Predatory arthropod community composition in apple orchards: Orchard management, landscape structure and sampling method

Studies on predatory arthropods in agricultural areas seldom include
Diptera other than hover flies, partly because common sampling methods are
less effective for capturing species that easily fly off when disturbed.
To study the effect from this bias when describing the predator community,
we compared traditional beat sampling of branches and suction sampling for
describing the community of predatory arthropods in Swedish apple
orchards, both organic orchards and orchards using integrated pest
management. Our results indicate that the proportion of both predatory
dipterans and parasitic hymenopterans increase dramatically when using
suction sampling (Diptera: 32% vs 20%, Hymenoptera: 25% vs 7%). In fact,
predatory dipterans were the most abundant predatory group when using
suction sampling, in contrast to beat sampling where spiders were the most
abundant group. One group of predatory flies that was particularly rich in
both species and individuals in the surveyed apple orchards was dance
flies in the family Hybotidae. Even though the bias of sampling method was
evident, it was encouraging that the method choice did not affect the
conclusions concerning management on predatory arthropod communities. With
both methods, dipteran and coleopteran predators were more abundant in
organic apple orchards whereas opilionids were more abundant in orchards
managed according to integrated pest management. The inclusion of
landscape variables further indicated effects of landscape diversity and
of deciduous forest cover, but the response varied in sign between
predatory groups. Whereas both Coleoptera and Heteroptera were more
abundant in orchards surrounded by more complex landscapes (high landscape
diversity and/or high deciduous forest cover), spiders, opilionids and
dipterans were rather less abundant in these orchards. To conclude, our
study points to the potential importance of predatory dipterans in apple
orchards, and we highly recommend future studies of arthropod predators in
apple and other crops to actively include predatory Diptera.