Reduced foraging efficiency of a parasitoid under habitat complexity: implications for population stability and species coexistence

Summary Habitat complexity may stabilize interactions among species of different trophic levels by providing refuges to organisms of lower trophic levels. Searching behaviour of the parasitoid,Diadegma semiclausum, was followed in different semifield set‐ups, a low and high‐density monoculture ofBrassica oleraceaand two intercrops,B. oleraceawithSinapis alba(also a member of the Brassicaceae) andB. oleraceawithHordeum vulgare(Poaceae). When a low‐density monocrop ofB. oleraceawas compared with a high‐density monocrop, no differences were found in the ability of the female wasps to locate a host‐infested plant,B. oleracea, infested withPlutella xylostellathat was placed in the centre of the set‐up. The efficiency of the parasitoid to locate the host‐infested plant was differentially affected by the species composition of the vegetation. Wasps entered theSinapis‐Brassicaset‐up faster, but took more time to find the host‐infested plant than in theHordeum‐Brassicaset‐up. The horizontal arrangement, i.e. by mixingS. albaorH. vulgarewith, or placing them as rows betweenB. oleracea, did not affect host‐finding efficiency. Plant height did influence host finding. Wasps found the host‐infested plants earlier in the set‐up with shortSinapisplants compared with tallSinapisplants. Once the wasps had landed on the host‐infested plant, the surrounding vegetation did not affect time needed to parasitize five consecutive hosts on the same infested plant, regardless of the composition or horizontal/vertical arrangement of the set‐up. Chemical and structural refuges in complex landscapes may play an important role in the persistence of this system through dampening oscillations of parasitoid and host populations.