratocystis ulmi, on the UK and North American
landscapes has been well documented and needs little introduction (Brasier, 1990;
Liebold et al., 1995). However, other tree diseases of fungal origin are less well known
but have had equally detrimental socio-economic and environmental effects. For exam-
ple, the Asian chestnut blight fungus, Cryphonectria parasitica, after its arrival on
infected nursery stock in New York at the beginning of the 20th century, spread in
several decades over large areas of the eastern seaboard, destroying almost all native
American chestnut trees (Castanea dentata) by the early 1950s (Hepting, 1974). Before
this invasion, American chestnut had dominated many forest ecosystems, forming up
to 25% of the tree cover and constituting not only a high-value timber but also a vital
source of food for wildlife (Anagnostakis, 1987). Whilst these pathogens have shown
host specicity at the generic level and therefore have had an impact only on segments
of the forest community, the root pathogen, Phytophthora cinnamomi, has a much
broader host range, attacking plants from at least 48 different families, and has caused
up to 75% losses in the native eucalyptus forests of western Australia (Weste and Marks,
1987). Since its suspected arrival in the 1920s, this plant pathogen has transformed
the sclerophyll forests with a species-rich understorey into open woodland dominated
by sedges (Burdon, 1987).
An even more recent threat from an invasive plant pathogen involves anthracnose
disease of native dogwood (Cornus spp.) in the USA, caused by the fungus Discula
destructiva, thought to have been imported on contaminated ornamentals from Asia
in the 1970s (Daughtrey and Hibben, 1994). The devastation by this pathogen is wors-
ening, as demonstrated by an increase in tree mortality in some areas from 33% in
1984 to nearly 80% 4 years later. Clearly, the long-term detrimental effects from the
elimination of an important component tree on native forest ecosystems in North
America will be immense, following changes in species composition and community
dynamics (Hiers and Evans, 1997).
The Potential Hazards and Safety Concerns
The safety of fungi being contemplated for use in biological control must be consid-
ered at many levels, with primary concern for direct effects on vertebrates, especially
humans. In considering safety toward all organisms, vertebrates, invertebrates and
plants not intentionally being affected by the biological control fungus are referred to
as non-target organisms. The North American Microbial Biocontrol Working Group
350 Safety of Fungal Biocontrol Agents
identies the following potential safety issues: (i) competitive displacement of non-
target organisms; (ii) allergenicity; (iii) toxigenicity to non-target organisms; and (iv)
pathogenicity to non-target organisms (Cook et al., 1996). In addition, indirect effects,
such as those that could come about through the depletion of the target host itself,
must be considered as a potential safety issue (Goettel and Hajek, 2000). The poten-
tial unintended effects will differ depending on the potential target and non-target
organisms and the ecosystems they inhabit.
Allergenicity
Fungi are capable of producing spores that cause allergies or allergic reactions; how-
ever, actual or potential fungal microbial control agents are not among the species that
are responsible for the production of common allergens (Latg and Paris, 1991).
Nevertheless, there are reports of allergic reactions to microbial control fungi, princi-
pally with those exposed during mass production and application (Austwick, 1980).
For instance, a group of scientists working with B. bassiana reported moderate to severe
allergic reactions, and consequently the US Environmental Protection Agency (EPA)
lists this fungus as a dermal sensitizer (Saik et al., 1990). Furthermore, it has been
demonstrated that crude extracts of Metarhizium anisopliae contain one or more potent
allergens (Ward et al., 1998). Since all fungi are potentially allergenic, it is necessary
to avoid exposing unprotected humans during production and application.
Toxicity
Fungi secrete a wide variety of compounds, many of which are toxic to plants, inver-
tebrates or vertebrates. M. anisopliae produces destruxins and cytochalasins; B. bassiana
oosporein, beauvericin, bassianolide and beauveriolide, Hirsutella thompsonii hirsutellin,
and Trichoderma harzianum peptaibols, to name a few. The role that many of these
toxins play in pathogenesis is little understood; however, some may contribute to the
ability of the fungus to overcome its host (see Chapter 12). However, toxin produc-
tion varies according to isolate and does not necessarily play an important role in vir-
ulence in every case.
The production of toxins by a candidate fungal biocontrol agent should not nec-
essarily immediately preclude its use in biocontrol. Considerations must be made as
to possible effects from the presence of the toxin in the formulated product (e.g. dan-
ger to applicator), fate of the toxin after application (i.e. rate of degradation) and pos-
sible bioaccumulation of the toxin within the host or environment (e.g. danger to
scavengers feeding on cadavers). For instance, the fungal pathogen Fusarium nygamai
has been considered as a promising mycoherbicide for control of witch-weed (Striga
hermonthica), a serious constraint on grain production in Africa (Abbasher and
Sauerborn, 1992). However, recent work has shown that this fungus produces myco-
toxins, including several novel compounds (Capasso et al., 1996). Due to the poten-
tial threat to vertebrates, particularly humans and their animals, interest in developing
this fungus as a mycoherbicide has waned.
M.S. Goettel et al. 351
Pathogenicity
Pathogenicity towards the target host is usually the desired effect. However, patho-
genicity towards non-target organisms could be an unintended effect. Fungi, includ-
ing species intended for biological control, can infect a wide variety of hosts, which
sometimes include mammals. For instance, there are reports of B. bassiana infecting
captive American alligators and a giant tortoise that had been stressed by chilling
(Heimpel, 1971; Saik et al., 1990; Semalulu et al., 1992). Results of laboratory assays
demonstrate that a strain of this fungus can also be pathogenic to embryos of the
inland silverside sh, Menida beryllina, and the grass shrimp Palaemonetes pugio; coni-
dia attached and germinated and hyphae subsequently penetrated the chorionic mem-
brane within 5 days (Genthner and Middaugh, 1992; Genthner et al., 1997). Using
a similar laboratory assay system, Genthner and Middaugh (1995) reported that, M.
anisopliae conidia adversely affected both embryos and newly hatched larvae of the
inland silverside sh. These effects included decreased cardiac output, chorionic rup-
ture and teratogenic expressions in embryos and larvae.
More recently, several cases of human infection by M. anisopliae have been reported
in both immunocompetent and immunoincompetent individuals, with one fatality
in an immunoincompetent child (Burgner et al., 1998; Revankar et al., 1999)
There are numerous case reports that the nematode-destroying fungus Paecilomyces
lilacinus is a causative agent of human infections in both immunoincompetent and
immunocompetent individuals (Itin et al., 1998; Gutierrez-Rodero et al., 1999). This
species was listed in 1999 as an important emerging nosocomial fungal pathogen by
the National Foundation for Infectious Diseases (www.nd.org/publications/
clinicalupdates/fungal/noso.html) and has been recovered from tortoises, lizards, snakes,
crocodiles and alligators at the London Zoo (Austwick, 1983). Another entomopath-
ogenic fungus, Conidiobolus coronatus, is commonly associated with lesions in both
humans and horses (Saik et al., 1990). Clearly, evaluation of potential fungal micro-
bial control agents must include an evaluation of their virulence towards non-target
organisms, especially vertebrates, with consideration given to potential human expo-
sure scenarios.
