Effect of agricultural management on rosy apple aphid biocontrol in Swedish apple orchards

Abstract: The rosy apple aphid (Dysaphis plantaginea Passerini) is regarded as a highly harmful insect of great economical importance for Swedish apple growers. We studied the natural pest control service of small size colonies of D. plantaginea under different management regimes in a situation of ant attendance inhibition. Early stage rosy apple aphid colonies were established on potted trees in five organic and four conventional apple orchards in southern Sweden. Ant attendance was inhibited by means of sugar feeders. A net sleeve was mounted around the shoot hosting a control colony to avoid exposition to natural enemies. The number of aphids, ants and associated natural enemies (including eggs) were counted weekly. In addition, the arthropod community was assessed weekly by suction sampling in all the orchards. There was a drastic reduction in the amount of exposed colonies compared to the exclusion controls in both conventional and organic apple orchards. Despite this reduction, differences in colony survival were observed between agricultural managements. The size of surviving colonies was also observed to be larger in conventional orchards compared to organic. The most recorded natural enemies within the established colonies were Anthocoris sp., followed by predatory midges, earwigs, spiders and lacewing larvae. Hoverfly eggs and larvae were observed in lower numbers than expected. Suction samples revealed strong differences in Anthocoridae and Miridae populations between management schemes whilst no differences were observed for other important aphidophagous natural enemies such as earwigs, predatory midges and lacewings. Anthocoridae population in the different orchards correlated with the registered D. plantaginea colony decline. Additionally, the visitation by predatory Heteroptera was found to be higher in organic orchards. Our results indicate that predatory Heteroptera, in particular those of the family Anthocoridae, play an important role in the suppression of the rosy apple aphid at initial infestation phases in Nordic conditions. The use of pesticides appears to impact their population and thus the biological control service they provide under an artificially reduced ant attendance.