Virginia wine grape susceptibility to Drosophila suzukii oviposition and resulting larval survivorship

Virginia viticulturists have seen several new invasive species impact wine grape production.  Drosophila suzukii (SWD) causes a decrease in crop yield as well as grape quality by spreading sour rot.  Grapes have a long growing season in which they are exposed to SWD from late June to harvest in September.  This study investigated when grapes first become susceptible to SWD attack and the resulting offspring survivorship.  Six varieties of wine grapes were exposed to SWD adults weekly from green pea stage to maturity.  Grapes were placed in cups and exposed to SWD adults for 48 hrs.  Eggs were counted and grapes were monitored for 14 days to observe any emerging adults.  Grape characteristics evaluated for SWD oviposition susceptibility included skin thickness, penetration force and degrees brix.  Three of the six varieties tested were susceptible to SWD oviposition, which occurred 17 August for two varieties and 25 August for another.  Laval survivorship was evaluated for eggs that were oviposited directly into the grapes as well as from grapes that had been mechanically wounded.  SWD were able to complete their lifecycle in intact fruit, but with high rates of mortality.  All grape varieties had SWD emerge from the mechanically damaged grapes and had significantly higher survivorship.  Larvae on mechanically wounded fruit were able to complete their life cycle on grapes measuring a degrees brix as low as five.  This shows that given an opportunity early on in the growing season SWD can utilize severely immature grapes as a host.