For many pathogens of invertebrates and weeds, the ability to infect is usually
constrained within a host group (e.g. many pathogens of weeds can only infect other
plants and cannot infect animals). Among non-vertebrate non-target organisms, safety
concerns rst focus on safety towards invertebrates and crop amenity plants used by
humans. In addition, plants and animals that constitute the ora and fauna of release
areas but with little direct relation to humans could be affected and safety evaluations
should consider direct effects at this level also.
Depletion of hosts
The goal of any biological control programme is to lower the population of a pest.
However, this reduction in the pest population may in turn detrimentally affect other
non-target organisms that in one way or another depend on this pest. The extent of
harm to the non-target population will very much depend on the extent and speed
of the depletion of the host and on the length of time that the host has been in its
targeted location (e.g. is it introduced or endemic?).
Over the years, the biocontrol of weeds and specically classical biological control
352 Safety of Fungal Biocontrol Agents
has consistently been plagued by conicts of interest, centring on the misconcep-
tion that there will be a sudden depletion of the target weed host (Harris, 1985). Any
exotic plant that has been deliberately imported for economic or ecological benet or
has subsequently acquired local added value, but which later invades and dominates
native ecosystems or agriculture, may still have its supporters. Thus, any threat to that
plant resulting from the introduction of a natural enemy, such as a fungal pathogen,
can create controversy and seriously or irrevocably disrupt a weed biocontrol pro-
gramme. As Harris (1985) concluded, classical biological control must be done as a
matter of public interest and with enabling legislation since the long-term effects can-
not be restricted to individual properties or release sites. Thus, even if a biocontrol
programme is clearly in the interest of the general public and supported by it, this
support must be unanimous and unambiguous because of the rule-of-law principle.
Although depletion of alien weed populations may well be popular with both
farmers and conservationists alike, individuals or organizations who derive or perceive
(more usually, misperceive) nancial or ecological benets from the presence of the
weed have the power to prevent the release of biocontrol agents. Because this process
usually involves court cases, there are a number of examples in which lengthy and
often costly battles have had to be fought (McFadyen, 1998). Such a conict of inter-
est still one of the most contentious disputes in the history of biocontrol occurred
in Australia with the boraginaceous plant, Echium plantagineum. This plant is vari-
ously known as Patersons curse by farmers and Salvation Jane by bee-keepers, who
rely on it as a dependable source of pollen during dry years. The case was eventually
settled out of court after a considerable delay and cost to the biocontrol programme.
On the positive side, however, it did result in the Australian Biological Control Act,
which, for the rst time, provided a legal basis for the introduction of exotic biocon-
trol agents (Cullen and Delfosse, 1985). Ironically, of course, classical biocontrol aims
not to eliminate an alien plant indeed, this can never be achieved with a co-evolved
natural enemy but to gradually reduce its competitive ability and thus restore the
natural balance (see Chapter 6). In all probability, there will always be sufcient ow-
ers from the remaining weed populations to satisfy the needs of the bee-keepers.
An even more extreme example, in the USA, involves salt-cedars (Tamarix spp.);
alien shrubs from Eurasia, which have become the most serious threat to riparian
ecosystems in western states, often completely replacing the native ora. Such has been
the habitat change that the endangered southwestern willow ycatcher now relies almost
entirely on salt-cedars for nesting (DeLoach et al., 1996). Consequently, ornitholo-
gists, a powerful lobby in the USA, have thus far successfully blocked any attempts to
implement a classical biocontrol strategy, despite the fact that a suite of potentially
useful natural enemies has been identied in the plants native range (DeLoach et al.,
1996). Ironically, nine species of rare birds and at least ve other species of vertebrates
have been identied as being endangered as a direct result of the salt-cedar invasion.
It has taken more than a decade of research to overturn these well-meaning but sadly
misguided and even irresponsible objections. Further historical and ecological studies
have now demonstrated that the ycatcher populations had actually declined since the
arrival of salt-cedar and that experimental removal of the weed has resulted in increased
growth and density of willows, the preferred vegetation type of this bird species.
Biological control is expected gradually to reduce Tamarix populations, with a corre-
sponding increase in native shrubs, but without the intervening loss (originally
predicted by the ornithologists) of nesting habitats (DeLoach et al., 1999).
This issue is even more contentious when the target host is indigenous. The
M.S. Goettel et al. 353
arguments centring around the recent introduction of an Australian pathotype 3 (=
Entomophaga praxibuli) of the grasshopper-pathogenic fungus Entomophaga grylli into
the USA illustrate this point (Carruthers and Onsager, 1993; Lockwood 1993a, b).
Lockwood (1993a) speculated that suppression or even extinction of target as well as
non-target acridids may result in the loss of biodiversity, proliferation of new weed
species and otherwise innocuous acridid species, disruption of plant community struc-
ture, suppression of essential organisms vectored by grasshoppers and disruption of
food-chains and other nutrient cycling processes. In rebuttal, Carruthers and Onsager
(1993) pointed out that endemic E. grylli pathotypes 1 and 2 (= Entomophaga calopteni
and Entomophaga macleodii) already periodically produce epizootics in grasshopper
populations and reduce outbreak populations of grasshoppers. However, pathotype 3
from Australia, although biologically very similar to pathotype 1, differed in that it
could produce conidia in Melanoplus spp., an attribute that was lacking in pathotype
1. This was seen as advantageous as it would allow infection of two major pest species
by a single species within a single season.
Extensive evaluations of the E. grylli species complex in both the USA and Australia
provided detailed information on the biology and epizootiology of the fungus, and
models were used to predict the response to the introduction of the Australian patho-
type into the USA (Carruthers and Onsager, 1993). A detailed proposal was submit-
ted to the US regulatory agency and permits were granted for the eld release of this
pathotype into North Dakota and Alaska. Releases were made and the pathogen was
monitored. Despite initial evidence that the fungus established and increased in preva-
lence (Carruthers and Onsager, 1993), current evidence suggests that the frequency of
infection has declined to levels such that long-term survival of this pathotype in North
America is questionable (Bidochka et al., 1996). Only time will tell if the fungus will
resurface in future years.
Competitive displacement
Fungi introduced or applied as biocontrol agents have the potential to competitively
occupy a niche, thereby adversely affecting one or more native organisms within that
niche. This may be the intended effect with some fungal biocontrol agents. For instance,
the saprophytic fungus Phlebia gigantea applied to freshly cut pine stumps competi-
tively displaces Heterobasidion annosum, the cause of root rot of pine (Rishbeth, 1975).
Unintended effects would occur if this competitive displacement were to seriously affect
a non-target organism, perhaps even leading to its extinction or in some other way
detrimentally affecting a component of the ecosystem. For example, fungal pathogens
and insect parasitoids may compete within the tissue of the host insects (Goettel et
al., 1990; Vinson, 1990). The pathogen usually out-competes younger parasitoids,
while older parasitoids are often capable of completing their development within fun-
gus-infected insects.
Lockwood (1993a) suggested the possibility that the introduction of an exotic
pathotype of E. grylli from Australia could suppress native North American grasshop-
pers, seriously affecting the natural control of those species currently being regulated
by the native E. grylli species through competitive displacement of the native E. grylli
pathotypes. However, Carruthers and Onsager (1993) point out that there should be
little concern if the introduced pathotype became the dominant grasshopper pathogen
in rangeland and crop environments. Although exotic natural enemies may displace
354 Safety of Fungal Biocontrol Agents
native species, there are usually habitats in which these native species are able to coex-
ist (Bennett, 1993).
