SWD parasitoid releases!

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Spotted wing drosophila, Drosophila suzukii (SWD), will have a new natural enemy to contend with in the Salinas and Santa Maria growing regions. After ten years of research by a determined group of entomologists to find a better parasitoid for SWD, Ganaspis brasiliensis (Gb for short) is a parasitoid that attacks the SWD larvae and is projected to have a positive impact on California strawberries.

Figure 1. Strawberry fruit are collected from strawberry fields to assess SWD population numbers prior to releasing Ganaspis brasiliensis.
Figure 2. Strawberries from the field are brought to the lab and SWD and parasitoids are counted weekly for 4 weeks.

We are targeting areas adjacent to strawberry and caneberry crops near riparian areas where native blackberries or similar native hosts grow year-round. The SWD living in riparian areas can serve as a source of reinfestation to nearby strawberries. After strawberry fields are mowed, SWD from those fields can move into the riparian habitat and continue their life cycle if native sources of caneberries are present. We are monitoring both the Gb riparian release sites, nearby strawberry fields’ SWD population, and all parasitoids emerging from SWD. We will continue to look for sites with successional wild fruiting hosts with SWD populations to release Gb.

Figure 3. Ganaspis brasiliensisis will be released in riparian areas with established SWD populations, Gb should fly out into the nearby strawberry crops to infest SWD there. They could also potentially lower the SWD that move from the riparian areas into the adjacent berry crops.

The Gb parasitoids are specialists on SWD and will only attack SWD. There are different groups or lineages of Gb found worldwide and even in the US already, but these had various levels of specificity to SWD. The Gb “G1 group” (from Japan and probably South Korea) was chosen for breeding and release in the US because it is highly specific to SWD and will not be interested in Drosophila melanogaster or other related flies.

Figure 4. The larval parasitoid, Ganaspis brasiliensis (Gb) is being released in strawberry growing regions to help fight spotted wing drosophila (SWD). We transport Gb in tubes (left) then tap them gently onto the berry plants to release them (right).

There are other parasitoids that attack SWD but they have a way of encapsulating and killing many of those that try to parasitize them. That’s one of the reasons SWD is so successful here. However, Gb can successfully parasitize them with each female Gb producing  ~98 eggs. Gb tends to prefer the younger SWD larvae and they can live for ~18 days.

Figure 5. The larval parasitoid, Ganaspis brasiliensis (Gb), is currently being released by a joint team of entomologists from the Cal Poly Strawberry Center, the CA Strawberry Commission, the University of California (Berkeley), and USDA-ARS.

It is clear SWD aren’t going to be eradicated even with this new parasitoid but we hope it will help reduce the number of SWD in our berry crops and that the riparian areas will serve as a permanent source of Gb that can move into our berry crops when SWD arrive.