Location: Invasive Species and Pollinator Health
Project Number: 2030-22300-032-007-R
Project Type: Reimbursable Cooperative Agreement
Start Date: Nov 1, 2018
End Date: Aug 31, 2021
The primary objective of this project is to determine if augmenting numbers of resident parasitoids in crop fields and non-crop habitats can increase parasitism of Spotted Wing Drosophila (SWD) in California cash crops such as cane berries and strawberry. Our specific objectives are to: 1) examine whether numbers of resident parasitoids in non-crop habitat can be increased through augmentation; 2) assess dispersal rates of parasitoids from non-crop habitat into the crop; 3) determine if parasitoid augmentation in non-crop habitat reduces SWD damage in the crop; 4) examine whether augmentation of SWD parasitoids in non-crop habitat can increase parasitism rates at the landscape level.
We will conduct the proposed research in organic berry (raspberry, blackberry and strawberry) fields owned or managed by Driscoll’s, which is based in Watsonville, California, and by Naturipe, which is based in Salinas, California. Two generalist pupal parasitoids of spotted wing drosophila (SWD), Pachycrepoideus vindemiae and Trichopria drosophilae, will be mass reared and released into plots of non-crop habitat adjacent to berry fields (i.e., riparian areas harboring Himalayan blackberry). Both P. vindemiae and T. drosophilae are cosmopolitan generalists that readily attack SWD pupae in its invaded range in California and southern Europe, and in its native range in Asia, and were the only parasitoids that emerged from field-collected SWD in recent surveys in California. Studies in Europe and North America have shown that these pupal parasitoids are capable of attacking and developing in SWD, but that strong immune responses by SWD prevent resident larval parasitoids in these areas from completing development in SWD larvae. Both P. vindemiae and T. drosophilae are cosmopolitan and co-occur in California. Rearing both species in the laboratory is straightforward, and both can be stored at 12°C for 2 months. Parasitoid numbers will be augmented in non-crop habitats starting in November in 2018 and 2019, after berry production has stopped in crop fields and SWD has returned to overwinter in non-crop habitats. There will be two treatments, with parasitoids not augmented or augmented. There will be a minimum of five sites of each treatment, for a total of at least 10 sites. Berry fields at sites will be at least 2 ha in area. To minimize variation between control and augmentation sites, sites will be paired based on geographic proximity, before randomly assigned one site within each pair to each treatment. Paired sites will be at least 500 m apart, but will be in the same geographic area, and will be as similar as possible in climate, elevation, berry variety, and SWD population levels. In the first year, both parasitoid species will be released at augmentation sites, and the most abundant species in our samples from the first year will be the only parasitoid released in the following year. A minimum of 10,000 parasitoids per month of each species will be distributed within each non-crop plot adjacent to berry fields. Releases will continue for six months, until strawberry plants begin to bear fruit. Sampling will take place in control and augmentation non-crop habitat plots, and in adjacent strawberry fields. For one-week periods during every month from March to October in 2019 and 2020, we will monitor SWD numbers in traps baited with apple cider vinegar, parasitism rates in sentinel SWD pupae, and numbers of SWD per gram of collected fruit (when fruit is present). Sampling will occur along transects extending from source habitats into strawberry fields, to assess whether parasitoid augmentation in SWD source habitats can help control SWD across the crop landscape.