Specic Fungal Attributes and Use Patterns Related to Hazard
The potential hazards and the degree of difculty in assessing hazards will very much
depend not only on the pathogen in question, but also on its intended use. Fungal
biocontrol agents can be used in augmentative, classical and conservation biological
control. In augmentative approaches, the fungi are introduced either in low numbers
(i.e. inoculative augmentation) or in very large numbers (i.e. inundative augmenta-
tion), essentially as pesticides. In the classical approach, fungi are introduced into geo-
graphical habitats where they have not previously occurred. The intent here is that
they become established and provide self-perpetuating, long-term control. In the con-
servation approach, the habitat or management practices are manipulated in order to
favour the naturally occurring fungi.
Host range
As a group, fungi exhibit a very wide range of host specicity. Some species are very
host-specic, while others are generalists and are known from a very wide array of
hosts. Many species within Hyphomycetes are facultative pathogens and consequently
have broad host ranges. However, individual strains generally exhibit specicity for a
limited number of hosts (Glare and Milner, 1991). In contrast, many obligate
pathogens, such as E. grylli, are restricted to several closely related host species (i.e.
several species within Acrididae) (Carruthers and Onsager, 1993).
The range of species that a fungus can infect often differs between that found in
the laboratory (physiological host range) and that found in nature (ecological host
range) (Hajek and Butler, 2000). The physiological host range is determined from lab-
oratory infection assays and demonstrates which hosts could potentially be infected
under eld conditions. The ecological host range can only be determined from eld
studies. Differences between physiological and ecological host ranges are thought to
be the result of the complex biotic and abiotic conditions that occur in the eld and
have an impact on both pathogen and host susceptibility. Such conditions are not nor-
mally replicated in laboratory host-range tests. Consequently, in order to make labo-
ratory-acquired data as applicable to the eld situation as possible, every effort must
be made to mimic the eld situation (Butt and Goettel, 2000; Hajek and Goettel,
2000).
Dose-related susceptibility
Host susceptibility to fungal pathogens is, for the most part, dose-dependent. A thresh-
old for infection is presumed to exist whereby a minimum number of fungal propag-
ules is required. Thereafter, increasing numbers of propagules increase the probability
of a successful infection. This dosemortality relationship provides a built-in safety
factor in the inundative-augmentation use of many facultative fungal pathogens as
microbial insecticides, because high doses are present for only a relatively brief time.
M.S. Goettel et al. 355
Even though infection may be dose-dependent, susceptibility and disease trans-
mission are also very much dependent on many abiotic factors. For instance, most
fungi depend on high relative humidity for sporulation. If humidity conditions dur-
ing periods when large numbers of host cadavers are present are not conducive to
sporulation, adequate numbers for infection of subsequent generations will not become
available and the induced epizootic will subside.
Persistence and dispersal
Spore dispersal in most fungi is passive, relying on wind and water. In the Oomycetes,
however, the spores are motile and are reliant on the presence of water (e.g. a lm of
water on a leaf surface). In the Entomophthorales, spores are forcibly discharged and
can land centimetres from the host or be carried longer distances on air currents.
For the most part, most spore types are very sensitive to ultraviolet (UV) radia-
tion and consequently spores exposed to sunlight are short-lived. The half-life of some
spores exposed to direct sunlight can be a matter of minutes. Persistence is gener-
ally much increased in shaded habitats and in soil. In contrast, the thick-walled rest-
ing spores produced by species within the Oomycetes and Zygomycetes and some
ascomycetous fungi are capable of persisting for many years under adverse abiotic
conditions.
Genetically altered fungi
Some of the goals of genetic modication of fungi intended for biological control are
to expand the host range, increase the speed of kill by incorporating more toxic modes
of action and extend persistence (see Chapter 8). Through this direct genetic manip-
ulation, and especially through the introduction of foreign genetic material from organ-
isms within and outside the fungal kingdom, it may be possible, either directly or
indirectly, to change many attributes of a given fungus, which could drastically change
its safety. Genetically altered organisms are unique in that they have never existed in
nature in their altered form and consequently their potential hazards can only be den-
itively evaluated once the pathogen has been released into the environment. Changes
in the ecological attributes of a fungus which, for instance, favour its virulence against
a target host may also inadvertently allow the fungus to persist in new niches and affect
non-target hosts previously not encountered. Consequently, it would be prudent to
engineer fungi in such a way as to minimize their ability to persist in nature.
Nevertheless, there is still the ever-present concern that the genetically modied organ-
ism may pass its newly acquired traits to another microorganism.
Although there are several entomopathogenic fungi that have been or soon will
be genetically altered, none to date have been released in the wild, and therefore any
predictions as to their potential detrimental effects would be purely speculative.
Permission has recently been granted to release a genetically altered strain of M. aniso-
pliae (R. St Leger, personal communication), which overproduces the cuticle-degrading
Pr1 protease required for pathogenesis. This strain causes more rapid death of its host
and the subsequent melanization response of the host prevents the fungus from fur-
ther colonizing it and subsequently sporulating on the cadaver (see Chapter 8).
356 Safety of Fungal Biocontrol Agents
Non-indigenous vs. indigenous organisms
Fungal pathogens have the ability to replicate, spread, persist and adapt to new envi-
ronments. Regardless of whether pathogens are detrimental or benecial within their
area of endemicity, their population levels are more or less in ecological balance. In
other words, the pathogens themselves, as well as their hosts and other organisms
dependent on the hosts, adapt to the ecosystem and coexist together. If inoculum lev-
els of indigenous fungi are articially raised, as in inundative augmentation, there can
be a relatively short-term perturbation of the ecosystem, which is usually the desired
effect on the target host. Inoculum levels eventually return to pre-augmentation lev-
els, and the hostpathogen balance is eventually restored. Consequently, Goettel
(1995) argues that very little attention need be paid to the host range of an indige-
nous fungus that is to be used inundatively in its native area. Similar arguments are
brought forth by Keller (1998) who suggests that the closer the use of a fungus is to
the natural situation, the fewer the requirements that should be imposed on hazard
identication.
However, when a pathogen is introduced into a new environment that is, to an
area where it was previously non-indigenous three outcomes are possible. The
pathogen can simply not adapt and die out. It may survive and reproduce, but with
little consequence to the overall ecosystem. Or the microorganism can spread, unim-
peded, sometimes with devastating consequences to its host. These devastating conse-
quences are usually the desired effect in classical control if the effects are restricted to
the target host. But, if the host range was not as predicted, or if the host is benecial,
the ecological or economic results can be devastating, as seen in the example of the
accidental introduction of Dutch elm disease into North America (see examples in the
section on plant pathogens).
Mode of use: inundative vs. inoculative release
These very different strategies differ signicantly in concerns regarding non-target
effects. For classical biological control, the establishment of natural enemies is gener-
ally considered permanent and irreversible, so predicting the host range in the area of
release is critical before release. The intent of this strategy is that, after establishment,
the fungus will increase in response to host increases; in particular, highly host-spe-
cic pathogens are sought for these programmes so that their life cycles are closely tied
to host populations.
