Independent and Combined Effects of Bacillus thuringiensis and the Parasitoid Cotesia plutellae (Hymenoptera: Braconidae) on Susceptible and Resistant Diamondback Moth (Lepidoptera: Plutellidae)

We evaluated mortality to larvae of diamondback moth, Plutella xylostella (L.), caused by the microbial pathogen Bacillus thuringiensis Berliner and the endoparasitic wasp Cotesia plutellae Kurdjumov, separately and in combination. Each of 3 colonies of diamondback moth (susceptible, moderately resistant, and highly resistant to B. thuringiensis) received the following 4 treatments: (1) control, (2) B. thuringiensis only, (3) parasitoids only, and (4) B. thuringiensis plus parasitoids. With increasing colony resistance, the effect of B. thuringiensis decreased but the effect of parasitoids remained the same. The effect of B. thuringiensis plus parasitoids depended on the susceptibility of the host to B. thuringiensis. For the susceptible colony, highest diamondback moth mortality was caused by B. thuringiensis plus parasitoids, but for the 2 resistant colonies, parasitoids alone caused as much mortality as B. thuringiensis and parasitoids combined. For the susceptible and highly resistant colonies, we also evaluated the effect of varying the time interval between parasitism by C. plutellae and exposure to B. thuringiensis on diamondback moth mortality and C. plutellae survival. For the susceptible colony, diamondback moth mortality and C. plutellae survival did not vary as a function of the time interval between parasitism and exposure to B. thuringiensis. For the resistant colony, delaying B. thuringiensis treatment from 0 to 4 d after parasitism occurred did not affect diamondback moth mortality, but it significantly increased parasitoid survival. Efforts to integrate B. thuringiensis and C. plutellae must consider mortality of immature parasitoids inside of susceptible hosts.