Aphelinus abdominalis Dalman (Hymenoptera: Aphelinidae), a solitary endoparasitoid of cereal aphids [e.g. Sitobion avenae (F.)] and aphids in greenhouses [e.g. Myzus persicae (Sulzer) and Macrosiphum euphorbiae (Thomas)] is available as biological control agent against aphid pests in greenhouses. As little is yet known about its long-range host location after release, the in-flight orientation of female A. abdominalis was investigated with regard to the effects of post-emergence experience, using a wind tunnel bioassay. In no-choice tests experienced females responded to the odour of M. euphorbiae-infested sweet pepper and aubergine plants while naive females exhibited mostly random flights. In a choice test, offering infested and uninfested plants of the same species, experienced wasps were able to recognize the plant-host complex (PHC) and selected it as landing site. In contrast to uninfested plants, host-damaged plants (infested plants with aphids removed) attracted experienced females just as well as infested plants. When the responses of groups of parasitoids with experience on two different plant-host complexes were studied, specifically trained wasps were observed to orientate significantly better towards the infested target plant than wasps with previous experience on the non-target plant. A final choice test, with an infested pepper and an infested aubergine plant as odour sources, showed that females trained on one of the offered plant-host combinations significantly preferred the odour of the learnt PHC to that of the different PHC. The results suggest that A. abdominalis females employ specific volatile signals emitted by host-infested plants (synomones) during long-range host location. These odours must be learnt, e.g. in association with a successful oviposition.
The tritrophic interactions between two different plant-host complexes, Avena sativa-Sitobion avenae, Triticum aestivum-S. avenae and the aphid parasitoid Aphidius rhopalosiphi were studied with respect to odour learning and recognition by the parasitoid. The orientation behaviour of females towards odours from either uninfested or aphid-infested oat or wheat plants was tested in a series of dual choice Y-tube olfactometer experiments. Female parasitoids had the opportunity to gain a single oviposition experience on either the oat-S. avenae or wheat-S. avenae complex before the experiment. In the first set of experiments, where A. rhopalosiphi was reared on the oat-S. avenae complex, eight odour-bait combinations were tested. The females did not discriminate between uninfested oat and wheat. After oat complex experience, females responded to odours from the oat complex, but not to odours from the wheat complex. Consequently, in a direct comparison the oat complex was preferred over the wheat complex. After wheat complex experience, the parasitoid's orientation responses gave a different picture. Both, the wheat complex and the oat complex, were then shown to be equally attractive. Hence, in direct comparison no preference was recorded between the oat and wheat complexes. In a second set of experiments, where A. rhopalosiphi was reared on the wheat- S. avenae complex, a possible influence of any pre-adult or emergence-related host plant experience could be excluded as the same results were obtained as before. At first glance the responses towards the different odour baits seem inconsistent. However, the results may be explained using a simple model with two key odour components.