Prey availability, pesticides and the abundance of orchard spider communities

In a 4 year study, in southern England, the abundance of apple orchard canopy spiders and their potential prey was manipulated using two pest management strategies based on broad spectrum (highly toxic both to spiders and pests) and selective (moderately toxic to spiders but highly toxic to pests) insecticides in the first part of the growing season. The spider community was left to develop freely afterwards. Apple orchard plots untreated by pesticides served as control. The effect of insecticides was detrimental to spider populations as the treatments coincided with the peak abundance of adults in May and early June. Within adults, the treatments were harmful to female spiders, whereas, male spiders were much less affected. As a result the proportion of males increased in all of the sampled spider families. The use of selective insecticides resulted in a higher spider abundance compared to the use of broad spectrum compounds while the highest spider abundance was found in the pesticide free trees, i.e. three significantly different spider abundance levels were produced in spring. Spider abundance began to increase unequally between the treatments afterwards and became identical in the two pesticide treated plots due to the immigration of juveniles from surrounding habitats. However, a similar equalisation of abundance was not observed between the pesticide treated plots and untreated control. Analysing the abundance pattern of potential prey in the plots of the studied orchard we concluded that the post-disturbance increase in spider abundance was regulated by prey availability. (C) 2008 Elsevier Inc. All rights reserved.