Effect of aphid-infested plants on the host location and learning behaviour of the parasitoid Aphelinus abdominalis

Aphelinus abdominalis Dalman (Hymenoptera: Aphelinidae) is a generalist parasitoid of cereal (e.g. Sitobion avenae) and greenhouse aphids (e.g. Macrosiphum euphorbiae). It differs from Aphidius spp. (Hymenoptera: Braconidae) by its robust constitution, longevity and its habit to use aphids not only as hosts for its progeny but also as prey (host feeding). In addition, A. abdominalis does not evoke Aphidius-characteristic defence responses by attacked aphids as this wasp approaches its hosts very carefully and then inserts its ovipositor nearly unnoticed by the aphids. In order to obtain more information on the host location behaviour of A. abdominalis, the effect of aphid-infested plants on its orientation and learning behaviour after mummy emergence was first studied in a series of Y-tube olfactometer experiments. When females reared on S. avenae were offered a 2 h foraging experience on M. euphorbiae-infested sweet pepper (Capsicum annuum) and then had the choice between the odour of uninfested sweet pepper plants and plants infested by aphid hosts (M. euphorbiae), they were significantly attracted to the odour of infested plants. Naive A. abdominalis, i.e. females that prior to the experiments were not offered any contact with a plant-host complex (PHC) after their emergence from S. avenae mummies and hence had no chance to learn the odour of the PHC, were not attracted by aphid-infested plants. The effects of adult experience on long-range host location by A. abdominalis (again reared on S. avenae) were then investigated in wind tunnel experiments by using M. euphorbiae-infested and uninfested sweet pepper and aubergine (Solanum melongena) plants. Females experienced with one of the offered PHCs significantly preferred the odour of the learnt PHC to that of the different PHC. A. abdominalis females obviously learn specific volatile signals emitted by aphid-infested plants for long-range host location.