WOOLLY APPLE APHID (ERIOSOMA-LANIGERUM HAUSM, HOM, APHIDAE) PARASITISM BY APHELINUS-MALI HAL (HYM, APHELINIDAE) IN RELATION TO HOST STAGE AND HOST COLONY SIZE, SHAPE AND LOCATION

The host specific (solitary) endoparasitoid, Aphelinus mali, will parasitize all stages of woolly apple aphid but apparently prefers third stage nymphs and older hosts. Female parasitoids are generally larger than males, emerge from larger mummies and take about 1/2 day longer to complete development. The sex ratio is strongly male biased when only small hosts are available for parasitization, and strongly female biased when only large hosts are available. Pre-adult mortality decreases with increasing mummy size suggesting higher mortality for males than females which may explain, in part, the slightly female biased sex ratio in the field. Rates of parasitism are inversely proportional to host colony size because woolly aphids in the centers of large colonies are relatively more protected from parasitoid attack than aphids on colony margins due to the dense crowding of aphids and the white, waxy secretion that covers colonies. In very small woolly aphid colonies (surface area < 37 mm2) few, if any, individuals are protected from A. mali. Rates of parasitism are greater in long, narrow colonies than in round colonies of the same size because a higher proportion of woolly aphids are located on the colony margins where parasitoids concentrate their attacks. The likelihood of successful biological control is discussed in relation to the observed patterns of parasitism.