Inorganic Insecticides

Inorganic Insecticides are compounds that are not carbon based. They are stable chemicals which do not
evaporate and are frequently soluble in water. Arsenical compounds such as calcium and lead arsenate
were frequently used insecticides prior to the wide-scale development of synthetic-organic insecticides
which began in the 1940s. Other examples of inorganic compounds with insecticidal properties are
sodium fluoride, sodium silicofluoride and boric acid. These insecticides are not used in modern
production agriculture.

Biological and Botanical Insecticides

Biological insecticides are bacteria, viruses, fungi and other microorganisms that attack insects.
Naturally occurring pathogens are often very important in preventing the outbreak of pests. Some
biological insecticides have been developed and marketed for the control of pests in field crops. These
biological control agents generally have the advantage of being highly selective, and thus safe to the
environment and non-target organisms. Common examples are nuclear polyhedrosis viruses (NPVs) and
Bacillus thuringiensis (Bt insecticides), a bacteria. Some NPVs have activity of important pests such as
bollworm, tobacco budworm and various armyworms species. However, they are not frequently used
because of short persistence, slow activity and low efficacy relative to synthetic-organic insecticides. Bt
products such as Dipel® and MPV® are still marketed and used widely by homeowners and organic
producers. They were used extensively in cotton during the 1980s and 1990s, tank-mixed with other
insecticides, to help control pyrethroid resistant tobacco budworms. Now, genetically modified crops
such as Bt cotton and Bt corn have been adopted by many producers. Bt crops possess one or more
genes from Bacillus thuringiensis, and plants produce their own insecticide. Depending upon the gene
inserted into the plant genome, Bt crops provide control of very important pests such as tobacco
budworm, bollworm, pink bollworm, armyworms, corn borers, and corn rootworms.

Botanical Insecticides are naturally-occurring, organic compounds which are produced by plants.
Examples include nicotine (from tobacco) and pyrethrins (from Chrysanthemum). Derivatives of these
natural compounds were used extensively for insect control prior to the development of synthetic
insecticides. However, botanical insecticides are not currently used in the production of field crops
because of their relatively low efficacy, short persistence, and high cost of production. Although most
botanical insecticides are relatively safe to use, some plant-derived toxins are acutely toxin to humans.

Synthetic-organic Insecticides

Synthetic-organic insecticides were widely developed beginning in the 1940s. These compounds are the
primary insecticides used for the control of insect pests. The first class of synthetic-organic insecticides
used on a large scale were the organochlorines, followed by organophosphate and carbamate
insecticides. Synthetic pyrethroid insecticides were first marketed in the late 1970’s, and on a
worldwide basis, and they are probably the most useful insecticides for the control of agricultural insect
pests. Beginning in the mid-1990s, several new classes of insecticides were introduced for insect control
in agriculture. These include neonicotinoid insecticides such as Centric®, Admire Pro® and Intruder®
that are frequently used in the production of agricultural crops. Other new classes of insecticides used
in field crops are represented by spinosad (e.g., Tracer®), emamectin benzoate (e.g., Denim®) and
indoxacarb (e.g., Steward®). Also recently, new insect growth regulators have been developed and are
being widely used for insect control in many agricultural systems. There are other new insecticide
classes which are used to a limited extent in the production of cotton, corn, soybean and other field
crops. With some exceptions, mostly with IGR insecticides, synthetic-organic insecticides affect nerve
processes of insects. The toxicity to humans and other non-target organisms often varies widely among
and within the various classes of synthetic-organic insecticides.

• Organochlorine Insecticides: Organochlorine Insecticides (OC or chlorinated hydrocarbons)

were the first synthetically produced insecticides used on a large scale in production agriculture
and in other areas of insect control. Representatives of this rather diverse chemical class
include DDT, toxaphene, endosulfan, chlordane, mirex, lindane (BHC), and the cyclodienes (e.g.,
heptachlor, aldrin, dieldrin). OC insecticides are generally broad spectrum, affecting sodium
channels within the nervous system of insects. The general public generally believes that DDT
and other organochlorine insecticides are acutely toxic to humans and other non-target
organisms, but in reality, the biggest problem with these compounds is their negative effects on
the environment. OC insecticides are generally persistent and tend to bioaccumulate in the
ecosystem, being stored and concentrated within food chains, and having chronic toxicological
impacts on non-target organisms. Thus, most organochlorine pesticides have been banned from
use. However, because they were the first highly-effective insecticides, they are an important
component of insect control history.

The evolution of insect resistance to organochlorine insecticides demonstrated the potential

risks of relying solely on insecticides for pest control. However, the use of DDT and similar
compounds for control of disease vectoring insects has saved countless lives. Worldwide,
several of organochlorines are still important today. Chlordane is excellent for protecting wood
from termites but is banned in the United States.