In inundative augmentation, it is assumed that the fungus is already resident in
the release area and that organisms in that area will only experience higher levels for
relatively short periods. Therefore, this strategy would only have a potentially tempo-
rary impact on the release area. Even if there were detrimental effects to non-target
organisms, these effects would only be temporary and, in essence, no different from
using a synthetic chemical insecticide, although fungal pathogens are invariably much
more host-specic. The strategy for inundative or augmentative biocontrol typically
involves the mass production and application of a formulated product (e.g. a myco-
herbicide). This, of course, can pose signicant hazards, especially as regards human
exposure. Potential human health effects include allergenicity, pathogenicity and expo-
sure to toxic metabolites. However, these hazards are minimal if the same precautions
in the application of mycopesticides are used as those that have been adopted for the
M.S. Goettel et al. 357
application of chemical pesticides. Nevertheless, it is indisputable that basic knowledge
on possible effects on humans needs to be evaluated prior to the registration of a
product based on a fungal pathogen, as is the basic requirement for any microbial con-
trol agent today.
Addressing the Hazards Examples of Safe Use
Despite the many hazards that fungi possess as a whole, numerous species are being
safely mass-produced and used for inundative augmentation to control pestiferous
arthropods, nematodes and weeds without any apparent detrimental environmental or
safety effects. Fungal pathogens have also been used for classical biological control of
insects and weeds, although use of this latter strategy has been much more common
with introductions of arthropods as natural enemies. Effects on the environment are
considered before releases, irrespective of use strategy. Effects could differ by release
strategy (e.g. introductions for classical biological control are permanent and this strat-
egy usually focuses on perennial or natural systems while inundative augmentation
usually involves release of high doses of a native pathogen in annual systems (see
above)). The effects of outbreak pest populations on non-targets if no control is under-
taken versus use of alternative controls (e.g. synthetic chemical pesticides) must always
be weighed against potential effects of biological control agents on non-target organ-
isms.
Hazards must be assessed and addressed at every step of the development and use
of a fungal biocontrol agent. Steps must be undertaken in the manufacturing process
to minimize human exposure, especially as concerns potential allergenicity or toxicity
to workers, as is required in any industrial process involving allergens or toxic chem-
icals. This concern can be resolved by monitoring workers combined with production
procedures that minimize human contact with the fungal agent. Formulations must
be developed that maximize pathogen targeting and minimize drift or exposure to the
applicators. Labels must clearly dene the environments and targeted hosts where appli-
cation is suitable. Potential dangers to non-target organisms must be addressed.
For preliminary estimates of host specicity, laboratory studies are frequently con-
ducted exposing a diversity of species to a fungal pathogen, especially including pred-
ators and parasitoids that are already important in providing partial control of the pest.
As previously mentioned, such assays should mimic the eld situation as much as pos-
sible. Another type of preliminary data on host range is that gathered by collecting
infected organisms in the eld and identifying the cause of death. Although not always
possible, the most accurate method for determining non-target impact is data from
actual trials when fungi were released. However, few eld studies have been conducted.
Below we discuss issues and the relevant data that were used to determine safety of
fungi that are currently used for biological control.
Fungi against insects
Inundative
At present, there are over a dozen commercial products based on nine principal fungal
species registered worldwide for inundative use against invertebrate pests (Shah and
358 Safety of Fungal Biocontrol Agents
Goettel, 1999; see also Chapter 3). No apparent detrimental effects have been reported
due to their use. The most common insect-pathogenic fungus produced for inunda-
tive augmentation is the hyphomycete B. bassiana (see Chapter 3). B. bassiana has been
recorded from over 700 species of arthropods, many of which are non-target or ben-
ecial hosts (Li, 1988; Goettel et al. 1990). However, most isolates are much more
host-specic. For instance, isolate GHA, currently registered in several countries against
an array of pests (Shah and Goettel, 1999) has been demonstrated as being innocu-
ous under eld conditions to many species that are included in the host list of B.
bassiana (Goettel and Jaronski, 1997).
B. bassiana has been extensively tested, both in the early 1960s, by Nutrilite
Products, Inc., and more recently, by Mycotech Corporation and Troy Biosciences, Inc.
(Goettel and Jaronski, 1997). Tests have included repeated subcutaneous, intravenous
and intraperitoneal injection of rats, as well as feeding studies. A human volunteer
even taped 0.2 g of conidia on one arm for 8 h day
1
every other day for 12 doses
with no evidence of dermatitis. No incidents of human hypersensitivity reactions have
occurred despite years of mass production of the fungus by Mycotech Corporation
(Goettel and Jaronski, 1997). Clearance studies in mice reported that a low dose
(2 10
5
spores) delivered subcutaneously cleared within 2 days (Saik et al., 1990;
Semalulu et al., 1992). No mortality occurred in embryos and larvae of the fathead
minnow, Pemephales promelas (Goettel and Jaronski, 1997). Although there is no evi-
dence that B. bassiana is infectious in these laboratory studies, there are reports in the
literature of B. bassiana infecting vertebrates, especially reptilians (see section on
Pathogenicity above). However, there has not been an upsurge in reptilian infections
since B. bassiana was commercialized. In fact, Mycotech and Troy Biosciences obtained
exemptions from the requirement of a tolerance for residues of their strains in or on
all food commodities when applied or used as a ground spray and aerial foliar sprays
on terrestrial crops (Anon., 1995, 1999).
Field studies conducted in rangeland treated with B. bassiana to control grasshop-
pers demonstrated no infection among 2500 non-target arthropods killed at the time
of eld collection and assessed for overt colonization by the fungus (Goettel et al.,
1996). In lucerne, 10 days after application with B. bassiana, c. 20% of the leaf-cut-
ting bees killed at time of collection demonstrated overt colonization by B. bassiana.
However, there was no evidence that the fungus affected leaf-cutting bee larvae, dia-
pausing prepupae or emerging next-generation adults. In lucerne, prevalence of B.
bassiana colony-forming units in coccinellids and phalangids increased 2 days after
application, but these effects were later demonstrated to be the result of ingested coni-
dia within the digestive tract, rather than blastospores within the haemocoel, which
would have been indicative of infection (M.S. Goettel, unpublished). It was concluded
that the application of B. bassiana during their studies in rangeland and lucerne caused
only minimal effects on non-target organisms.
M. anisopliae is another entomopathogen that is currently registered against sev-
eral hosts, including cockroaches, grasshoppers, weevils and scarabs (Shah and Goettel,
1999; see also Chapter 3). This fungus has undergone extensive mammalian safety
testing including inhalation and intraperitoneal injection in mice and rats, intraocu-
lar injection in rabbits, oral dosing of frogs and long-term feeding studies in rats
(Burges, 1981; Shadduck et al., 1982; Saik et al., 1990) and birds (Smits et al., 1999).
No adverse effects were reported and there was no evidence of infectivity. However,
Mycotech observed extreme toxicity to mice by an isolate of M. anisopliae and one of
M. anisopliae var. acridum (Goettel and Jaronski, 1997). In addition, another isolate
M.S. Goettel et al. 359
of M. anisopliae var. acridum was associated with severe dermal hyperallergenic response
in humans by Mycotech (S. Jaronski, personal communication).
