Location: Invasive Species and Pollinator Health
Project Number: 2030-22000-033-021-A
Project Type: Cooperative Agreement
Start Date: Jun 1, 2024
End Date: Mar 31, 2025
Objective:
The objectives of this project are to: 1) Mass rear the exotic Spotted Wing Drosophila (SWD) parasitoid Ganaspis brasiliensis; 2) Continue releases of G. brasiliensis at an areawide implementation site; 3) Monitor establishment and spread of G. brasiliensis across the landscape; 4) Determine areawide impacts of G. brasiliensis on SWD levels and insecticide use; 5) Examine areawide patterns and seasonal population dynamics of SWD and its naturally occurring pupal parasitoids.
Approach:
The study will be conducted at two sites managed by Driscoll’s Berries in Watsonville, Santa Cruz Co. The SWD parasitoid G. brasiliensis will be released into one “implementation” site, and the other site will serve as a control. Sites were established in 2021, and consist of large acreages of organic and conventional cane berries (raspberry and blackberry) adjacent to noncrop habitats infested with wild Himalayan blackberry, a widespread host of SWD. Each site is ~250 acres and is comprised of ~25% non-crop vegetation or woody areas. Sites are 13 km apart and are as similar as possible in landscape features, grower practices and SWD pressure.
To prepare for mass releases, G. brasiliensis will be reared at large scale in Dr. Daane’s laboratory at UC Berkeley. Open field releases of G. brasiliensis will start in late spring/early summer when ripe fruits and SWD appear in crop fields. At least 2,000 to 5,000 parasitoids will be released at the implementation site 5-8 times per year. To track spread, releases will be from one consistent spot, which will be immediately adjacent to semi-natural habitat to provide favorable conditions for parasitoid survival. Recent work with the naturally occurring SWD pupal parasitoid P. vindemmiae suggested that it moved at least 664 m from the release point, and likely accumulated in the non-crop borders.
SWD levels and parasitism rates of G. brasiliensis and the naturally occurring SWD pupal parasitoids P. vindemmiae and T. drosophilae will be monitored from early June to November in each year when ripe fruit is available and SWD is most abundant in California. Each site will have 25 sampling points spaced in a grid at a rate of one per 10 acres, for a total of 50 samples. A third of the sampling points will be in the crop interior, a third in the crop edge, and a third in non-crop habitat.
Adult SWD will be monitored at each sampling point using a Trece cup trap baited with a 50:50 mixture of apple cider vinegar and white wine. Sampling of adults will occur weekly until late August, and monthly starting in September. To monitor numbers of SWD larvae and provide a measure of crop damage, and to assess larval parasitism by G. brasiliensis, ten ripe berries
will be collected within 3 m of each trap every two weeks. Fruit will be weighed in the laboratory and then held in vented containers for at least six weeks for emergence of flies and parasitoids. Pupal parasitism will be monitored at each sampling point using sentinel traps consisting of banana slices infested with pupating SWD larvae. Banana slices will be held in small plastic cups, which will be placed in larger plastic containers. A hole on one side of the larger container will be covered in 0.9 mm mesh to allow parasitoid movement into traps while restricting entry by flies. Banana slices will then be held in the lab for parasitoid and fly emergence. Sentinel traps will be placed at sites every four weeks.
