Plant structure and the foraging success of Aphidius rhopalosiphi (Hymenoptera: Aphidiidae)

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Abstract
1. A study of searching behaviour on wheat and host preference of Aphidius rhopalosiphi (DeStefani‐Perez) is described. 2. Parasitoids divided their time equally between the leaves but spent very little time on the ear. 3. After contact with honeydew or an aphid host, parasitoids were arrested in a particular area and increased the time spent searching. 4. Aphids feeding on the ear were parasitized less successfully, since their position between the grains protected them from parasitoid attacks. 5. A.rhopalosiphi exhibited no preference for Metopolophium dirhodum (Walker) or Sitobion avenae (Fabricius), although the handling time for the latter was significantly longer; this resulted in fewer S.avenae being parasitized when it was abundant. Parasitoids did not switch between hosts in these experiments. 6. As a result of its searching behaviour, A.rhopalosiphi will encounter and parasitize M.dirhodum feeding on the leaves more frequently than S.avenae, which feeds on the ear. This will limit the parasitoid's ability to regulate populations of the cereal aphid S. avenae.

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