Host-Mediated Competition between the Pathogen Bacillus thuringiensis and the Parasitoid Cotesia plutellae of the Diamondback Moth (Lepidoptera: Plutellidae)

We evaluated the 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. The outcome of interactions between the parasitoid and pathogen depended on host phenotype. Host resistance to the pathogen reduced the success of both the parasitoid and the pathogen. 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 symmetrical and competitive; the parasitoid had a significant negative effect on pathogen performance and the pathogen had a significant negative effect on parasitoid performance. Highly resistant hosts were not susceptible to infection by the pathogen, which created a refugium from competition for the parasitoid. The results show that, similar to host-plant-mediated competition between herbivores, host phenotype can influence competitive interactions between parasitoids and pathogens.