Natural allies: Independent and interactive effects of two specialist natural enemies on woolly apple aphid colonies

Classical approaches to biological control have often focused on single species of natural enemies, but there is increasing recognition that diverse multi-agent systems are important for effective biological control of pests. Previous studies have shown evidence of both synergism and interference occurring in multi-agent systems. In this study we examined the combined effects of two specialist natural enemies, a parasitoid wasp, Aphelinus mali Halderman (Hymenoptera: Aphelinidae), and an aphidophagous hoverfly, Heringia calcarata Loew (Diptera: Syrphidae), on their shared host, woolly apple aphid, Eriosoma lanigerum Hausmann (Hemiptera: Aphidae). The natural enemies were introduced, independently or in combination, to caged aphid colonies on potted apple trees maintained under controlled conditions in a growth chamber. After 14 days we recorded and compared the number of live aphids and the percentage of parasitized aphids among colonies containing one, both, or neither of the natural enemies.In addition, we conducted laboratory feeding studies to examine intraguild predation of A. mali by H. calcarata and field studies comparing oviposition by female H. calcarata on colonies that were or were not parasitized by A. mali.  Our findings are discussed in the relation to the potential to use H. calcarata as an additional classical biological control agent to manage E. lanigerum in New Zealand apple orchards.