Exclusion netting affects apple arthropod communities

Abstract

Exclusion cages can quantitatively demonstrate the value of natural enemies and the biological control services they provide. Currently, in Washington State, enclosing orchards in netting is being evaluated as a pest control method, but these enclosures may also serve as an exclusion cage to natural enemies, thereby disrupting biological control. To test the effects of net enclosures on apple arthropod communities, we conducted a two-year study comparing apple herbivore, predator, and parasitoid densities in caged and uncaged plots (both receiving insecticides) to an untreated check. Effects on the various groups were influenced by size and motility. Densities of larger motile arthropods, including codling moth (Cydia pomonella (L.)), chrysopids, and syrphids, were lower in the caged plots, demonstrating exclusion. Woolly apple aphid (Eriosoma lanigerum (Hausmann)) and its parasitoid Aphelinus mali (Haldeman) densities in the caged plots were over 100-fold greater and 5-fold greater, respectively, than the other treatments. European earwig (Forficula auricularia (L.)), a generalist predator, was more abundant in the caged plots in 2016 but not in 2017. Spider mites were more abundant in the cage treatments for both years, while predatory mites had higher densities in the cages in 2017 only. These outcomes show that net enclosures change the composition of apple arthropod communities through exclusion of certain herbivores and natural enemies. In the case of some pest herbivores (e.g., codling moth), this exclusion is beneficial, but in the case of predators, it is detrimental. This study further demonstrated that exclusion of only part of the natural enemy complex may be sufficient to cause herbivore outbreaks.