EFFECTS OF NATURAL ENEMIES ON RELATIVE FITNESS OF HELIOTHIS-VIRESCENSGENOTYPES ADAPTED AND NOT ADAPTED TO RESISTANT HOST PLANTS

We investigated the potential of two natural enemies of Heliothis vire scens (F.) (Noctuidae) to affect its rate of adaptation to tobacco con taining Bacillus thuringiensis Berliner toxin. Larval fitness of two l aboratory strains of H. virescens, one adapted to B. thuringiensis tox in and one not adapted, was compared on toxic and nontoxic plants, in the presence of the parasitoid Campoletis sonorensis (Cameron) (Ichneu monidae) or the entomopathogenic fungus Nomuraea rileyi (Farlow) Samso n. By exposing larvae to plants and enemies for no more than 24 h, we focussed on the behavioral rather than physiological component of thei r interaction with toxic plants and natural enemies. Parasitism of H. virescens larvae by C. sonorensis during exposure periods of 1-4 h was lower on toxic plants than nontoxic plants and was lower for nonadapt ed larvae than for toxin-adapted larvae. Decreased larval feeding dama ge on toxic versus nontoxic plants, and by nonadapted versus adapted l arvae, may explain differences in parasitism, because C. sonorensis lo cates host larvae using cues from damaged plants. Effects of plant tox icity and larval strain on H. virescens survival were numerically cons istent with effects on parasitism, but they were not statistically sig nificant. When mean larval survival is used to estimate fitness of the nonadapted genotype relative to the toxin-adapted genotype, we find t hat C. sonorensis is expected to delay adaptation to toxic plants. Per cent infection by N. rileyi of H. virescens larvae exposed to fungus-t reated plants for 24 h was greater when plants were toxic, and was gre ater for nonadapted larvae than toxin-adapted larvae. There were corre sponding decreases in larval survival on toxic compared to nontoxic pl ants, and of nonadapted compared to adapted larvae. Interaction of eff ects of plant line and larval strain on survival was significant in th e presence of fungus, but not in the absence of fungus, which indicate s that the effect of toxic plants on the relative fitness of toxin-ada pted and nonadapted larvae was mediated by fungus. As in the interacti on with C. sonorensis, behavior of larvae on plants may explain differ ences in susceptibility to N. rileyi. Because nonadapted larvae moved more than toxin-adapted larvae on toxic plants, nonadapted larvae may have been more likely to encounter a lethal dose of conidia. In contra st with C. sonorensis, N. rileyi, which decreased the fitness of the n onadapted genotype relative to the adapted genotype, is expected to ac celerate adaptation to toxic plants.