Bacillus thuringiensis (Bt) is a valuable source of insecticidal proteins for use in conventional sprayable formulations and in transgenic crops, and it is the most promising alternative to synthetic insecticides. However, evolution of resistance in insect populations is a serious threat to this technology. So far, only one insect species has evolved significant levels of resistance in the field, but laboratory selection experiments have shown the high potential of other species to evolve resistance against Bt. We have reviewed the current knowledge on the biochemical mechanisms and genetics of resistance to Bt products and insecticidal crystal proteins. The understanding of the biochemical and genetic basis of resistance to Bt can help design appropriate management tactics to delay or reduce the evolution of resistance in insect populations.

Biochemistry and Genetics of Insect Resistance to Bacillus thuringiensis

Evolution of resistance in pests can reduce the effectiveness of insecticidal proteins from Bacillus thuringiensis (Bt) produced by transgenic crops We analyzed results of 77 studies from five continents reporting field monitoring data for resistance to Bt crops, empirical evaluation of factors affecting resistance or both Although most pest populations remained susceptible, reduced efficacy of Bt crops caused by field-evolved resistance has been reported now for some populations of 5 of 13 major pest species examined, compared with resistant populations of only one pest species in 2005 Field outcomes support theoretical predictions that factors delaying resistance include recessive inheritance of resistance, low initial frequency of resistance alleles, abundant refuges of non-Bt host plants and two-toxin Bt crops deployed separately from one-toxin Bt crops The results imply that proactive evaluation of the inheritance and initial frequency of resistance are useful for predicting the risk of resistance and improving strategies to sustain the effectiveness of Bt crops

Insect resistance to Bt crops: lessons from the first billion acres

Evolution of insect resistance threatens the continued success of transgenic crops producing Bacillus thuringiensis (Bt) toxins that kill pests. The approach used most widely to delay insect resistance to Bt crops is the refuge strategy, which requires refuges of host plants without Bt toxins near Bt crops to promote survival of susceptible pests. However, large-scale tests of the refuge strategy have been problematic. Analysis of more than a decade of global monitoring data reveals that the frequency of resistance alleles has increased substantially in some field populations of Helicoverpa zea, but not in five other major pests in Australia, China, Spain and the United States. The resistance of H. zea to Bt toxin Cry1Ac in transgenic cotton has not caused widespread crop failures, in part because other tactics augment control of this pest. The field outcomes documented with monitoring data are consistent with the theory underlying the refuge strategy, suggesting that refuges have helped to delay resistance.

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Insect resistance to Bt crops: evidence versus theory

Transgenic crops producing Bacillus thuringiensis (Bt) toxins for insect pest control have been successful, but their efÞcacy is reduced when pests evolve resistance. Here we review the deÞnition of Þeld-evolved resistance, the relationship between resistance and Þeld control problems, the theory underlying strategies for delaying resistance, and resistance monitoring methods. We also analyze resistance monitoring data from Þve continents reported in 41 studies that evaluate responses of Þeld populations of 11 lepidopteran pests to four Bt toxins produced by Bt corn and cotton. After more than a decade since initial commercialization of Bt crops, most target pest populations remain susceptible, whereas Þeld-evolved resistance has been documented in some populations of three noctuid moth species: Spodoptera frugiperda (J. E. Smith) to Cry1F in Bt corn in Puerto Rico, Busseola fusca (Fuller) to Cry1Ab in Bt corn in South Africa, and Helicoverpa zea (Boddie) to Cry1Ac and Cry2Ab in Bt cotton in the southeastern United States. Field outcomes are consistent with predictions from theory, suggesting that factors delaying resistance include recessive inheritance of resistance, abundant refuges of non-Bt host plants, and two-toxin Bt crops deployed separately from one-toxin Bt crops. The insights gained from systematic analyses of resistance monitoring data may help to enhance the durability of transgenic insecticidal crops. We recommend continued use of the long- standing deÞnition of resistance cited here and encourage discussions about which regulatory actions, if any, should be triggered by speciÞc data on the magnitude, distribution, and impact of Þeld-evolved resistance.

https://bioone.org/journals/journal-of-economic-entomology/volume-102/issue-6/029.102.0601/Field-Evolved-Insect-Resistance-to-Bt-Crops--Definition-Theory/10.1603/029.102.0601.pdf

Field-Evolved Insect Resistance to Bt Crops: Definition, Theory, and Data

Helicoverpa armigera and Helicoverpa zea are major caterpillar pests of Old and New World agriculture, respectively. Both, particularly H. armigera, are extremely polyphagous, and H. armigera has developed resistance to many insecticides. Here we use comparative genomics, transcriptomics and resequencing to elucidate the genetic basis for their properties as pests. We find that, prior to their divergence about 1.5 Mya, the H. armigera/H. zea lineage had accumulated up to more than 100 more members of specific detoxification and digestion gene families and more than 100 extra gustatory receptor genes, compared to other lepidopterans with narrower host ranges. The two genomes remain very similar in gene content and order, but H. armigera is more polymorphic overall, and H. zea has lost several detoxification genes, as well as about 50 gustatory receptor genes. It also lacks certain genes and alleles conferring insecticide resistance found in H. armigera. Non-synonymous sites in the expanded gene families above are rapidly diverging, both between paralogues and between orthologues in the two species. Whole genome transcriptomic analyses of H. armigera larvae show widely divergent responses to different host plants, including responses among many of the duplicated detoxification and digestion genes. The extreme polyphagy of the two heliothines is associated with extensive amplification and neofunctionalisation of genes involved in host finding and use, coupled with versatile transcriptional responses on different hosts. H. armigera’s invasion of the Americas in recent years means that hybridisation could generate populations that are both locally adapted and insecticide resistant.