Interestingly, Genthner and Middaugh (1995) reported that, in their laboratory
assay system, M. anisopliae conidia adversely affected both embryos and newly hatched
larvae of the inland silverside sh (see section on Pathogenicity above). It is difcult
to judge the utility of the assay system of Genthner and Middaugh for predicting the
hazard posed by a fungal control agent in the environment. Their system is highly sen-
sitive and demonstrates the ability of certain fungi to penetrate the chorion, but reject-
ing a candidate agent based on their assay seems unwarranted. It is more useful to use
their assay to label candidates that may need more extensive vertebrate safety testing.
Although several cases of human infection have been reported (see section on
Pathogenicity above), M. anisopliae has been registered and deemed safe when used
according to the label instructions. If reports of human infections increase, it is
inevitable that the fungus will need to be re-evaluated.
The hyphomycete Beauveria brongniartii has been developed for application
against the scarab pest Melolontha melolontha in Switzerland (Baltensweiler and Cerutti,
1986). After aerial applications to the forest/pasture ecozone where adult beetles aggre-
gate to mate, 10,165 insects and spiders were collected and reared to detect infection.
Overall infection among non-targets was only 1.1%, with no infections in coccinel-
lids and neuropterans but up to 9% infection among spiders.
Classical
There are relatively few examples where fungi have been used for classical control of
invertebrate pests. The most recent has been the attempted establishment of E. grylli
from Australia for control of native North American grasshoppers. Although many pre-
dictions of detrimental effects were made, indications are that the fungus failed to
establish (see section on Depletion of hosts above). Another example is the intro-
duction of the entomophthoralean fungus Zoophthora radicans into Australia from Israel
to control the introduced spotted alfalfa aphid. Non-target effects were never seen,
especially with regard to the hymenopteran parasitoid Trioxys complanatus, which was
also introduced as part of this control programme (Glare and Milner, 1991).
Releases of the Asian entomophthoralean pathogen Entomophaga maimaiga were
made in 19101911. However, the pathogen was not found to be established in the tar-
get North American populations of Lymantria dispar until 1989 (Hajek, 1999). It is
unknown whether this pathogen became established during releases in 19101911 (it
was not detected from 1911 to 1989) or from a more recent accidental introduction.
Initial studies demonstrated that this fungus would only infect lepidopteran larvae.
Further evaluations of the host range demonstrated low levels of infection in a number
of lepidopteran families (Hajek et al. 1995). These studies were followed by collecting
and rearing non-target Lepidoptera from foliage during epizootics caused by E. maimaiga
in L. dispar populations (Hajek et al., 1996). Among the > 1500 insects collected and
reared, only two individuals of two common species were found to be infected by E.
maimaiga. Because later instars of L. dispar spend signicant amounts of time in the leaf
litter, where large titres of E. maimaiga spores occur, lepidopteran larvae in the litter were
also sampled. Although high levels of infection occurred among L. dispar larvae, only
two other litter-dwelling lepidopteran larvae, of two different species, were infected by
E. maimaiga (Hajek et al., 2000). It was concluded that this pathogen is highly specic
with very little risk to non-target organisms, including non-target lepidopterans.
360 Safety of Fungal Biocontrol Agents
Nevertheless, an ecologist questioned whether fungal epizootics causing declines
in L. dispar populations might have an adverse impact that was indirect (i.e. due to
rapid host depletion (see section on Depletion of hosts above). With huge declines
in L. dispar populations, would the other natural enemies linked with L. dispar then
decline due to lack of hosts and not be able to respond quickly enough in order to
control this pest if the fungus were not active? In this hypothesized scenario, the result-
ing unstable system would be even more prone to occasional outbreak populations of
L. dispar (Valenti, 1998). This theory was proposed based on 1992 epizootics that dev-
astated L. dispar populations, but to date (2000) there is absolutely no evidence that
E. maimaiga drives other natural enemies of L. dispar to such low levels that they can-
not respond if L. dispar begins once again to increase (A.E. Hajek, unpublished data).
In fact, semi-eld (Malakar et al., 1999) and empirical eld (Hajek, 1997) studies of
interactions between E. maimaiga and the L. dispar nuclear polyhedrosis virus suggest
minimal to no negative interactions between these two virulent pathogens.
Fungi against nematodes
Many studies have been conducted investigating the use of fungi for control of plant-
parasitic nematodes and, more recently, nematodes parasitizing livestock. In fact, nem-
atode-attacking fungi are considered to be fairly common members of the soil
community. Several fungal species attacking plant-parasitic nematodes are now mass-
produced for control. Care must be taken so that fungi introduced to control plant-
parasitic nematodes do not also affect insect-parasitic nematodes, benecial fungi, such
as mycorrhizae, or other benecial members of the rhizosphere (see Chapter 5). To
date, no detrimental effects on the rhizosphere microbial community have been doc-
umented, but few studies have been conducted to address non-target effects of
nematophagous fungi. Due to the great diversity of soil inhabitants and the complex-
ity of interactions within the soil, non-target studies are difcult to conduct. However,
we can draw inferences from knowledge of the host specicity of nematophagous fungi.
Nematode-attacking fungi utilize a diversity of types of associations with their hosts
but these can generally be grouped as predatory or parasitic (Stirling, 1991). Predatory
fungi have specialized structures, e.g. sticky rings and/or pegs for trapping nematodes.
Among the predatory fungi, Arthrobotrys has received the most attention and meth-
ods for mass-producing and applying Arthrobotrys irregularis have been developed
(Cayrol, 1983). Predatory fungi are not considered very specic to certain species of
nematode prey (Barron, 1977; Kerry, 2000). Rosenzweig et al. (1985) found that, for
nine nematodes tested, including free-living as well as plant and insect parasites, seven
adhesive-producing nematode-trapping fungi were non-selective and were able to trap
and consume all of the different nematodes. For a different fungus (Monacrosporium
ellipsosporum), which traps nematodes using sticky knobs, ten of the 15 nematode
species tested were trapped (Gaspard and Mankau, 1987).
More recently, emphasis has shifted to the use of parasitic fungi attacking seden-
tary stages of nematodes, e.g. saccate females and eggs. Among these parasitic fungi,
much research has been directed toward two species: P. lilacinus and Verticillium chlamy-
dosporium (Kerry, 2000). These fungi are opportunistic parasites, showing little host
specicity, although isolates can differ in their ability to attack eggs of different nem-
atode species. In addition, these fungi do not only attack nematodes; in particular,
V. chlamydosporium is known to infect other organisms, including fungal spores and
M.S. Goettel et al. 361
eggs of snails and slugs, and it occurs in soil when root-knot and cyst nematodes are
not present (Kerry and Crump, 1998; see also Chapter 5). Interestingly, although
V. chlamydosporium populations do not depend only on populations of nematode hosts,
a direct association has been found between the numbers of chlamydospores and the
numbers of the plant-parasitic Heterodera avenae in the soil (Kerry and Crump, 1998).