• Organophosphate Insecticides: Organophosphorus Insecticides (OP) are a rather diverse group

of chemicals that were derivatives of compounds developed for chemical warfare in WW II. OP
insecticides were relied upon when insects developed resistance to the chlorinated
hydrocarbons. These compounds may be classified into several chemical groups (phosphonates,
phosphates, etc.). Examples include dicrotophos (Bidrin®), methyl parathion, chlorpyrifos
(Lorsban®), acephate (Orthene®), and diazinon. Many agricultural uses of OP insecticides have
been restricted in recent years, but OPs are still commonly used in many field crops and on a
worldwide basis because they provide a broad spectrum of insect control. Organophosphorus
insecticides are classical inhibitors of acetylcholinesterase, the enzyme which stimulates
breakdown of acetylcholine at the post-synaptic nerve endings. In general, these insecticides
are toxic to insects and mammals and should be handled with care. However, they are less
persistent than organochlorine insecticides, and thus have less long-term environmental
impacts.

• Carbamate Insecticides: Carbamate Insecticides were developed and used after insect
populations had developed resistance to the chlorinated hydrocarbons and OP insecticides.
Similar chemical compounds are used extensively in other areas of agriculture (herbicides,
fungicides, etc.) and medicine. One of the members of this group is carbaryl, commonly sold as
Sevin®. Carbaryl is widely used on many crops and is available to home gardeners and other
 applicators of non-restricted use insecticides because of its rather low mammalian toxicity.
Conversely, methomyl (Lannate®), carbofuran (Furadan®) and aldicarb (Temik®), three
carbamate insecticides that once were commonly used in row crop agriculture, are among the
most toxic insecticides. The carbamates act similarly to the organophosphorus insecticides in
that they inhibit acetylcholinesterase.

• Synthetic Pyrethroid Insecticides: Botanically-produced pyrethrins were seldom used for insect
control purposes because of high production costs of and their instability in sunlight. When
synthetic alternatives were developed, they were rapidly adopted for use in agriculture. Today,
synthetic pyrethroids are used for many purposes because of their efficacy, low use rates, low
cost, broad spectrum of control, and relatively low mammalian toxicity. The historical
development of pyrethroids was a long process. The first widely-used pyrethroid insecticides
were allethrin (Pynamin®), tetramethrin (Neo-Pynamin®), resmethrin, bioallethrin, fenvalerate
(Pydrin®) and permethrin (Ambush®). The next generation of pyrethroids greatly influenced
agriculture. They are still very important for insect control in commercial agriculture and
include cypermethrin (Ammo®), bifenthrin (Brigade®), lambda-cyhalothrin (Warrior®), among
others. The mode of action of pyrethroids is similar to that of DDT, affecting the function of
sodium channels in nerve cells. Most entomologists consider the synthetic pyrethroids to be the
most versatile and effective insect control materials ever developed. However, some important
insect species have developed resistance to these chemicals, including the tobacco budworm.

• Neonicotinoid Insecticides: Neonicotinoid, or chloronicotinyl, compounds are a relatively new

class of insecticides now used widely for insect control in field and vegetable crops. Examples
include imidacloprid (Admire Pro® and Gaucho®), thiamethoxam (Centric® and Cruiser®) and
acetamiprid (Intruder®). The class name originates from the mode of action. These compounds
affect nicotinic receptors, similar to nicotine, essentially mimicking the function of acetylcholine
within insect nerve cells. Neonicotinoids are characterized by more specialized activity and are
most often used for the control of sucking insect pests such as aphids, plant bugs and whiteflies.
They are also used extensively to control fleas on pets. This class of insecticides has relatively
low toxicity to humans and other non-target organisms, but they are often criticized for their
impact on pollinators.

• Insect Growth Regulators: These synthetic materials interfere with normal growth and
development processes of insects which are regulated by hormones. In general, insect growth
regulators are selective, having significant activity on a relatively small group of insects, and thus
play a limited role in the protection of agricultural crops. The oldest examples are diflubenzuron
and altosid. Diflubenzuron was used extensively in some early boll weevil eradication efforts. It
has significant activity of some species of caterpillar and grasshopper pests and is commercially
available as Dimilin®. Diflubenzuron blocks synthesis of chitin, an important component of
insect cuticle. It is also used as a chemosterilant of boll weevil. Altosid, and the newer
insecticide pyriproxyfen (Knack®), are juvenile hormone mimics (i.e., agonists). Juvenile
hormone is a chemical that encourages or regulated the maturing process of insects.
Methoxyfenozide (Intrepid®) and tebufenozide (Confirm®) are ecdysone agonists and are highly
 active on many species of armyworms and other foliage feeding caterpillars. Ecdysone is the
hormone in insects which stimulates molting.

• Other, Relatively New Insecticides: There are many other, relatively new classes of insecticide
chemistry that have recently been introduced and used in commercial agricultural.
Representatives of these new classes include spinosad (BlackHawk®, class = Naturalytes),
indoxacarb (Steward®, class Oxadiazine), emamectin benzoate (Denim®, class = Synthetic
Avermectin), and chlorantraniliprole (Prevathon® and Besiege®, class Diamide). Like older
insecticide classes, these newer insecticides are nerve poisons. However, they are more
specific, having less impact on non-target organisms and a reduced risk of causing negative
environmental impacts. The relatively narrow spectrum of activity of these compounds limits
their use to the control of specific kinds of pests, but many are very useful in controlling
populations of important insects that are resistant to older classes of insecticides.

More classes of insecticide and additional details can we found at the website for the Insecticide
Resistance Action Committee.