Alert: Spotted Wing Drosophila (SWD) in Grapes
Matthew Clark1, Eric Burkness2, Drew Horton1, and Bill Hutchison2
1Grape Breeding & Enology Program, Dept. of Horticulture, University of Minnesota
2MN Extension IPM Program, Dept. of Entomology, University of Minnesota
Typically, when grape berries have intact skin, female SWD are not able to penetrate the berry skin to lay eggs. However, despite the relatively low susceptibility of grapes to being infested by SWD, as grapes age, skins become thinner and there is a greater risk for splitting and other fruit injury to occur (hail, birds, disease, general deterioration). With the very wet growing season in 2016, we have observed increased berry splitting in tight cluster cultivars. In varieties like Marquette and the Frontenac series, we have observed fruit breakdown. This degradation in berries (skin and pulp) could likely be attributed to the increased berry water content. Additionally, we have been observing necrosis on Marquette rachis leading to some berry decay but have not been able to determine if this is pathogenic or a stress response. In most years, harvest of Frontenac can be delayed as a benefit to juice chemistry as it often “hangs well” allowing acids to decline. This is not recommended in 2016.
Once fruit skin is compromised, or the fruit skin has deteriorated sufficiently, female SWD will begin laying eggs in the fruit. Despite typically low populations of adult flies, relative to other fruit crops, if fruit becomes susceptible to egg lay, populations can grow exponentially in a few short weeks reaching thousands per trap per week (http://www.fruitedge.umn.edu/swdtrap#grapes). With the limited success and/or options for insecticide use, the best options for managing SWD populations in grapes is to use production practices that minimize fruit splitting, disease, bird damage, and fruit deterioration. If fruit starts showing high levels fruit damage or deterioration, growers should consider harvesting fruit as soon as possible.
Vineyard monitoring can help inform the grower if and when the SWD are present. Although grape is not a preferred host, SWD may be present in other fruits adjacent to vineyards, including in gardens, greenhouses, composting fruit, and native plant populations. These SWD are opportunistic and will exploit damaged or deteriorating grape berries. Through monitoring, the grower can determine what steps are needed to use insecticides, although there is inconclusive evidence that insecticides are effective. Weekly monitoring will help determine what insect pests are a problem, and if their is increased presence of SWD or other pests.Traps can be purchased from catalogs such as Great Lakes IPM (http://www.greatlakesipm.com/2016%20Catalog%20Web.pdf) or traps can be constructed using the following instruction sheet (http://www.ipm.msu.edu/invasive_species/spotted_wing_drosophila/monitoring)
Vineyard sanitation is also critical to minimize fallen fruit or missed clusters during harvests. Most wine makers are familiar with fruit flies in the winery and in discarded grape must. One should consider strategies to restrict the fly movement. Consider covering waste must with fine mesh or tarps until all fruit has been harvested from the vineyard as this compost could serve as a source of continued fly infestation. Small amounts of damaged fruit/must can be placed in clear plastic bags where heat from the sun will kill larvae and flies, although this may have limited success in the fall as temperatures decline. Do not compost or bury the material as neither of these methods have been shown to eliminate SWD. Winter conditions in Minnesota are not favorable for SWD as the insects are not able to survive.
In the Winery
Fruit flies, including SWD, are adept at transferring microbes including acetobactor from berry to berry in the vineyard and from juice to wine and to equipment. These microbes are responsible for the volatile acidity (VA) that taints the wine with aromas of acetic acid and ethyl aldehyde. You will smell and taste this in the vineyard, and these berries should be discarded (although not to the vineyard floor). The use of traps in the winery fermentation area can help eliminate flies that either enter the winery on their own or those that come in with fruit at harvest. The traps used for monitoring are suitable. Simple vessels with a small volume of vinegar, sweet wine, or red wine, and a drop of unscented dish detergent are cheap, effective traps that can be set out daily. Removing must and waste regularly and sanitizing winery surfaces will help to reduce fruit flies and microbial problems.
Winery practices to control VA with sulfur dioxide (SO2) are really no different than controlling wild yeast and other bacteria that come in on the berries and are controlled through managed fermentations. However, tainted juice prior to fermentation will be difficult to manage unless it falls below detection thresholds after winemaking through blending. During primary fermentation, racking and pressing may eliminate some of the VA in addition to more SO2. Other advanced techniques include reverse osmosis and filtration to reduce the microbe population. During and after fermentation, some of the bacteria may live in a film on the juice/wine surface and not be effected by the dissolved SO2. Again, best practices to reduce oxygen at the surface in the vessel will reduce the chances for VA. Select vessels with minimal head space and consider sparging with inert gases.