Spotted-wing Drosophila (SWD) trap catches continued to remain high at our four remaining locations in the last week of August. In addition to fall raspberry risk of damage, a key concern for grape growers should be the high numbers of SWD at Chanhassen (Hort. Research Center, HRC), and at Hastings. Traps in our network are effective at catching both male and female flies. As we approach wine grape harvest in the next 2-3 weeks, growers should remain vigilant in monitoring SWD (ideally, using traps located at on-farm vineyards), and if flies are present (e.g., trap catch >10 flies/week), consider using one of four alternating insecticides prior to harvest (e.g., Delegate or Entrust [organic-certified], Mustang Maxx, and Malathion). Among these, Mustang Maxx is the only Restricted Use Pesticide (RUP) that requires additional applicator certification. Importantly, growers must also follow the label for pre-harvest intervals (PHI) and re-entry intervals (REI), specific for wine grapes, and specific to each insecticide. Such sprays near harvest, particularly with Mustang Maxx (pyrethroid), will also help suppress the multicolored Asian lady beetle, which has been an important pest species for the past 15 years as well.
In recent years, particularly with cooler, wet (humid) weather in late Aug. to early Sept., SWD numbers can increase substantially. As blueberries and summer raspberry availability subsides, SWD will be opportunistic and readily disperse to other fall crops, to continue their development. In brief, with wine grapes, there is the risk of direct damage to berries by larval feeding (and a loss of quality ripe berry yield), but most important for most vineyards, is the concern for an unwelcome taint to the wine.
Following a 2-year research project (Dominique Ebbenga), we found that among the vast majority of cold-hardy Minnesota grape varieties and breeding lines tested (34 total) using healthy (skin intact) berries, only 4 of these were found to be infested with larvae. The key word here is intact berries. With a healthy berry skin, not previously damaged by birds, wasps, or disease, these results indicate that when the vineyard is managed well, the SWD females will not be able to penetrate the outer skin to lay eggs, and will therefore not be infested with larvae at harvest. Additional tests, where 8 of the most popular grape varieties were exposed to SWD females over a period of 4-8 weeks (e.g., Itasca, La Crescent, Marquette, Frontenac), only resulted in 3 berries infested or <0.9% of the total exposed.
Despite the low risk of grape infestation when berries are intact, there are years where disease, birds, or even physiological splitting can cause pre-mature injury to grapes, and then allow an opening for SWD females to lay eggs; once eggs are laid, it is highly likely that berries will be infested, typically with one to two larvae surviving per berry. More important, however, is the indirect injury to grapes and final wine quality. The primary concern that we now have with SWD is that it has been shown to be a vector of Acetobactor spp., and specifically the acetic acid bacterium (AAB), also known as the source of sour rot on grapes, and be transmitted to berries as flies forage on clusters. This concern was also recently confirmed with SWD in Italy. High levels of AAB on grapes can lead to increased levels in juice and final wine product. When AAB is found in wine, the risk of alcohol being converted to acetic acid increases, resulting in the “vinegar taint.” Based on Ebbenga’s research to date (1st year results), we found that when SWD was caged on berry clusters for 2 weeks under field conditions, fly densities of both 10 and 20 flies each resulted in statistically significant increases in volatile acidity (VA), compared to the uninfested caged berries. Although some of the AAB concern can be addressed in the winery during processing by the addition of sulfide, it is highly recommended to reduce the risk of SWD in the field prior to harvest. In addition to SWD, other fruit/vinegar flies are capable and more likely to cause sour rot in wine grapes. Although SWD may play a role in this disorder, management of all fruit flies in the vineyard can help reduce risk of AAB/sour rot — University of Minnesota Extension