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The evolution of resistance to Bacillus thuringiensis in greenhouse Trichoplusia ni populations Janmaat, Alida Francisca

Abstract

The microbial insecticide, Bacillus thuringiensis (Bt), has become the mainstay of nonchemical control of Lepidopteran pests either as sprays or through the incorporation of Bt toxins into transgenic crops. Findings in the present study, report the frequent and rapid development of resistance to Bt subsp. kurstaki in populations of cabbage loopers, Trichoplusia ni, in commercial greenhouses in British Columbia, Canada. Studies of the genetic inheritance of resistance to Bt (DiPel) in these populations suggest that the Bt resistance is inherited as an autosomal, partially recessive trait and is due to more than one gene. However in a second study, dominance of Bt resistance varied with the host plant on which Bt was provided suggesting that the host plant will impact resistance evolution. Cucumber, tomato and sweet pepper are the three principal crops grown in commercial greenhouses. In laboratory studies, T. ni performance varied considerably among the three crops with the most rapid growth and highest fecundity on cucumber leaves and the least rapid growth and lowest fecundity on pepper leaves. This finding suggests that there is intense selection pressure on T. ni populations in pepper environments. Suprisingly, a negative relationship between fecundity and offspring size was observed across the three host plant treatment groups. Offspring of the most fecund cucumber treatment group were significantly smaller than offspring of the least fecund pepper treatment group. Resistance traits are often assumed to be associated with fitness costs and the presence of such costs may depend on the environment. In herbivorous insects, the host plant is a pivotal component of the herbivore's environment and it is likely that resistance-associated fitness costs are magnified by poor nutritional resources. Therefore, the performance of four genotypic lines (resistant, susceptible and reciprocal hybrids) and their progeny were compared among the three greenhouse crops. Interestingly, the magnitude of fitness costs associated with Bt resistance increased with declining host plant suitability. Moreover, no viable progeny were produced by resistant lines fed the least suitable host plant. Therefore, tritrophic interactions between T. ni, Bt, and the host plant will play a significant role in the evolution of resistance.

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