Efficacy and non-target effects of net enclosures in apple orchards

Building shade netting structures over Washington orchards has become increasingly popular as a method to reduce sun damage to fruit. If netting material is used to enclose the orchard, it can also be used to exclude larger arthropod pests such as brown marmorated stink bug, Halyomorpha halys. However, exclusion cages may exclude both pests and natural enemies, disrupting biological control in the case of the latter. To test the hypothesis that net cages would exclude/prevent damage by stink bugs, we used the native consperse stink bug, Euchistus conspersus, as a surrogate for the exotic pest, H. halys. Despite the release of captured stink bugs in close proximity to the plots, our preliminary experiments were unsuccessful in that no damage occurred in either the caged or uncaged trees. We also studied the nontarget effects of the cages on secondary pests and natural enemies.  Although no treatment differences in density of the parasitoid Aphelinus mali were noted, woolly apple aphid densities were significantly higher (»73´) in the caged plots. Earwigs, an important aphid predator, were significantly higher in caged plots, whereas lacewings caught in plant-volatile baited Delta traps were »85´ higher in uncaged plots.  Spider mites were significantly higher, and predatory mites significantly lower, in the caged plots. These results show that biological control is being disrupted by the netting.