The effects of refuge size and number on acarine predator-prey dynamics in a pesticide-disturbed apple orchard.
In apple orchards, the use of pesticides such as pyrethroids for the control of lepidopteran and dipteran pests can severely disturb natural equilibria by killing or repelling acarine predators. After spraying, phytophagous mite populations will often subsequently increase to densities which are above economic thresholds. In order to conserve predator populations, the size and number of predator refuges were manipulated. Refuges were made with various sizes of polyethylene sheeting placed over 0, 10, 30, 60 or 100% of the leaves on trees before spraying with the pyrethroid permethrin. At the time of spraying, the phytoseiid predator Typhlodromus caudiglans [Anthoseius caudiglans] and the stigmaeid predator Zetzellia mali were present in similar densities. The main phytophagous mites present were the tetranychid mites Panonychus ulmi and Tetranychus urticae. After spraying, A. caudiglans was virtually eliminated from sprayed leaves, but not from refuge leaves. Recolonization of sprayed leaves by A. caudiglans was slow, probably due to toxic or repellent effects of the pyrethroid residue. Densities of Z. mali were only slightly affected by the pyrethroid, but this species was unable to control tetranychid densities on sprayed leaves in the absence of A. caudiglans. After spraying, P. ulmi and Tetranychus urticae increased on sprayed leaves to densities well in excess of crop economic thresholds, while densities remained low on refuge leaves. The major effect of refuge size was a high positive correlation between entire-tree densities of A. caudiglans and refuge size, and a high negative correlation with P. ulmi densities. The refuge size necessary to control P. ulmi to below economically damaging densities was predicted to be in excess of 60% of each tree. Thus, only 40% of each tree would receive a pyrethroid spray. This is not practical for management purposes as the reduced spray coverage is unlikely to control target lepidopteran and dipteran pests. No significant effects of refuge number were observed. The application of a pyrethroid spray resulted in a major disturbance to natural acarine population dynamics in this orchard for up to 11 weeks after spraying. While refuges were useful in conserving a population of the predator A. caudiglans and reducing entire-tree densities of phytophagous mites, the use of refuges may be more valuable when using pesticides with a lower residual effect.