P. lilacinus has been registered as a product for the control of nematodes.
Mammalian safety tests included acute oral, dermal and pulmonary toxicity tests in
rats and irritation studies in rabbits, and they indicated the relative safety of this
microorganism (http://www.ticorp.com.au/safety.htm). However, there are numerous
case reports that P. lilacinus is a causative agent of human infections (see section on
Pathogenicity above). This fungus is currently undergoing the tests necessary for
registration in Australia. Given the numerous citations of its ability to infect humans,
it would not be surprising if more extensive vertebrate testing is required for registra-
tion. It has been suggested that possibly the isolates of P. lilacinus collected from
nematodes do not present a human health risk (see Chapter 5).
Perhaps the most unusual use of a fungus for nematode control is that of
Myrothecium verrucaria. This soil-dwelling hyphomycete is mass-produced for the
control of numerous species of endo- and ectoparasitic nematodes, but, after the fun-
gus has been grown in vitro, it is killed before application (Warrior et al., 1999). Anti-
nematode effects are caused by multiple active ingredients that act synergistically to
cause indirect effects; after application, the soil microcosm becomes an inhospitable
habitat for nematodes, affecting motility, host/mate nding and egg development and
increasing the parasitism of nematode eggs. Applications of this heat-killed fungus have
been shown to enhance antagonism toward root-knot nematodes and are associated
with structural and functional changes in the rhizosphere bacterial community
(J.W. Kloepper, personal communication). Toxicological testing has demonstrated no
effect of this product on aquatic invertebrates and tests have demonstrated no effects
on an animal-parasitic nematode (Nippostrongylus brasilense) or free-living species of
nematodes (Caenorhabditis elegans and Panagrellus redivivus) (Warrior et al., 1999).
Fungi against weeds
Inundative
As discussed previously, the fungal pathogens currently being used or evaluated as
mycoherbicides can either have narrow or wide host ranges and this will be depend-
ent upon the crop ecosystem in which that particular product is being targeted and/or
the dispersal capacity of the constituent pathogen. One of the rst products, Collego,
developed for use against the leguminous weed, Aeschynomene virginica, in rice ecosys-
tems in the southern USA, was based on a supposedly host-specic strain or forma
specialis of Colletotrichum gloeosporioides (Templeton, 1982). Subsequently, however, it
was shown to have an expanded host range within the Leguminosae, attacking several
economically important plants, including several bean species (TeBeest, 1988).
Nevertheless, this spectrum has been assessed as posing no danger to agriculture, since
the product is used exclusively within rice-based cropping systems, far removed from
potentially susceptible crops, and, in addition, the pathogen has poor dispersal ability.
A similar strategy has been adopted for the use of the high-prole crop pathogen
Phytophthora palmivora and, in what was a pioneering venture, a product (DeVine)
362 Safety of Fungal Biocontrol Agents
was developed for the control of strangle-vine (Morrenia odorata) in citrus orchards in
Florida (Ridings, 1986). Although the strain used is also pathogenic to cucurbitaceous
crops, the product can be safely applied as long as there are no susceptible crops within
200 m of the treated orchard: appropriate labelling to this effect is, of course, manda-
tory (Charudattan, 1991).
A more recent and potentially more hazardous venture has involved the develop-
ment of a mycoherbicide based on the silver-leaf fungus, Chondrostereum purpureum,
which was once a notiable disease in Europe and subject to strict legislation. This
was targeted specically at the invasive North American black cherry (Prunus serotina),
but is also used to control not only this woody weed but also other exotic hardwood
species in the Netherlands. The product, Biochon, is applied as a mycelial suspen-
sion to cut stumps to prevent re-sprouting. The risk analysis of C. purpureum, which
is a well-documented pathogen of plantation and ornamental Prunus spp., was based
almost entirely on epidemiological data (De Jong et al., 1990).
Conceptual and simulation models, supplemented by inoculation experiments,
were developed to predict the risks posed to non-targets by articially increased pop-
ulations of pathogens (De Jong et al., 1991). It was demonstrated that, although the
risk to susceptible crops is high up to 500 m from the treated area, this falls dramat-
ically thereafter and is negligible at 5000 m. Thus, the product has been approved for
safe use as long as there are no Prunus orchards within this range.
Classical
The inherent safety and stability of co-evolved natural enemies has been emphasized
recently (Marohasy, 1996; McFadyen, 1998). After more than 100 years of experience
of classical biocontrol of weeds, involving the release of over 600 exotic agents, there
are only eight examples of damage to non-target plants. Five were anticipated, all were
entirely predictable behavioural responses and not the result of host shift and, most
importantly, none had any signicant economic or environmental impact. All these
examples involved insect agents, whilst all of the 20 or more fungal pathogens so far
used for classical weed biocontrol have proved to be extremely safe, with a high suc-
cess rate (Evans, 2000). Nevertheless, there is still a general fear or mistrust of the con-
cept of exploiting exotic plant pathogens by many countries, which is rarely expressed
if the project involves insects for weed control. For various reasons, plant pathogens
are adjudged to pose more of a risk, probably based on historical associations with
invasive crop diseases (see section on Plant pathogens above).
In contrast, Australia has shown some degree of plasticity or leniency in vetting
or approving the release of exotic plant pathogens that have extended host ranges,
attacking plant species other than the target weed, and these cases have been docu-
mented recently (Evans, 2000). An example is the introduction of a rust, Uromyces
heliotropii, into Australia for control of the alien weed Heliotropium europaeum. Despite
the fact that endemic Heliotropium spp. had proved to be susceptible in greenhouse
tests, Hasan and Delfosse (1995) argued successfully that the Australian and alien
Heliotropium spp. never overlapped in their ranges and that climatic conditions in the
non-target areas were unsuitable anyway for rust establishment and development, and,
therefore, that the risks were low. A similar, potential impasse was also faced and sur-
mounted in the case of rubber-vine weed, Cryptostegia grandiora (Asclepiadaceae), in
Queensland. The co-evolved rust Maravalia cryptostegiae, from Madagascar, was shown
to be pathogenic to a rare, endemic asclepiad (Evans and Tomley, 1994). However, it
M.S. Goettel et al. 363
was concluded that the risks involved were minimal and that the rust was safe to
release, since the symptoms were not severe, and probably due to the severity of the
testing protocol (Wapshere, 1989), and the only known habitat of the native species
is itself endangered, as well as being both geographically distant and climatically dis-
tinct from the predicted range of the weed. Moreover, the threat posed by the weed
to entire ecosystems far outweighed the potential loss of one species (Evans, 2000).
Even more recently, the microcyclic rust, Puccinia melampodii, from Mexico, has
been approved for importation into Australia as part of an integrated strategy for man-
agement of the composite weed Parthenium hysterophorus, even though it has been
found to sporulate on a related indigenous, albeit weedy, plant species, as well as on
several sunower and marigold cultivars (Evans, 2000). Once again, it was concluded
by the Australian Quarantine and Plant Inspection Service that the actual and poten-
tial hazards involved in not attempting to control this allergenic weed, especially the
threat to human health, were signicantly greater than the perceived risks to non-target
plants.
In summary, based on the protocols now in place and the experience gained, the
classical introduction of fungal pathogens for the control of alien invasive plants is a
safe, environmentally benign, economic and potentially sustainable strategy for long-
term weed management.
