Effect of induced tolerance to Bt toxin on the egg size of Helicoverpa armigera and parasitism by Trichogramma pretiosum

Larvae of Helicoverpa armigera (Hubner) can develop a form of Bt tolerance after exposure to sub-lethal doses of Bt-toxin subclass Cry1Ac. Increasing levels of tolerance are produced over generations of larval exposure, which are not related to DNA sequence changes, and are largely maternally transmitted. The characteristic of maternal transmission, combined with the importance of egg parasitoids to cotton pest management, raises questions about the effects of Bt tolerance/exposure on the eggs of H. armigera and on some key metrics of egg parasitism. In the present study, the effect of inducible tolerance on eggs of H. armigera and parasitism by Trichogramma pretiosum (Riley) is investigated. First, the volumes of eggs laid by susceptible and tolerant H. armigera females are compared. In addition, the effect of inducible tolerance on egg parasitism is determined by comparing parasitism success, the number of adult wasps emerged per host egg, and the proportion of male and female offspring emerged per host egg. The results obtained suggest that Cry1Ac-tolerance is associated with increased egg volume, even after just one generation of sub-lethal exposure. When tolerant H. armigera are freed from ongoing sub-lethal exposure, a corresponding decrease in egg volume is not detected. Although there is no difference in the percentage of eggs parasitized, there is an increase in the number of emergent parasitoids, especially males, from eggs laid by tolerant H. armigera. These results confirm that maternally-transmitted Bt tolerance is reflected in the phenotype of the eggs of tolerant offspring, which affects egg parasitism.