Evaluation of the Predatory Wasp, Ancistrocerus gazella, for Biological Control of Leafrollers in Otago Fruit Crops: 1. Prey Composition, Nest Structure and Wasp Productivity from Artificial Nests

Groups of artificial nest tubes were deployed in the field for colonization by the accidentallyimported wasp, Ancistrocerus gazella (Panzer), a reported important predator of lepidopterous larvae, especially pest tortricids. There were 3-4 nest sites in each of seven study areas, and four tube lengths (75, 100, 125, 150 mm) were compared. Nests were dissected at various stages after occupancy to determine the prey composition and nest structure, and the numbers and sex of adults produced. The wasps collected larvae of nine lepidopterous families, including many tortricids, but pest leafrollers comprised only 4.1% and 1.5% of all larvae collected respectively in 1995-96 and 1996-97. Prey included beneficial species feeding on weeds and the larvae of some non-pest native insects. Prey species composition was highly site-dependent and was a major determinant of the numbers of larvae per cell. Cells for male wasps were smaller and contained fewer larvae than those for females. Tube length did not affect colonization rates but more cells and adult wasps were produced in longer tubes. The results cast serious doubt on the proposed management of A. gazella for biological control of leafrollers, but suggest that this approach could be useful for other pests, such as Coleophora spp., which were collected in large numbers.