Border planting to enhance biological control of brassica pests by Hover flies (Diptera: Syrphidae) : A dissertation submitted in partial fulfilment of the requirements for the degree of Bachelor of Horticultural Science with Honours at Lincoln University

Abstract

Brassicas in New Zealand are attacked by the cabbage aphid (Brevicoryne brassicae (L.)), green peach aphid (Myzus persicae (Sulzer)) and by the larvae of white butterfly (Artogeia rapae (L.)) and diamond-back moth (Plutella xylostella (L.)). Prophylactic use of 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 non-crop flora is impoverished. In this work, parts of a cabbage-field border were drilled with the annual plant Phacelia tanacetifolia Benth. (Hydrophyllaceae) as a pollen source in the spring. Four strips were alternated with four control strips (naturally occurring vegetation). Aphid and white butterfly populations were assessed weekly in unsprayed plots adjacent to the eight replicated areas, and hover fly eggs were counted at the same time. Hover fly adults were trapped in yellow pan traps which were arranged across the field in transects from each of the eight sampling areas. There were clear effects of the P.tanacetifolia border-planting on numbers of adult hover flies and on aphid populations, and the potential for this type of habitat manipulation to increase 'diversity' as a component of integrated pest management is discussed in relation to parallel work in Europe.