Olfactory and visual cues trigger host location behavior of Torymus sinensis, a parasitoid of the globally invasive Asian chestnut gall wasp

Torymus sinensis (Hymenoptera: Torymidae) has been manipulated extensively in biological control programs targeting the globally invasive Asian chestnut gall wasp, Dryocosmus kuriphilus (Hymenoptera: Cynipidae). T. sinensis life cycle is synchronized with gall wasp larval development to allow effective gall wasp population suppression and a reduction in gall formation. In spite of its extensive use for biological control, relatively little is known about its host location and host acceptance behavior. We investigated T. sinensis host location behavior using a Y tube olfactometer. Adult females were tested for their response to olfactory and visual cues associated with D. kuriphilus galls and chestnut foliage. Parasitoids were not attracted by the odor of fresh galls alone (N=20; X²₁=0.2; P=0.66), and had a negative response to the visual cues of galls and chestnut foliage when odor cues were not provided (N=20; X²₁=5; P=0.03). However, the combination of olfactory and visual stimuli provided by a fresh gall coupled with chestnut foliage elicited a strongly positive response (N=20; X²₁=7.2; P=0.01). This positive response persisted even when the fresh gall was replaced by an inert surrogate gall (N=20; X²₁=5; P=0.03), provided the visual stimulus remained and the olfactory cues from fresh galls were available. Our results indicate that both visual and olfactory cues are required to enable T. sinensis to successfully find suitable hosts. These findings improve our understanding of the stimuli that influence T. sinensis host location behaviors leading to successful gall wasp parasitization, and may enhance our ability to manipulate T. sinensis for gall wasp management.