Bt crops are plants genetically engineered (modified) to contain the
endospore (or crystal) Bt toxin to be resistant to certain insect
pests. “Plant Genetic Systems”, in Belgium, was the first company
to produce a Bt crop (tobacco) in laboratory in 1985 but the crop
was not commercially successful (Vaek et al. 1987). However, in
1995, the Environmental Protection Agency (EPA) in the USA
approved the commercial production and distribution of
the Btcrops (corn, cotton, potato, and tobacco). Currently, the most
common Bt crops are corn and cotton (Vaek et al. 1987). In 2013,
four insect-resistant Bt brinjal (eggplant) varieties were approved
for seed production and initial commercialization in Bangladesh
(Koch et al. 2015). Recently, Bt soybean varieties expressing
Cry1Ac+ Cry1Ab were approved for commercial use in Latin
America to control lepidopteran insects (Koch et
al. 2015). Bt crops, containing Bt toxins, were planted in almost
100 million ha (Brookes and Barfoot 2017).
The most widely used Bt vegetable crop is sweet corn. Shelton et
al. (2013) compared sweet corn varieties grown in the USA where
the primary insect pest was Heliothis zea and demonstrated that
non-sprayed Bt varieties produced more clean marketable ears
than corn varieties sprayed with chemical insecticides up to 8
times.
Adoption of Bt cotton has greatly reduced the abundance of
targeted pests in cotton and other crops close to cotton that are
infested by polyphagous target insects (Naranjo 2011). In addition,
the reduction in insecticide use enabled IPM programs in Bt crops
fields and the increase of natural enemies populations.
Bt was first discovered in 1901 by the Japanese biologist Shigetane Ishiwatarias a cause of sotto
disease that was killing silkworms and named it Bacillus sotto (Milner 1994). In 1911, Ernst
Berliner isolated this bacterium from dead Mediterranean flour moth in Thuringia, Germany, and
named it Bt. In 1915, Berliner reported the existence of a parasporal body, or crystalline
inclusion (called crystal) close to the endospore within Bt spore (Fig. 1), but the activity of the
crystal was not then discovered (Milner 1994). In 1956, it was found that the main insecticidal
activity against lepidopteran insects was due to the parasporal crystal (Milner 1994). Zakharyan
(1979) reported the presence of a plasmid in a strain of Bt and suggested that the plasmids
involved in formation of endospore and crystal.
otton is one of the first crop protection products from biotechnology.
All Bt cotton plants contain one or more foreign genes derived from the soil-
dwelling bacterium, Bacillus thuringiensis; thus, they are transgenic plants.
The insertion of the genes from B. thuringiensis causes cotton plant cells to
produce crystal insecticidal proteins, often referred to as Cry- proteins. These
insecticidal proteins are effective in killing some of the most injurious caterpillar
pests of cotton, such as the larvae of tobacco
budworms and bollworms. This new technology for managing insect pests was
approved for commercialization in the United States by the U.S. Environmental
Protection Agency (EPA) in October 1995 and is now available from several seed
companies in this country, as well as in many other cotton-growing countries
around the world.
Cotton varieties containing the Cry1Ac Bt protein provide protection against three
major U.S.
Tobacco budworm, Heliothis virescens (F.), larva
BT Cotton is one of the first crop protection products from biotechnology.
All Bt cotton plants contain one or more foreign genes derived from the soil-
dwelling bacterium, Bacillus thuringiensis; thus, they are transgenic plants.
The insertion of the genes from B. thuringiensis causes cotton plant cells to
produce crystal insecticidal proteins, often referred to as Cry- proteins. These
insecticidal proteins are effective in killing some of the most injurious caterpillar
pests of cotton, such as the larvae of tobacco
budworms and bollworms. This new technology for managing insect pests was
approved for commercialization in the United States by the U.S. Environmental
Protection Agency (EPA) in October 1995 and is now available from several seed
companies in this country, as well as in many other cotton-growing countries
around the world.
Cotton varieties containing the Cry1Ac Bt protein provide protection against three
major U.S. proper use and long-term preservation of this valuable technology.
Frequent introduction of new transgenic cotton varieties creates a need to
continuously evaluate their cost-effectiveness and develop efficient plans for their
deployment. A goal of this publication is to answer questions about the technol-
ogy by explaining how Bt cotton is developed, how it controls insect pests, and how
it can be most effec- tively used in insect pest management. Restrictions on and
limitations to the use of Bt cotton are discussed, such as insects’ develop- ment of
resistance to it and approaches to preserving the technology for long- term profits.
T
he insect-disease-causing organism Bacillus thuringiensis (Bt) is a naturally
occurring soilborne bacterium found
worldwide. A unique feature is its produc- tion of crystal-like proteins that
selectively kill specific groups of insects and other organisms. When the insect eats
these Cry- proteins, its own digestive enzymes activate the toxic form of the
protein. Cry- proteins bind to specific receptors on the intestinal walls and rupture
midgut cells. Susceptible insects stop feeding within a few hours after taking their
first bite, and, if they have eaten enough toxin, die within 2 or 3 days.
Different Bt strains produce different Cry- proteins, and there are hundreds of
known strains. Scientists have identified more than 60 types of Cry-proteins that
affect a wide variety of insects. Most Cry-proteins are active against specific
groups of in- sects, such as the larvae of certain kinds of flies, beetles, and moths.
