Abstract
Parasitoids need to find their plant-feeding hosts in complex environments that contain multiple other plant and insect species. They usually rely on herbivore-induced plant volatiles to locate herbivore-infested plants from a distance and their foraging efficiency may be reduced when volatile information is not available. Downwind foraging during times when high wind speeds prevent odour-guided upwind flights may create foraging situations with limited accessibiliy to volatile information. We hypothesized that parasitoids forage less efficiently by landing on nonhost-damaged or undamaged plants when they are forced to fly downwind and tested this in a wind tunnel experiment. We released the parasitoid Cotesia glomerata (Hymenoptera, Braconidae) either upwind or downwind of a plant stand and observed their foraging behaviour. During downwind foraging, parasitoids were less successful in host finding and needed more time until they managed to oviposit in a host caterpillar compared to the upwind foraging situation. The observed increase in foraging time was caused by prolonged foraging on nonhost-infested and undamaged plants in the downwind situation, indicating that parasitoids leave an unprofitable patch that does not contain host caterpillars earlier, when they perceive volatiles from other herbivore-infested plants located upwind. Volatile information on the availability of herbivore-infested plants within a plant stand seems to be crucial for efficient foraging in plant stands that contain a mixture of host-infested, nonhost-infested and undamaged plants. Parasitoid foraging efficiency may thus be strongly reduced when high wind speed prevents odour-guided upwind flight.
Highlights
Parasitoids need to find their plant-feeding hosts in complex environments that contain multiple other plant and insect species
The herbivore-induced plant volatiles (HIPVs) emitted by plants infested with nonhost caterpillars are as attractive to the parasitoid Cotesia glomerata as plants infested with their host caterpillars
Parasitoid responsiveness was lower in the downwind than in the upwind foraging situation (GLMM: c2 1⁄4 20.483, P < 0.001) and increased after oviposition experience (GLMM: c2 1⁄4 9.825, P 1⁄4 0.002; Fig. 3a)
Summary
Parasitoids need to find their plant-feeding hosts in complex environments that contain multiple other plant and insect species. The observed increase in foraging time was caused by prolonged foraging on nonhost-infested and undamaged plants in the downwind situation, indicating that parasitoids leave an unprofitable patch that does not contain host caterpillars earlier, when they perceive volatiles from other herbivore-infested plants located upwind. Foraging parasitoids use plant volatiles that are emitted upon insect feeding to detect herbivore-infested plants from a distance and to evaluate patch and habitat quality (Mumm & Dicke, 2010; Oudenhove, Mailleret, & Fauvergue, 2017; Tentelier & Fauvergue, 2007) These so-called herbivore-induced plant volatiles (HIPVs) often provide information about the plant species under attack as well as the attacking herbivore (Danner, Desurmont, Cristescu, & Dam, 2017; Dicke & Hilker, 2003; Hare, 2011). Since parasitoid host-finding behaviour is usually studied by releasing parasitoids downwind from the host-infested plant and observing their upwind flight towards it, we lack information on their behaviour in situations that do not allow for upwind flights, such as during high wind speeds
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