Amanda L. Patin

Development time and resistance to Bt crops

Crop plants genetically engineered to produce insecticidal toxins derived from the bacterium Bacillus thuringiensis (Bt) are being grown on millions of hectares, but their success will be short-lived if pests adapt to them quickly,. The primary strategy for delaying insect resistance to transgenic Bt plants is to provide refuges of host plants that do not produce Bt toxins. This potentially delays the development of insect resistance to Bt crops by providing susceptible insects for mating with resistant insects. But our laboratory results with a worldwide pest of cotton, pink bollworm moths (Pectinophora gossypiella), contradict an important assumption of the refuge strategy. We find that a resistant strain of larvae on Bt cotton takes longer to develop than susceptible larvae on non-Bt cotton. This developmental asynchrony favours non-random mating that could reduce the expected benefits of the refuge strategy.

Effects of Bt Cotton and Cry1Ac Toxin on Survival and Development of Pink Bollworm (Lepidoptera: Gelechiidae)

Abstract We evaluated the effects of Bacillus thuringiensis (Bt) toxin Cry1Ac on survival and development of a susceptible strain and laboratory-selected resistant strains of pink bollworm, Pectinophora gossypiella (Saunders). For susceptible and resistant strains tested on artificial diet, increases in Cry1Ac concentration reduced developmental rate and pupal weight. In greenhouse tests, survival of resistant larvae on transgenic cotton that produces Cry1Ac (Bt cotton) was 46% relative to their survival on non-Bt cotton. In contrast, Bt cotton killed all susceptible larvae tested. F1 hybrid progeny of resistant and susceptible adults did not survive on Bt cotton, which indicates recessive inheritance of resistance. Compared with resistant or susceptible larvae reared on non-Bt cotton, resistant larvae reared on Bt cotton had lower survival and slower development, and achieved lower pupal weight and fecundity. Recessive resistance to Bt cotton is consistent with one of the basic assumptions of the refuge strategy for delaying resistance to Bt cotton. Whereas slower development of resistant insects on Bt cotton could increase the probability of mating between resistant adults and accelerate resistance, negative effects of Bt cotton on the survival and development of resistant larvae could delay evolution of resistance.

Overwintering Cost Associated with Resistance to Transgenic Cotton in the Pink Bollworm (Lepidoptera: Gelechiidae)

Abstract Fitness costs associated with resistance to transgenic crops producing toxins from Bacillus thuringiensis (Bt) may have important effects on the evolution of resistance. We investigated overwintering costs in pink bollworm, Pectinophora gosypiella (Saunders), strains with different degrees of resistance to Bt cotton. Frequency of resistant individuals in a strain was not associated with induction of diapause or emergence from diapause in early winter. Emergence from diapause in the spring was 71% lower in three highly resistant strains than in two heterogeneous strains from which the resistant strains were derived. This underestimates the overwintering cost because the frequency of the resistance allele was relatively high in the heterogeneous strains. Emergence in the spring in hybrid progeny from crosses between the resistant and heterogeneous strains was greater than in resistant strains but did not differ from susceptible strains, showing that the overwintering cost was recessive to some extent.

Fitness Costs and Maternal Effects Associated with Resistance to Transgenic Cotton in the Pink Bollworm (Lepidoptera: Gelechiidae)

Abstract Transgenic cotton producing a Bacillus thuringiensis (Bt) toxin is widely used for controlling the pink bollworm, Perctinophora gossypiella (Saunders). We compared performance of pink bollworm strains resistant to Bt cotton with performance of their susceptible counterparts on non-Bt cotton. We found fitness costs that reduced survival on non-Bt cotton by an average of 51.5% in two resistant strains relative to the susceptible strains. The survival cost was recessive in one set of crosses between a resistant strain and the susceptible strain from which it was derived. However, crosses involving an unrelated resistant and susceptible strain indicated that the survival cost could be dominant. Development time on non-Bt cotton did not differ between the two related resistant and susceptible strains. A slight recessive cost affecting development time was suggested by comparison of the unrelated resistant and susceptible strains. Maternal effects transmitted by parents that had eaten Bt-treated artificial diet as larvae had negative effects on embryogenesis, adult fertility, or both, and reduced the ability of neonates to enter cotton bolls. These results provide further evidence that fitness costs associated with the evolution of resistance to Bt cotton are substantial in the pink bollworm.

Resistance Management: Slowing Pest Adaptation to Transgenic Crops

Transgenic crops that are genetically modified to produce insecticidal proteins from the common bacterium Bacillus thuringiensis (Bt) can help to control pests while reducing reliance on insecticide sprays. So far, no insects have evolved resistance in the field to Bt transgenic crops. However, diamondback moth populations have evolved resistance to Bt sprays in the field and many pests have evolved resistance to Bt toxins in the laboratory. To delay resistance, the refuge strategy provides host plants that do not produce Bt toxins, thereby promoting survival of susceptible pests. In Arizona, Bt cotton has been extremely effective in controlling the pink bollworm (Pectinophora gossypiella), a major pest. Despite a surprisingly high frequency of resistance in 1997, resistance did not increase in Arizona field populations of pink bollworm from 1997 to 1999. Nonetheless, pink bollworm and other insects will eventually evolve resistance, so any particular transgenic crop variety is not a permanent solution to pest problems. Instead, transgenic crops can be used in harmony with other tactics as part of integrated pest management. Evaluations of transgenic crops should consider their advantages and disadvantages compared with alternatives. If transgenic crops can greatly reduce use of hazardous insecticides, as achieved in Arizona cotton, great benefits may occur.