For example, Colorado potato beetle larvae are affected by Cry3A proteins; Cry1Ac
is used against tobacco budworms; and European corn borers can be killed with
Cry1Ab, Cry1F, Cry1Ac, and Cry9c proteins. Other Cry- proteins are active against
mosquito larvae, flies, or even nematodes. Some Cry-proteins have been
used for more than 30 years in various liquid and granular formulations of natural
Bt insecticides, mainly to control caterpillars on a variety of crops. The Bt cotton
varieties presently used against tobacco budworms, bollworms, and certain other
caterpillars produce the Cry1Ac protein.
The Why’s and How’s of Creating Bt
Cotton
B
ioinsecticides like Bt that are sprayed on crops may perform as well as
synthetic insecticides in very limited situations, but the
performance of Bt insecticides has been inconsis- tent in many instances.
The erratic performance in cotton is attributed to four reasons:
The toxin is rapidly degraded by ultraviolet light, heat, high leaf pH, or desiccation.
Caterpillars must eat enough treated plant tissue to get a lethal dose of the toxin, since
the toxin has no contact effect.
The sites where tobacco budworms and bollworms feed are difficult to cover with the
foliar-applied sprays.
Bt Cry-proteins are less toxic to older larvae. A cotton plant modified to produce
Cry-protein
within the plant tissues that caterpillars eat
overcomes most of the aforementioned limita- tions. The plant-produced Bt
proteins are pro- tected from rapid environmental degradation since they are not
directly exposed to the environ- ment. Incomplete coverage is not usually a
problem because the plants produce the proteins in all tissues where larvae feed,
thus ensuring that the larvae will eat the Cry-protein. The protein is always present
whenever newly hatched larvae feed, eliminating the timing problem associated
with foliar application. The result is that Bt cotton has a built-in system that
efficiently and consistently delivers Cry-toxins to the target pests from the time a
newly hatched larva takes its first bite (fig. 1).
Bt cotton offers a vastly improved method for delivering Cry-insecticides to target
insects, compared to traditional Bt sprays. Bt cotton may also be considered a form
of host plant resistance, in that the Cry-protein trait is carried in the plant’s genes,
as is traditional plant resistance to insects.
4
Figure 1. Mode of action for Bt toxin after eaten by a tobacco budworm larva. Modified with permission
from Ostlie et al. 1997.
5
Biotechnologists created Bt cotton by inserting selected exotic DNA, from a Bt
bacterium, into the cotton plant’s own DNA. DNA is the genetic material that
controls expression of a plant’s or an animal’s traits. Following the insertion of
modi- fied Bt DNA into the cotton plant’s DNA, seed companies moved the Cry-
protein trait into high- performance cotton varieties by traditional plant breeding
methods. Agronomic qualities for yield, harvestability, fiber quality, and other
important characteristics were preserved at the same time the Cry-protein gene
was added to commercial varieties.
The three primary components of the genetic package inserted into cotton DNA include:
Protein gene. The Bt gene, modified for im- proved expression in cotton, enables the cotton
plant to produce Cry-protein. The first varieties of Bt cotton produced in the United States
contained one Cry-protein gene—Cry1Ac. Other varieties contain a “stacked” gene complex, for
example— one gene for insect control (Cry1Ac) and one gene to protect the cotton from
application of the herbicide glyphosate. Future cotton varieties may include these genes, other
genes that allow the plant to produce different Cry-proteins, or insecti- cidal proteins from
sources other than Bt. There are many possible combinations for crop im- provement traits.
Promoter. A promoter is a DNA segment that controls the amount of Cry-protein produced and
the plant parts where it is produced. Some pro- moters limit protein production to specific parts
of the plant, such as leaves, green tissue, or pollen. Others, including those used in Bt cotton
and certain Bt corn varieties, cause the plant to produce Cry-protein throughout the plant. Pro-
moters can also be used to turn on and turn off protein production. Current varieties of Bt cotton
produce some Bt protein throughout the growing season.
Genetic marker. A genetic marker allows re- searchers to identify successful insertion of a
gene into the plant’s DNA. It also assists plant breeders in identifying and developing new
cotton lines with the Bt gene. A common marker is an herbicide tolerance gene linked to the Bt
gene. Following a transformation attempt to place the Bt and marker gene into the plant’s DNA,
plants are treated with herbicide. Plants that were success- fully transformed have the Bt gene
and the herbicide resistance gene and will survive herbi- cide treatment; plants without the
marker gene, and hence without the linked Bt gene, will be killed by the herbicide.
This genetic package—a Bt gene plus a promoter and marker—can be inserted into
cotton plant DNA through a variety of plant transformation techniques.
Transformed plants may be affected by the genetic package, as well as the location
of the new genes in the plant DNA. The insertion site may affect Bt protein
production and other plant functions as well. So biotechnology compa- nies
carefully scrutinize each transformation to ensure adequate production of Bt
protein and to limit possible negative effects on agronomic traits.
Following a successful transformation, plants are entered into a traditional
backcross breeding program with the variety chosen to receive the foreign Bt gene
package. The final product, a Bt cotton variety, is developed after four or five
backcross generations. Even though the new transgenic Bt cotton variety may be
named after the parent variety, agronomic qualities can be considerably different.
The Safety of Bt and Bt Cotton
B
efore registering Bt cotton, EPA reviewed data on Bt insecticides that had
been accu-
mulated for decades. Bt Cry-proteins were found to be toxic only to certain insect
groups and to have no known negative effects to humans, domestic animals, fish,
wildlife, or other organ- isms. EPA exempted Bt-produced Cry-proteins
from the requirement of tolerance in food because of their history of safety and
because they de- grade rapidly in the environment. These Cry- proteins are
considered among the safest and most environmentally friendly insecticides
known.
