SWD has been a challenge for berry growers in Ontario for several years. In 2017, when the population increased earlier than normal, it caused losses in cherry crops. We don’t know a lot about its impact on grapes in Ontario, but we do know that it is present in our vineyards. Research in other grape growing regions suggests that SWD is implicated with injury to fruit, the attraction of other sour rot-bearing Drosophila species and contamination of fruit and wine.
The following are excerpts from an article written in 2016 by Jody Timer, Penn State Dept. of Entomology. A summary of options for controlling SWD in grapes is provided at the end of the article.
The potential infestation rate of spotted wing Drosophila differs from other vinegar flies because the female possess a serrated ovipositor that cuts into healthy fruit to lay eggs. Consequently, spotted wing Drosophila (SWD) larvae can be found in fruit that is just ripening: https://youtu.be/dPr61VC2gyo
During egg-laying, it is believed that sour rot and fungal disease can also be introduced, further affecting the fruit quality. SWD are thought to overwinter primarily as adult females, and they prefer moderate, cool wet climates similar to the Lake Erie grape belt. Adults live approximately two to nine weeks. During this time, one adult female can lay 100 to 600 eggs in fruit. During peak temperatures, a female can lay more than 100 eggs a day. Such a high reproduction rate indicates the SWD’s high potential for fruit infestation and their potential for spreading rapidly through a field or a vineyard.
Because this pest is similar in appearance to common vinegar flies, the greatest problems have occurred when populations went unnoticed and thus remained untreated until they caused considerable damage to crops.
In our research we sent up 25 traps for SWD though out the region. By harvest SWD were found in all of the traps. They began to be attracted to grapes at verasion, and by harvest the SWD outnumber the other vinegar flies (fruit flies) in all of the traps. Over the entire season, the percentage of SWD to other vinegar flies caught in our traps over the last three years is approximately 25-30%.
We then conducted 2 and 4 choice and no-choice test with common wine and juice grape varieties SWD infested all of the grapes we tested. They showed no strong preference for cultivar of grape, color of grape, or brix (as long as the variety was past verasion). An interesting side discovery from our research was that SWD does not appear to attack native wild grapes. Even given no other source (no choice testing) it laid only a few eggs on the wild grapes. SWD attack injured grapes before non-injured.
These flies become a greater problem the later the grapes are harvested due to late season rots, which makes the later ripening wine grapes particularly at risk. Although this insect is of decided concern to wine grape growers. Besides the problem of the late season rots this insect can initiate, there is the problem of wine taint.
SWD is of special concern because of their ability to lay eggs in otherwise healthy fruit. Often the fruit will not look damaged until the larval populations, which have hatched from the internally laid eggs, grow and feed internally on the grape berry till it eventually collapses. This can happen after seemingly healthy fruit has been harvested and sent to wineries.
The alcohol in the wine softens the fly’s body and it releases a nasty-smelling enzyme into the wine. SWD can also transmit large Acetobacter populations. Acetobacter is one of the organisms that causes sour rot in grapes and it can cause of host of other problem with winemaking.
Click here for the full article (and some really cool videos): https://psuwineandgrapes.wordpress.com/2016/08/05/is-spotted-wing-drosophila-a-problem-in-my-wine-grapes/
Products registered in Canada for SWD in grapes include Mako, Malathion 85E, Entrust, Success and Harvanta 80SL. Delegate has also shown good control of Drosophila species in our research trials. Consider using one of these products in pre-harvest insecticide sprays to manage SWD and other Drosophila flies when you are deciding on products for management of grape berry moth, multi-coloured Asian ladybeetle or wasps. For details on these products refer to Pesticide registrations for SWD in grape.
The images below show magnified egg-laying injury to Petit Verdot (top) (photo credit Doug Pfeiffer, Virginia Tech) and Viognier (bottom) (photo credit Meredith Shrader, Colorado State U.) . Note the slight depression and the white filaments extending from the egg-laying site. Other Drosophila species also produce eggs with filaments; however, these eggs are laid only on the surface. SWD eggs are inserted through the skin into the flesh with only the filaments outside.