The influence of an umbelliferous understory on the parasites of organic apple orchards in Canterbury

Abstract

The influence of a wild parsnip (Pastinaca sativa L.) ground cover on the parasites of organic apple orchards was examined. The indirect influences on the major pest species, codling moth (Cydia pomonella L.) and lightbrown apple moth (Epiphyas postvittana (Walker)) were also investigated.

Sampling was carried out in the Lincoln University Biological Husbandry Unit, Canterbury, New Zealand, from November 1990 until March 1992. Three different orchards were sampled, one with a natural grass understorey, and two with a managed wild parsnip understorey.

Adult parasites were trapped using yellow pitfall traps, elevated water traps and aspiration from parsnip inflorescences. 'Delta' pheromone traps were used to trap male codling moth and lightbrown apple moth. Ground dwelling predators were collected by yellow pitfall trap. The influence of other potential regulating factors on parasite abundance and activity were also examined. This involved measurement of meteorological factors, behavioural observation and records of orchard management activities. The amount of damage to fruit from the major apple pests was assessed at different tree heights.

Yellow pitfall traps were a reliable method of monitoring seasonal parasite phenology. Large numbers of parasites utilised the wild parsnip infloresences for their nectar resource. Numbers of parasites in general, and of leafroller parasites, were greatest in orchards with a wild parsnip understorey, indicating a beneficial effect of parsnip presence on parasite numbers.

The organic orchards were found to be a highly complex system with many interrelated temporal and spatial influences affecting the patterns of insect abundance and activity. Meteorological factors had a profound influence on parasite activity. Orchard management activities did not adversely affect parasite abundance or activity.

No correlation was found between parasite numbers and pest abundance or damage levels. However, pest trends were greatly influenced by differing varietal susceptibilities between orchards studied.

The overall conclusion was that wild parsnip increases the numbers of parasites in apple orchards but the impact on pests is largely governed by varietal characteristics of the apple. The use of wild parsnip to enhance parasites in apple orchards is advocated, but the parsnip density must be such that pathogens are not encouraged.