Streams and rural abandonment are related to the summer activity of the invasive pest Drosophila suzukii in protected European forests

Abstract

Protected native-forested areas may be occupied by fruit pests, and so, studies exploring the biotic and abiotic determinants of fruit-pest abundance in forested areas may reduce damages in crops and wild forest frugivores. The Spotted Wing Drosophila (SWD) Drosophila suzukii is an economically important fruit pest in many temperate regions around the world. During the dry summer in northwestern Spain, we assessed 24 native riparian and 32 non-riparian chestnut forest patches as non-crop habitats for the SWD. We surveyed chestnut forests in 2017 and found a positive association between spatial proximity of forest patches to streams and SWD captures, which led us to study in 2019 the stream-SWD associations in greater detail. We explored whether native-insect communities and changes in vegetation structure related to rural abandonment were associated with variation in SWD captures, while accounting for the effects of covariates, including stream distance. There were no significant associations in the riparian and non-riparian-habitat surveys between the captures of SWDs and those of native insects, including 22 families of flies and 10 families of parasitic wasps. However, captures of SWDs and of other drosophilid flies were positively related to each other and the direction of the association was reversed by stream distance, which suggests the potential role of streams in regulating interactions among non-riparian insects, including SWD. We also found correlative evidence that degraded riparian forests and the abandonment of traditional forest practices in chestnut forests may be contributing to the spread of SWD. Given the numbers of SWDs in our forest samples were similar to values in August in crop areas, it is advisable that future studies address the impacts of SWD invasion on native forest frugivores, which have been overlooked in studies of this widely distributed invasive species.