Parasitoid foraging in a 'volatile' environment: Effects of host and host-plant species on Cotesia marginiventris (Hymenoptera: Braconidae) behavior

When plants are damaged by insect feeding they protect themselves by releasing volatile organic compounds (VOCs) which are utilized by natural enemies of the herbivores, such as parasitoid wasps. Because VOCs aid in parasitoid host-finding, elucidation of their limitations is of major importance. This dissertation explores the chemical ecology of two economically important caterpillar pests, Spodoptera exigua (beet armyworm) and Trichoplusia ni (cabbage looper), and a shared parasitoid natural enemy Cotesia marginiventris. S. exigua is a preferred host of C. marginiventris, while T. ni is reportedly only parasitized if feeding in fields near S. exigua. Parasitoids exhibit sophisticated and flexible associative learning capabilities that can persist for several days, and positive or aversive experiences related to host suitability may have important ecological impacts because they affect the efficiency of the foraging parasitoid. This doctoral dissertation combines chemical, behavioral, and ecological methods to explore the foraging behavior and realized host range of this important biological control agent. First, I show that S. exigua and T. ni differentially induce VOCs in cotton, collards, and Palmer pigweed. Using behavioral bioassays conducted in a wind tunnel, I demonstrate that C. marginiventris associates both host and non-host odors more frequently with the preferred host S. exigua than the sub-optimal host T. ni. Furthermore, wasps can differentiate the odor blends of plants damaged by S. exigua or T. ni. Lastly, I show that host species suitability can be dependent on which plant species the herbivore is consuming, and this can affect egg deposition and development of C. marginiventris . Combined, these studies illustrate the complexities underlying generalist parasitoid foraging in environments where multiple host and host-plant species occur.