PREVALENCE OF SPIDERS AND THEIR IMPORTANCE AS PREDATORS IN ONTARIO PEACH ORCHARDS

Abstract

Philodromus spp., especially praelustris Keyserlmg, and sometimes Theridion murarium were the only abundant spiders in peach orchards, though small numbers of many other species were present. No satisfactory method of sampling the population density of all species was discovered. The spider population as a whole reached a minor peak in June and a much higher peak in early August. A single tree may contain more than 200 spiders. When fed only on Panonychus ulmi (Koch), young P. praelustris eventually died without moulting; young of T. murarium moulted once or twice but their development was slower than that of those in the orchards. Chironomids were the commonest prey of P. praelustris and apparently of other spiders in the orchards.Spiders collected from peach plots with different densities of P. ulmi and Bryobia arborea Morgan and Anderson were examined for pigments of these mites by paper chromatography. Some spiders in all collections had fed on the mites, and the percentage that had done so usually varied directly with mite density in the plots. Spiders form part of the complex of minor predators that aid the major predators in regulating the density of P. ulmi at endemic levels.