PATTERNS OF HOST SELECTION BY 4 SPECIES OF APHIDIID (HYMENOPTERA) PARASITOIDS - INFLUENCE OF HOST SWITCHING

1. We tested switching behaviour in four species of aphidiid parasitoids, using a two-aphid experimental system consisting of second-instar nymphs of pea aphid (Acyrthosiphon pisum (Harris)) and alfalfa aphid (Macrosiphum creelii Davis) feeding on broad beans in the laboratory. 2. Aphidius ervi Haliday, A.pisivorus Smith, A.smithi Sharma & Subba Rao, and Praon pequodorum Viereck showed an innate preference for pea aphid when both host species were provided in equal numbers. 3. Wasps encountered both aphid species equally but differed in their acceptance of alfalfa aphid. Females of A.pisivorus and P.pequodorum accepted alfalfa aphids when few pea aphids were available, but A. smithi always concentrated attacks on pea aphid. Aphidius ervi super-parasitized an increasing proportion of pea aphids as their availability declined. 4. Switching to the alfalfa aphid occurred in A. ervi and P.pequodorum (but not in A.pisivorus and A.smithi) under the condition of a 1:3 ratio of pea aphids:alfalfa aphids. Wasps did not switch when more pea aphids than alfalfa aphids were provided (3:1 ratio). 5. Alfalfa aphids were more likely than pea aphids to escape from parasitoid attack. 6. Switching to the most abundant host may not be adaptive in these four species of aphid parasitoids. A foraging wasp incurs a potentially higher cost in lost opportunity time when attacking (and failing to oviposit in) alfalfa aphids. In addition, alfalfa aphids may have lower host quality than pea aphids, a difference that could influence offspring fitness.