Regulations and Registration
We have seen how fungi can be successfully and safely used as both classical and inunda-
tive biological control agents. However, we have also seen that fungi as a whole can
possess properties that make them potentially hazardous both to the user and to the
environment in general. Consequently, the development and use of fungi as biocon-
trol agents requires an assessment of their potential hazards. In most countries, regu-
lations and registration requirements serve two major purposes: (i) to ensure the safety
of the agent; and (ii) to ensure efcacy. The major challenge facing the regulatory com-
munity is to adequately address safety issues without at the same time unduly slow-
ing research and impeding the development and implementation of microbial control.
In many cases, the intended use of the organism and its origin will determine the
type of regulatory oversight that will be required. For instance, in most countries, the
importation of a non-indigenous organism intended for classical biological control will
be regulated differently (and in many cases fall under different legislation) from the
regulation of microorganisms intended for inundative use, be they indigenous or not.
Here we attempt to provide an overview of the regulations and registration require-
ments for microbial control in general, while emphasizing as much as possible the spe-
cic requirements or concerns related to fungal microbial control agents.
Regulation of fungi as classical biocontrol agents
Most countries regulate the importation and release of biological control agents, includ-
ing pathogens, through legislation designed specically for the exclusion of noxious
agents. For example, the Animal and Plant Health Inspection Service (APHIS) of the
US Department of Agriculture (USDA) until recently regulated the importation and
release of microorganisms under the Federal Plant Pest Act and the Plant Quarantine
364 Safety of Fungal Biocontrol Agents
Act (OTA, 1995). Regulation of biological control agents through such plant pest
statutes often posed difculties, in that the acts addressed biological control agents in
the context of noxious weeds or other concerns. For instance, in the USA, there were
jurisdictional problems concerning the granting of permits for release of agents because
the acts only covered the movement of agents and not their release (OTA, 1995). On
20 June 2000, a new Plant Protection Act was passed by the US Congress. This Act
repealed ten statutes, including the Plant Quarantine Act and the Federal Plant Pest
Act. For the rst time, it recognizes biological control agents as benecial tools, sepa-
rating them from pests in the context of the denitions (Henstridge, 2000; P.
Henstridge, personal communication). It adds a new denition of biological control
based on the denition under the International Plant Protection Convention. It also
separates the denition entirely from the denition of plant pest. This changes the
focus from negative to positive and places the correct emphasis on biocontrol as a ben-
ecial control method. It contains language that emphasizes the need to avoid imped-
ing commerce where there is no risk. For example, it includes provisions that clarify
the Secretarys authority to allow for organisms or groups of organisms to be exempt
from regulation once it is determined that they do not present a risk. In addition,
there is a provision clarifying an individuals right to petition the Secretary to have an
organism added or removed from regulation and contains language that emphasizes
the need to facilitate commerce in benecial organisms.
Australia is the only country that has enacted legislation the Australian Biological
Control Act that deals specically with biological control. Nevertheless, biological
control agents are still regulated under other acts, such as the Quarantine Act and the
Wildlife Protection Act; the Biological Control Act is invoked only as a last resort in
controversial situations.
Recently, the Food and Agriculture Organization of the United Nations devel-
oped a code of conduct for the import and release of exotic biological control agents
(FAO, 1996). The objectives of the code are to facilitate the regulation of exotic bio-
logical control agents through introducing internationally acceptable procedures and
practices.
The most important part of the code as far as safety is concerned is paragraph
4.5. It states that the importer of biological agents must include:
an analysis of the risks posed to possible nontarget organisms and to the environment
generally and should entail available emergency procedures should the biological control
agent after release display unexpected adverse properties. The dossier should also contain
a report detailing laboratory tests, and or eld host range of the candidate agent. Testing
should be based on recommended procedures and approved by the authority. These tests
should relate to the candidate agent only and different procedures should apply to any
additives used in formulations of products which contain biological control agents.
The difculty lies in that the tests and information that are appropriate and are ade-
quate to ensure that these conditions are met are not apparent and are certainly open
to interpretation. Nevertheless, this code addresses the importance of conducting a risk
assessment prior to the introduction of an exotic agent.
When the concept of employing plant pathogens for weed control was rst put
into practice in the 1970s, it was almost taken for granted that the fungal agents had
to have an extremely narrow host range and, preferably, that they should be specic
to the target weed. As was discussed (see section on Addressing the Hazards above),
this is no longer a necessary prerequisite. For inoculative or classical biological con-
M.S. Goettel et al. 365
trol, the main emphasis is, and must always be, placed on high specicity to the weed
target or its near relatives. Thus, the most intensive, time-consuming and expensive
part of the programme involves determining the host range of the candidate pathogen,
for which the centrifugal, phylogenetic screening protocol or methodology has been
developed and subsequently modied (Wapshere, 1975, 1989). The strategy that
underpins the classical biological control approach is based on the theory that only
those natural enemies that have co-evolved with the target weed in its centre of ori-
gin or diversity will have the necessary attributes of high virulence and specicity for
successful classical agents. The initial biocontrol programmes concentrated on testing
plants of economic value in the release area instead of the more relevant concept of
genetic relatedness. The centrifugal, phylogenetic screen now offers a scientic and
dependable method of host-range testing with a proven track record (McFadyen, 1998).
In fact, the aforementioned Australian Biological Control Act does not demand that
the exotic agent selected for importation be host-specic, but only that it should not
cause any signicant harm to any person or to the environment. The Act thereby allows
a certain degree of latitude, both in interpreting the results of host-range screening and
in the predicted host range of the candidate agent (see section on Addressing the
Hazards above).
One of the main difculties that remain as far as entomopathogenic fungi are
concerned is the lack of acceptable evaluation methods that would adequately address
host range (Hajek and Goettel, 2000). Laboratory bioassays, which are at present almost
exclusively relied upon to establish host range as far as most regulations for the impor-
tation of exotic entomopathogens are concerned, measure the physiological host range.
However, it has been established that the physiological host range does not adequately
predict the ecological host range, the range manifested under eld conditions (Hajek
and Butler, 2000). But, by the careful design and use of laboratory and semi-eld
assays that incorporate pertinent parameters and through detailed knowledge of
the ecology and epizootiology within the area of endemicity of the pathogen in ques-
tion, information for pertinent risk assessment for non-target organisms as a result of
the introduction of entomopathogenic fungi can be obtained (Hajek and Goettel
2000).
Because agents intended for classical control are expected to have minimal con-
tact with humans, they are generally exempt from mammalian safety testing. Concerns
would arise if a potential agent were hyperallergenic and were spread aerially in large
numbers during epizootics, but such fungi are not being pursued for introductions.
Regulation of fungi as microbial pesticides
Mycopesticides are often based on an indigenous rather than an exotic fungal pathogen.
Hence, the selected pathogen is already in the system but, for one reason or another,
is not effective in controlling its host. The emphasis here is no longer on host speci-
city but on ensuring, through epidemiological analysis, that the mycopesticide poses
no danger to neighbouring useful plants or insects and especially that it should not
establish in a new, non-target host (see section on Addressing the Hazards above). It
is highly unlikely that an indigenous pathogen would establish itself in a new host as
a result of inundative use. If some non-target organisms were to be affected, it would
be expected that such an effect would be only temporary and only occur within the
immediate area of application, as is currently the case with most chemical insecticides
366 Safety of Fungal Biocontrol Agents
used at present. Unfortunately, most registration requirements for indigenous micro-
bial control agents still require relatively extensive laboratory host-range testing; this is
a requirement that is not generally demanded of most chemical pesticides because, a
priori, a wide host range is expected.
