Positive associative learning experiences supersede herbivore-induced plant volatiles in generalist parasitoid host-finding

Insect herbivores often induce plant volatile compounds that can attract natural enemies. Cotesia marginiventris (Hymenoptera: Braconidae) is a generalist parasitoid wasp of Noctuid caterpillars and is highly attracted to Spodoptera exigua-induced plant volatiles. The plasticity of C. marginiventris associative learning to volatile blends of various stimuli, such as food or host presence, has also been shown, but little is known about how this generalist parasitoid distinguishes between host species of varying suitability. Our data shows that S. exigua is an excellent host that yields high parasitoid emergence, while Trichoplusia ni serves as a sub-optimal host species due to high wasp mortality. We have also found that S. exigua and T. ni induce different volatile blends while feeding on cotton. Here, wind tunnel flight assays were used to determine the importance of differentially induced volatiles in host-finding by C. marginiventris. We found that this generalist parasitoid wasp does not distinguish between the two discrete volatile blends. Instead, positive oviposition experiences on the preferred host species (S. exigua) is required to elicit subsequent flight behavior towards plants damaged by either host species.