Interactions between the pathogen Bacillus thuringiensis, the parasitoid Cotesia plutellae, and the herbivore Plutella xylostella

Effects of variation in phenotype of Plutella xylostella (L.) (diamondback moth) on within-host interactions between the bacterial pathogen Bacillus thuringiensis Berliner and larvae of the parasitoid wasp, Cotesia plutellae Kurdjumov, were examined. The outcome of interactions between the parasitoid and pathogen depended on host phenotype. In susceptible hosts, the parasitoid did not affect performance of the pathogen, but the pathogen had a significant negative effect on the parasitoid. In moderately resistant hosts, the interaction between the parasitoid and pathogen was competitive; both had a significant negative effect on the other. Highly resistant hosts were not susceptible to infection by the pathogen, which created a refugium from competition for the parasitoid. I also found that the effects of B. thuringiensis + parasitoids combined on diamondback moth mortality depended on the hosts' susceptibility to B. thuringiensis. For susceptible hosts, highest diamondback moth mortality was caused by B. thuringiensis + parasitoids, but for the two resistant phenotypes, parasitoids alone caused as much mortality as B. thuringiensis + parasitoids combined.; I tested the direct and indirect effects of B. thuringiensis on adult C. plutellae longevity and oviposition choice. In two separate experiments with male and female parasitoids, B. thuringiensis treatment did not significantly effect parasitoids exposed to B. thuringiensis. I also observed the oviposition behavior of female C. plutellae given 5 larvae that had been treated with B. thuringiensis and 5 untreated larvae in a choice test. There was no effect of B. thuringiensis treatment on parasitoid oviposition choice. The mean number of ovipositions in treated larvae was not different from untreated larvae.; Computer simulation was used to study the effects of pesticide resistance development and management strategies on pesticide-parasitoid-host systems. From these analyses I concluded that C. plutellae and B. thuringiensis could be used together in an integrated pest management program to control diamondback moth, and that parasitoids would slow the evolution of pesticide resistance by decreasing the number of generations in which treatment is required. Also, parasitoids could be complementary to spatial refuges from pesticides, with parasitoids maintaining control of a pest while refuges slow the development of resistance by the pest.