Resistance of Trichoplusia ni Populations Selected by Bacillus thuringiensis Sprays to Cotton Plants Expressing Pyramided Bacillus thuringiensis Toxins Cry1Ac and Cry2Ab

Abstract

bTwo populations of Trichoplusia ni that had developed resistance to Bacillus thuringiensis sprays (Bt sprays) in commercial greenhouse vegetable production were tested for resistance to Bt cotton (BollGard II) plants expressing pyramided Cry1Ac and Cry2Ab. The T. ni colonies resistant to Bacillus thuringiensis serovar kurstaki formulations were not only resistant to the Bt toxin Cry1Ac, as previously reported, but also had a high frequency of Cry2Ab-resistant alleles, exhibiting ca. 20% survival on BollGard II foliage. BollGard II-resistant T. ni strains were established by selection with BollGard II foliage to further remove Cry2Ab-sensitive alleles in the T. ni populations. The BollGard II-resistant strains showed incomplete resistance to BollGard II, with adjusted survival values of 0.50 to 0.78 after 7 days. The resistance to the dual-toxin cotton plants was conferred by two genetically independent resistance mechanisms: one to Cry1Ac and one to Cry2Ab. The 50% lethal concentration of Cry2Ab for the resistant strain was at least 1,467-fold that for the susceptible T. ni strain. The resistance to Cry2Ab in resistant T. ni was an autosomally inherited, incompletely recessive monogenic trait. Results from this study indicate that insect populations under selection by Bt sprays in agriculture can be resistant to multiple Bt toxins and may potentially confer resistance to multitoxin Bt crops. Evolution of pesticide resistance in insect populations is a continuing threat to the efficacy of both synthetic and biological insecticides. Resistance of insects to the biological insecticide Bacillus thuringiensis (Bt), a naturally occurring soil bacterium, was first reported in the 1980s for a pest of stored grains, Plodia interpunctella (1). Resistance to Bt in Plutella xylostella and Trichoplusia ni has been identified following frequent applications of sprayable Bt formulations in agricultural situations (2-4). Since the mid-1990s, crops genetically engineered to contain insecticidal toxins from Bt have increasingly been adopted worldwide (5). With the wide adoption of Bt crops, insect resistance is a serious threat to their continuing success. Field populations of several major pest species have been reported to have developed resistance after Bt crop adoption, leading to increased crop damage (6, 7). More insect species have been reported to have increased frequencies of resistant alleles in field populations after the adoption of Bt crops (8-11).Selective application of pesticides with different modes of action has been a principal strategy for insecticide resistance management. Similarly, Bt gene pyramiding, i.e., simultaneous expression of multiple toxins that have different binding sites in the target insects, is an effective strategy to delay the development of resistance to Bt crops. This has been promoted for Bt crops currently deployed in the United States and some other regions of the world (7, 12). Bt toxins Cry1Ac and Cry2Ab do not share the same binding sites in target Lepidoptera insects (13,14) and ha...

Resistance to Bacillus thuringiensis Toxin Cry2Ab in Trichoplusia ni Is Conferred by a Novel Genetic Mechanism

“…The gene pyramiding strategy to combine Cry1Ac and Cry2Ab has proven to be effective in delaying the development of insect resistance to Bt toxins (25,26). Nonetheless, resistance to Cry2Ab has been reported in Heliothis virescens, Helicoverpa zea, Helicoverpa armigera, Helicoverpa punctigera, Pectinophora gossypiella, and Trichoplusia ni (8,(27)(28)(29)(30)(31)(32). These reports have indicated that Cry2Ab-resistant alleles are present in insect populations, and the frequency of resistant alleles will increase with selection.…”

“…These reports have indicated that Cry2Ab-resistant alleles are present in insect populations, and the frequency of resistant alleles will increase with selection. A recent study on the resistance to Cry2Ab in T. ni populations that evolved resistance to the Bt formulation DiPel in greenhouses showed that the DiPel-resistant T. ni populations not only were resistant to Cry1Ac but also had a high frequency of Cry2Ab resistance alleles, allowing their survival on Bt cotton pyramided with Cry1Ac and Cry2Ab (32).…”

“…At present, the understanding of mechanisms of resistance to Bt toxins is primarily on the resistance to Cry1Ab and Cry1Ac. Understanding of the molecular mode of action of Cry2Ab and the mechanism of resistance to Cry2Ab remains very limited (22,28,32,37). As Cry2Ab is a major Cry toxin in the current Bt crops widely deployed in the field, it is desirable to understand the characteristics of Cry2Ab resistance in insects and the underlying mechanisms that confer resistance to Bt crops.…”

“…Bt-resistant T. ni populations have been identified in pepper, tomato, and cucumber greenhouses under selection by Bt formulation sprays (38). These T. ni populations were highly resistant to Cry1Ac and also had a high frequency of Cry2Ab-resistant alleles (32,39). More importantly, the dual-toxin-resistant T. ni selected in vegetable production greenhouses can survive on the current generation of Bt cotton plants pyramided with Cry1Ac and Cry2Ab (32).…”

“…These T. ni populations were highly resistant to Cry1Ac and also had a high frequency of Cry2Ab-resistant alleles (32,39). More importantly, the dual-toxin-resistant T. ni selected in vegetable production greenhouses can survive on the current generation of Bt cotton plants pyramided with Cry1Ac and Cry2Ab (32). Therefore, the resistance to Cry2Ab in T. ni is an unique opportunity for studying mechanisms of insect resistance to Bt crops.…”