Spider abundance and diversity in apple orchards under three insect pest management programmes in Washington State, U.S.A.

Summary 1 Many apple growers in Washington State, U.S.A. use mating disruption (MD) for control of codling moth, Cydia pomonella (Linnaeus). Fewer applications of synthetic, broad-spectrum insecticides are made in MD orchards than in orchards under conventional (C) management. Spider abundance and diversity in MD, C and certified organic (O) orchards were compared. Spiders inhabiting the trees (arboreal), the understory vegetation, and the ground surface were studied. 2 Total arboreal spider density and total understory spider density were significantly higher in O orchards than in MD and C orchards. Many species occurred in both the trees and the understory. 3 Arboreal, visually orientated, hunting spiders and arboreal ambushers/runners were significantly more abundant in O orchards compared to C and MD orchards. Visual hunters were significantly more abundant in MD compared to C orchards. Numbers of spiders in two other guilds (web-makers and nocturnal hunters) showed no statistical differences with respect to orchard management type. 4 The highest density of ground surface-dwelling spiders occurred in one of the O orchards. Two C orchards had higher densities than any MD orchard. Ground surface species were distinct from those in the understory and the trees. 5 With one exception, an orchard's arboreal fauna was most similar to that of another orchard under the same type of pest management. Three exceptions were noted among comparisons of the understory faunas. The ground surface-dwelling fauna of one O orchard was distinctive, whereas that in the second O orchard was similar to the C and MD orchards. 6 Reduced use of synthetic, broad-spectrum insecticides in MD orchards did not result in arboreal spider densities comparable to those found in O orchards. A contributing factor may be that all species were univoltine. Spider populations may thus be severely reduced by even a small number of synthetic, broad-spectrum insecticide applications and the time required for recovery may be lengthy.