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Genomic innovations, transcriptional plasticity and gene loss underlying the evolution and divergence of two highly polyphagous and invasive Helicoverpa pest species

Susceptibility of Heliothis virescens (F.), Helicoverpa zea (Boddie), Pseudoplusia includens (Walker), Spodoptera exigua (Hubner), and Spodoptera frugiperda (J. E. Smith) to purified endotoxins and commercial formulations of Bacillus thuringiensis Berliner was measured in a wide range of colonies collected from 8 states in the U.S. Cotton Belt during 1992 and 1993. Results indicated that field-collected colonies of H. virescens, H. zea, P. includens, and S. frugiperda were as susceptible as laboratory-reared colonies and those reported in the literature to the purified endotoxin proteins Cry1Ac and Cry1Ab or the commercial formulations Javelin WG, Dipel ES, and Condor OF in diet-treated assays. Colonies of S. exigua collected from transgenic cotton expressing endotoxin protein had elevated median lethal concentrations (LC50s) compared with a colony collected from nontransgenic cotton or those from laboratory colonies. Ranges of LC50s for field-collected colonies of H. virescens generally were similar to the ranges observed for laboratory colonies and similar to those reported in the literature. Wider ranges of variation in LC50s were observed among populations of H. zea and S. exigua than among populations of the other species. However, the highest LC50 observed for H. zea was no higher than those reported in the literature. Only a few colonies of P. includens and S. exigua were tested. P. includens susceptibility was generally greater than that of H. virescens and less than that of H. zea. S. frugiperda was the least susceptible species studied. Variability in LC50s obtained with Cry1Ac ( r = 0.702) correlated with variability in LC50s for Cry1Ab across 13 colonies of H. virescens exposed to both proteins. Colonies of H. zea and H. virescens were pooled into single colonies and selected with the insecticidal proteins to produce endotoxin-resistant strains. Selection for resistance in H. virescens was not successful, but a strain of H. zea was selected that had elevated LC50s (10 times) after 2 generations and very high LC50s compared with a susceptible laboratory strain (100 times) after 8 generations, suggesting that H. zea has the genetic capacity to develop resistance to endotoxin proteins.

Variation in Susceptibility of Noctuid (Lepidoptera) Larvae Attacking Cotton and Soybean to Purified Endotoxin Proteins and Commercial Formulations of Bacillus thuringiensis

Efficacy of Bt cotton expressing two insecticidal proteins of Bacillus thuringiensis Berliner on selected caterpillar pests.

Impact of Nosema (Microsporidia) infection and fumagillin treatment on Lygus lineolaris (Hemiptera: Miridae).

Tarnished plant bug, Lygus lineolaris Palisot de Beauvois (Hemiptera: Miridae), has become a primary pest of cotton in the Midsouthern United States. Insect growth regulators such as novaluron are an important part of L. lineolaris management. While novaluron is lethal to nymphs, it does not kill adults, so it has been used when nymphs are the primary stage present. However, cotton yield protection was observed from an application of novaluron when adults were the predominant stage present. To explain this, a series of studies were conducted to examine sublethal impacts of novaluron to L. lineolaris adults. Novaluron ingestion by adults reduced hatch rate and sometimes reduced oviposition rate. Ingestion by either males or females reduced hatch rates, but the reduction was greater from female exposure. Contact exposure of adults with novaluron residues within 1 d of application reduced hatch rate by about 50%, but the impact on oviposition was inconsistent. A field study showed reduced hatch rate from contact exposure to mixed-age natural populations, but the overall net reproductive rate was not reduced. Surface exposure of eggs to novaluron did not reduce hatch rate. Overall, exposure of tarnished plant bug adults to novaluron, regardless of adult age or exposure route, reduced egg viability. However, the impact on oviposition rate and net reproductive rate varied with adult age and exposure route. This understanding of sublethal impacts of novaluron, in addition to lethal impacts on nymphs, should be considered when choosing application times to maximize effects on L. lineolaris populations.

Sublethal Impacts of Novaluron on Tarnished Plant Bug (Hemiptera: Miridae) Adults.

A laboratory experiment was conducted to examine the susceptibility of the nymphal stages and adult stage of the tarnished plant bug, Lygus lineolaris (Palisot de Beauvois), to a pyrethroid (permethrin) and an organophasphate (methamidophos) insecticide. One laboratory colony and the first generation of two field collected colonies were examined for their susceptibility to permethrin, while only the laboratory colony was used to examine susceptibility to methamidophos. Insects were individually placed into glass vials coated with various doses of each insecticide and mortality was recorded after three hours. Four replications of each of the first through fifth instar nymphs and adults were assayed with a different cohort of individuals used for each replication. For each insecticide, estimated LC50's of the fourth and fifth instar nymphs were approximately 1.5 to 2 times as high as that of adults for both insecticides examined, while the first through third instar nymphs were less than that of adults. Overall, this study indicates that the effectiveness of an insecticide application targeting tarnished plant bugs may be dependent upon the percentage of the various life stages present within a given field.

K. S. Knighten

Papers

Variation in Susceptibility of Noctuid (Lepidoptera) Larvae Attacking Cotton and Soybean to Purified Endotoxin Proteins and Commercial Formulations of Bacillus thuringiensis

Efficacy of Bt cotton expressing two insecticidal proteins of Bacillus thuringiensis Berliner on selected caterpillar pests.

Impact of Nosema (Microsporidia) infection and fumagillin treatment on Lygus lineolaris (Hemiptera: Miridae).

Sublethal Impacts of Novaluron on Tarnished Plant Bug (Hemiptera: Miridae) Adults.

Response of different life stages of the tarnished plant bug to various insecticides