Interaction of Resistant Plants and Wasp Parasitoids of Tobacco Budworm (Lepidoptera: Noctuidae)

Abstract

The interactions of 2 species of parasitoid wasps, their host, Heliothis virescens (F.), and transgenic tobacco producing a low concentration of Bacillus thuringiensis Berliner toxin were investigated in field tests. Campoletis sonorensis (Cameron) and toxic plants each decreased survival of larvae during the first 6 d on plants. C. sonorensis and toxic plants acted synergistically in combination, decreasing larval survival beyond the level expected for an additive interaction. Cardiochiles nigriceps Viereck did not significantly reduce 6-d survival of host larvae and did not interact with plant toxicity. Parasitism of H. virescens exposed for 1 d to natural populations of the same parasitoids was highly variable from day to day, and also varied between years. In 1990, C. sonorensis was the more prevalent species; in 1991, C. nigriceps was dominant. In 1990, single-day parasitism of neonates by C. sonorensis was lower on toxic plants than nontoxic plants. This result, which contradicted previous findings of increased parasitism among larvae exposed for several days on toxic plants, probably was caused by differences in initial behavior of larvae on toxic versus nontoxic plants. In 1991, position of larvae on plants did not affect parasitism by C. sonorensis ; however, parasitism by C. nigriceps was significantly higher among larvae collected from exposed positions on leaves compared with apical buds. Patterns of daily parasitism of 1- to 8-d-old larvae were extremely different for the 2 parasitoids: vulnerability to C. sonorensis among host larvae on nontoxic plants began to decline after 3 d; whereas, parasitism by C. nigriceps increased over the 1st few days and remained high through 8 d.