Semi-natural habitat as a source of <i>Drosophila suzukii</i> (Diptera: Drosophilidae) and its parasitoids in California cane berry fields

The invasive pest spotted wing drosophila, Drosophila suzukii (Matsumura), attacks many wild and ornamental hosts and may persist in semi-natural habitats when fruit is unavailable in the crop. Semi-natural habitat or non-crop habitat (NCH) may also provide shelter for D. suzukii or refuge from insecticide applications. We examined population dynamics of D. suzukii in berry fields and adjacent NCH over 3 growing seasons and sampled at different distances from NCH to determine whether NCH is a source for D. suzukii and its parasitoids in adjacent crop fields. Numbers of D. suzukii adults were higher in NCH on most sample dates and increased in berry fields with proximity to NCH in spring and fall, indicating that NCH was a refuge and source for D. suzukii. Proportions of female D. suzukii were higher in berry fields in spring and fall and increased with distance from NCH along transects, suggesting that females dispersed more widely in berry fields than males. NCH did not appear to be a significant source of D. suzukii in summer, when D. suzukii numbers tended to be similar between habitats and did not change along transects within berry fields. Parasitism of sentinel D. suzukii pupae by the dominant naturally occurring parasitoid Trichopria drosophilae (Perkins) did not differ between NCH and berry fields, indicating that T. drosophilae dispersed easily between habitats.