Most countries require registration of fungi destined to be used inundatively as
microbial pesticides. And, as with agents destined for the classical control approach,
one of the difculties in regulation of these agents is that, once again, they are regu-
lated by legislation initially designed for chemical pesticides. For instance, in the USA,
microbial pesticides are regulated and registered by the EPA under the Federal
Insecticide, Fungicide and Rodenticide Act. Even though the EPA developed special
data requirements for microbial pesticides in the early 1980s, many problems existed
until the mid-1990s when the EPA adopted a separate review system for microbial
pesticides from that for conventional pesticides (OTA, 1995). Some problems with the
methodology still exist.
An Organization for Economic Cooperation and Development (OECD) survey
of data requirements for the registration of biological pesticides in OECD member
countries found that there were real differences in certain data requirements among
the respondent countries, most notably in physical-chemical properties, ecotoxicology
and environmental fate. This situation exists even though a similar approach was used
to structure data requirements (Table 13.1) and emphasis was placed on the same test
areas or study categories (OECD, 1996). These differential data requirements have
been one of the most detrimental aspects of regulations of microbial control products.
In many cases, a company wishing to register a product in several countries would
have to provide different registration packages for each country, each with different
data requirements and formatting, thereby signicantly increasing the costs of regis-
tration of products, many of which are useful for niche markets at best.
A promising development in recent years has been the move towards harmoniza-
tion of regulatory requirements among countries. For instance, under the auspices of
a North American Free Trade Agreement Technical Working Group on Pesticides, the
USA, Canada and Mexico have worked together to harmonize data requirements for
registration of microbial pest control products between their countries (NAFTA, 1998),
making joint reviews possible (NAFTA, 1999). Similar attempts are being made to
develop common registration requirements for the European Union (Neale and
Newton, 1999). An even more ambitious project is to standardize and harmonize core
M.S. Goettel et al. 367
Table 13.1. Data requirements for the registration of a micro-
bial pathogen. (Adapted from OECD, 1996.)
Identity
Physical, chemical and biological properties
Function, mode of action and handling
Manufacturing, quality control and analytical methods
Residues
Efcacy
a
Toxicology
Ecotoxicology
Fate and behaviour in the environment
a
USEPA requires efcacy data only for pesticide products used
to control pests that threaten public health.
data requirements for the registration of microbial pesticides within the 29 countries
that are members of the OECD (OECD, 1999).
The goal of vertebrate safety testing is to assess the hazard posed by a candidate
microbial pest control agent (MPCA) by means of carefully selected laboratory tests.
The World Health Organization (WHO) was the rst organization to propose a tiered
testing strategy to evaluate the hazard posed by MPCAs to mammals (Anon., 1981b).
Elements of this proposal are incorporated into the current regulatory guidelines for
Canada, the USA, and the European Union. These tests replaced the long-term assays
that are used in assessing chemical insecticides with short-term (1 month) exposures
that utilize invasive routes, such as intravenous and/or intraperitoneal injection, as well
as feeding studies. This battery of tests is referred to as Tier 1 (US Code of Federal
Regulations 40 CFR 158.740c). If questions arise during Tier 1 testing, the candidate
then goes through a second battery of tests that are more extensive. A third tier of tests
is also available, but it is unlikely that an MPCA that does not clear Tier 2 will go to
that level, because it would most probably no longer be considered as a viable control
product at this stage. Unfortunately, many of these data are unavailable to the public
because they are considered proprietary. However, the results of some of these acute
tests have been published in peer-reviewed publications. Routes of exposure in these
published studies include inhalation, subcutaneous injection, intraperitoneal injection,
intravenous injection, dermal and ocular irritancy testing, dermal sensitization studies
and feeding studies (Anon., 1981b; Burges, 1981; Shadduck et al., 1982; Siegel and
Shadduck, 1987; Mier et al., 1989, 1994; Kerwin et al., 1990; Saik et al., 1990; Semalulu
et al., 1992; Goettel and Jaronski, 1997; Ward et al., 1998; Smits et al., 1999).
The MPCA does not have to be harmless in all tests and at all concentrations,
but, rather, the circumstances under which it produces infection or mortality must be
elucidated. Historically, these tests have emphasized assessing the infectivity and path-
ogenicity of a candidate organism, but recently concerns have been raised about assess-
ing the allergenicity of MPCAs and these tests may be added to the protocols necessary
for registration (Ward et al., 1998). Aspects of the biology (such as maximum
temperature tolerated) of the MPCA are considered when designing tests as well as
possibly vulnerable organ systems, based on literature reviews. This latter point may
be complicated by the taxonomic status of an MPCA. Medical reports may only identify
a vertebrate isolate to genus or, in some cases, the vertebrate isolate may have been
misidentied (Siegel et al., 1997). These questions can be addressed during safety test-
ing by choosing a particular route of exposure, such as inhalation or intravenous injec-
tion, and/or by extensively characterizing the isolate proposed as the MPCA.
Characterization includes both taxonomy (this can include differentiating between
entomopathogenic and non-entomopathogenic strains) and identication of toxins
and/or toxic metabolites.
Regulatory agencies utilize the data on hazard to determine risk (hazard expo-
sure). It is unlikely that an MPCA that successfully passes these screens will cause prob-
lems when released in the eld, but a possibility, perhaps remote, always exists that
there may be susceptible species or a unique scenario in the eld that may result in
infection or mortality. The perspective necessary for evaluating both safety data and
eld reports was succinctly stated by Burges (1981):
a no-risk situation does not exist, certainly not with chemical pesticides, and even with
biological agents one cannot absolutely prove a negative. Registration of a chemical is
essentially a statement of usage in which risks are acceptable and the same must be
applied to biological agents.
368 Safety of Fungal Biocontrol Agents
Concluding statements
History has already demonstrated that fungi can be effectively and safely used in bio-
logical control. As we become more familiar with microbial control agents, and micro-
bial ecology in general, regulations are being adopted to address the pertinent concerns.
A key is that registration requirements for biocontrol fungi must remain exible and
address the hazards of the specic candidate in question and its proposed use on a
case-by-case basis. The allowances for exemptions to many data requirements are of
paramount importance. And hazards must also be weighed in consideration of the
benets of microbials, which, to date, almost always outweigh those of chemical
pesticides.
Acknowledgements
We would like to thank B.R. Kerry, IARC-Rothamsted, and J.W. Kloepper, Auburn
University (Department of Plant Pathology), for sharing unpublished manuscripts, P.
Henstridge, USDA, APHIS, Plant Protection and Quarantine, Washington, DC, for
providing information on the new US Plant Protection Act, B. Brodie, USDA
Agricultural Research Service (ARS), Ithaca, New York, and L. Rehburger, Valent
Biosciences Corp., for helpful discussions, and Stefan Jaronski, USDA, ARS, Sidney,
Montana, for providing constructive criticism of the manuscript. This is LRC contri-
bution number 387-0045.
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