In a review
published in January in the Journal of Economic Entomology, Bruce Tabashnik,
Ph.D., and Yves Carrière, Ph.D., of the University of Arizona and Jeffrey
Fabrick, Ph.D., of the USDA Agricultural Research Service, examined
patterns of Bt resistance in agricultural pests around the globe.
Insecticidal proteins from Bacillus thuringiensis (Bt)
are toxic to some important pests but not to most nontarget organisms including
vertebrates and arthropod natural enemies. The area planted globally to
transgenic crops that produce Bt proteins increased from 1 million hectares
(ha) in 1996 to 109 million ha in 2019 (ISAAA 2019). In 2019, millions of
farmers in 27 nations planted Bt crops including corn, cotton, soybean,
sugarcane, and eggplant. Bt crops can suppress pests, decrease reliance on
conventional insecticides, and boost biological control However, evolution of
resistance by pests continues to reduce the benefits of Bt crops despite
widespread adoption of Bt crops called pyramids that produce two or more
distinct Bt proteins that are toxic to each target pest.
Three categories of resistance to Bt crops: (1) practical
resistance, (2) early warning of resistance, and (3) no decrease in
susceptibility (Tabashnik and Carrière 2019).
Some of the
important highlights of the review paper are:
· *Bt
crops have helped to suppress pests while also decreasing the need for
conventional insecticides and augmenting the effectiveness of biological
control species.
· *Two
stunning successes of Bt crops against invasive pests in the United States are
suppression of the European corn borer (Ostrinia nubilalis) to its lowest
levels in more than 75 years by Bt corn and eradication of the pink bollworm (Pectinophora
gossypiella) using Bt cotton together with sterile moth releases and other
tactics.
· *An
example of a success against a native pest is the control of the tobacco budworm
moth Chloridea virescens using Bt cotton in the U.S. and Mexico.
· *In
their review, authors examined 73 sets of data on monitoring resistance to Bt
crops, including information about responses to 10 Bt toxins in 22 species of
moth and two species of beetle. The 73 cases reviewed here are based on data from monitoring
responses to 10 Bt toxins by 22 moth species and 2 beetle species in 12
countries, yielding 26 cases of practical resistance, 30 cases of no decrease
in susceptibility, and 17 cases of early warning of resistance.
· *They
differentiated resistance found in these studies into the following three
categories :
a)practical resistance, in which more
than half of the individuals in a population are resistant and the field
efficacy of the Bt crop has decreased;
b) early warning of resistance, in which
resistance has evolved but fewer than half of individuals are resistant and
efficacy of the Bt crop has not decreased;
c)c)3no decrease in susceptibility, in
which there is no statistically significant decrease observed in
susceptibility.
·
In
the 73 data sets examined, they found 26 cases of practical resistance. The
average time from first planting of a particular Bt crop to the appearance of
practical resistance was 6.6 years, 17 instances of early warning of resistance.
The mean time of detection of early warning of resistance was 8.6 years after
exposure to Bt crops. Thirty instances of no significant resistance were found
after two to 24 years of exposure, with an average duration since exposure to
Bt crops of 12.2 years.
·
The
many instances of practical resistance led to the question of how resistance
can be delayed or prevented. One important way to reduce resistance is by
creating refuges consisting of non-Bt-modified plants that serve as hosts for
pest insects that are not resistant. Refuges were first envisioned to reduce
evolution of resistance to insecticide sprays, but they have been crucial for
slowing evolution of resistance to Bt insecticides. Because the refuge plants do
not produce Bt proteins, they allow survival of susceptible insects that can
mate with any resistant insects that emerge from Bt crops.
·
30
cases where monitoring data demonstrate no significant decrease in
susceptibility after 2 to 24 yrs. of pest exposure to Bt crops
·
Another
factor that can hinder the evolution of resistance is increasing the
concentration of Bt proteins enough to kill insects that are heterozygous for
resistance, i.e., they carry only one allele that confers resistance. This
“high-dose” strategy makes the resistance functionally recessive and less
likely to spread quickly.
·
Eight
Conditions Expected to Delay Evolution of Pest Resistance to Bt Crops :
ü the abundance of non-Bt host plant
refuges relative to Bt crops increases,
ü the inheritance of resistance becomes
more recessive,
ü the frequency of alleles conferring
resistance decreases,
ü the magnitude and dominance of
fitness costs associated with resistance increase, and
ü resistance becomes less complete
(i.e., more incomplete)
ü each toxin in the pyramid kills all
or nearly all susceptible insects (i.e., redundant killing is complete or
nearly complete),
ü little or no cross-resistance occurs
between toxins in the pyramid, and
ü pyramids are not grown simultaneously
with single-toxin plants that produce one of the toxins in the pyramid
Tabashnik says, “Theory and empirical evidence indicate that
recessive inheritance of pest resistance to Bt crops and abundant refuges of
non-Bt host plants can help to sustain the efficacy of Bt crops. When
inheritance of resistance is not recessive, the abundance of refuges relative
to Bt crops can be increased to effectively delay evolution of resistance.”
Strategies being explored to heighten the efficacy of Bt
crops include targeting each pest with two or more Bt proteins and using Bt
proteins together with RNA interference (RNAi) insecticides. In the close of
their review, Tabashnik and colleagues emphasize that rather than relying on
any one control tactic—such as transgenic crops—sustainable pest suppression
combines diverse integrated pest management tools.
Foot note : I am very happy to mention that one of my following
paper is one of papers which has been referred . “Singh, T. V. K., V. S.
Kukanar, and G. B. Supriya. 2021. Frequency of resistance alleles to Cry1Ac
toxin from cotton bollworm, Helicoverpa armigera (Hübner) collected from
Bt-cotton growing areas of Telangana state of India “. Journal of Invertebrate Pathology
. 183: 107559
Refer full length article for more details:
Journal of Economic Entomology, XX(XX), 2023, 1–13 https://doi.org/10.1093/jee/toac183
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