RECOGNITION OF HETEROSPECIFIC PARASITISM - COMPETITION BETWEEN APHIDIID (APHIDIUS-ERVI) AND APHELINID (APHELINUS-ASYCHIS) PARASITOIDS OF APHIDS (HYMENOPTERA, APHIDIIDAE, APHELINIDAE)

The solitary parasitoids Aphidius ervi Haliday (Hymenoptera: Aphidiidae) and Aphelinus asychis Walker (Hymenoptera:Aphelinidae) attacked but generally did not oviposit in pea aphids parasitized by the other species. Wasps selectively oviposited in unparasitized hosts when given a choice. Host discrimination depended on the recognition of internal cues. Females of A. asychis either could not recognize or ignored A. ervi's external host marking pheromone. Under most conditions, A. ervi survived in superparasitized hosts, killing competing A. asychis larvae by physical attack and possibly physiological suppression. The outcome of larval competition was not affected by oviposition sequence or age difference between larvae; A. asychis survived only when it had substantially completed larval development before the host was superparasitized by A. ervi. It is suggested that competition for host resources incurs a cost, for the winner in terms of reduced size or increased development time and for the loser in terms of lost progeny and searching time. Consequently, heterospecific host discrimination can be functional. Internal, and probably general, cues enable wasps to recognize and avoid oviposition in hosts already parasitized by an unrelated species.