HABITAT MANIPULATION TO ENHANCE BIOLOGICAL-CONTROL OF BRASSICA PESTS BY HOVER FLIES (DIPTERA, SYRPHIDAE)

Brassicas in New Zealand are attacked by the cabbage aphid, Brevicoryne brassicae (L.); the green peach aphid, Myzus persicae (Sulzer); and the larvae of Artogeia rapae (L.) and the diamondback moth, Plutella xylostella (L.). Use of prophylactic pesticides is the usual control strategy against such pests, but the invertebrate predatory community within fields can contribute to pest suppression. Larvae of New Zealand hover flies consume all of the above pests, but the requirement of the adult flies for pollen and nectar may limit their potential in fields where the noncrop flora is impoverished. In this work, parts of the borders of three cabbage fields were drilled with the annual plant P. tanacetifolia tanacetifolia Benth. (Hydrophyllaceae) as a pollen source in the spring. Four strips were alternated with 4 control strips (naturally occurring vegetation). Aphid and A. rapae populations were assessed weekly in unsprayed plots adjacent to the 8 replicated areas, and hover fly eggs were counted simultaneously. Hover fly adults were trapped in yellow pan traps that were arranged across the field in transects from each of the eight sampling areas. Border planting with P. tanacetifolia clearly affected numbers of adult hover flies and aphid populations. The potential for this type of habitat manipulation to increase diversity as a component of integrated pest management is discussed with reference to comparable research in Europe.