EFFECT OF AN ENTOMOPATHOGEN ON ADAPTATION OF HELIOTHIS-VIRESCENS POPULATIONS TO TRANSGENIC HOST PLANTS

The role of an entomopathogen in evolution of herbivore adaptation to partially resistant host plants was examined using a tritrophic system in the laboratory. We hypothesized that a pathogen should interact wi th herbivore behavior to accelerate herbivore adaptation to toxic plan ts: individuals not adapted to toxin tend to move more on toxic plants , and therefore are more likely to encounter a lethal dose of pathogen , further increasing the probability that they will be eliminated by s election. Heliothis virescens (F.) (Noctuidae) was selected for adapta tion to transgenic tobacco containing a sublethal concentration of Bac illus thuringiensis Berliner toxin under two treatment regimes: larvae placed on plants treated with infective conidia of the entomopathogen ic fungus, Nomuraea rileyi (Farlow) Samson, and larvae placed on plant s without fungus. Selection was initiated with a genetically heterogen eous strain created by crossing two laboratory strains of H. virescens , one not adapted to B. thuringiensis toxin, and one highly adapted (> 1000-fold) to toxin. This cross was performed four times to create fou r independent populations. Selection was initiated with F2 offspring f rom each cross and continued for 8-10 consecutive generations. Adaptat ion to toxin within each treatment population was quantified every gen eration by measuring survival and growth of larvae on artificial diet containing a low concentration of B. thuringiensis toxin. In three of four replicates, H. virescens populations exposed to N. rileyi adapted to B. thuringiensis toxin more quickly than populations not exposed. These results supported our hypothesis that the pathogen should accele rate adaptation to toxic plants. However, this hypothesis was contradi cted by the result in one replicate, in which the population not expos ed to fungus adapted to toxic plants faster. This opposite result coul d not be explained, but it suggests that there may be substantial vari ation in herbivore evolution in tritrophic systems. H. virescens popul ations selected in the presence of fungus and in the absence of fungus did not differ in feeding or in mortality when placed on leaf disks t reated with conidia. Thus, populations exposed to N. rileyi on plants for 7-8 generations displayed neither physiological nor behavioral ada ptation to N. rileyi.