Effects of quantitative and qualitative differences in volatiles from host- and non-host-infested maize on the attraction of the larval parasitoid Cotesia kariyai

Abstract

Cotesia kariyai Watanabe (Hymenoptera: Braconidae) is a specialist larval parasitoid of Mythimna separata Walker (Lepidoptera: Noctuidae). Cotesia kariyai wasps use herbivore-induced plant volatiles (HIPVs) to locate hosts. However, complex natural habitats are full of volatiles released by both herbivorous host- and non-host-infested plants at various levels of intensity. Therefore, the presence of non-hosts may affect parasitoid decisions while foraging. Here, the host-finding efficiency of naive C. kariyai from HIPVs influenced by host- and non-host-infested maize [Zea mays L. (Poaceae)] plants was investigated with a four-arm olfactometer. Ostrinia furnacalis Guenée (Lepidoptera: Crambidae) was selected as a non-host species. One unit (1 U) of host- or non-host-infested plant was prepared by infesting a potted plant with five host or seven non-host larvae. In two-choice bioassays, host-infested plants fed upon by different numbers of larvae, and various units of host- and non-host-infested plants (infestation units; 1 U, 2 U, and 3 U) were arranged to examine the effects of differences in volatile quantity and quality on the olfactory responses of C. kariyai with the assumption that volatile quantity and quality changes with differences in numbers of insects and plants. Cotesia kariyai was found to perceive quantitative differences in volatiles from host-infested plants, preferring larger quantities of volatiles from larger numbers of larvae or plants. Also, the parasitoids discriminated between healthy plants, host-infested plants, and non-host-infested plants by recognising volatiles released from those plants. Cotesia kariyai showed a reduced preference for host-induced volatiles, when larger numbers of non-host-infested plants were present. Therefore, quantitative and qualitative differences in volatiles from host- and non-host-infested plants appear to affect the decision of C. kariyai during host-habitat searching in multiple tritrophic systems.