Flexible parasitoid behaviour overcomes constraint resulting from position of host and nonhost herbivores

Parasitoids face several hurdles and distractions while foraging for their hosts, one of which is the presence of nonhost herbivores. Nonhost herbivores may interfere with plant volatile-mediated location of host-infested plants and reduce encounter rates with hosts on the plant. This results in a lower foraging efficiency. In this study, we tested whether the feeding position of a host and nonhost herbivore on the same plant influences foraging decisions and parasitism efficiency of parasitoids. We confined host and nonhost herbivores to either higher positions, i.e. younger leaves (preferred by the host) or lower positions on the plant, i.e. older leaves (preferred by the nonhost). Host and nonhost herbivores fed either on separate leaves or on the same leaf. Results from laboratory experiments show that during the first phase of foraging when plant volatiles are used to locate a host-infested plant, parasitoids were misled when host and nonhost were positioned in an unnatural way on the individual plant (host on the older leaves). The positions of host and nonhost partly influenced parasitoids during the second phase of foraging, when the host is located on the plant by using host cues. Total host-finding efficiency, as tested in a semifield set-up, was not affected by herbivore position. We conclude that parasitoid foraging behaviour has enough flexibility to overcome constraints resulting from an unexpected distribution of herbivores over a plant. •Position of host and nonhost herbivores on a plant influences parasitoid decisions.•Negative effect of unnatural distribution of host and nonhost herbivores on leaf choice.•Positive effect of combination of host and nonhost herbivores on behaviour on leaf.•No effect of position of host and nonhost herbivores on total foraging efficiency.•Parasitoid foraging behaviour overcomes constraint of unnatural herbivore position.