Is Spotted Wing Drosophila a Problem in My Wine Grapes?
By: Jody Timer, Penn State Dept. of Entomology, Lake Erie Regional Wine Research and Extension Center
Research has been conducted recently at the Lake Erie Grape Research and Extension Center, to determine the prevalence of spotted wing Drosophila throughout the Lake Erie grape growing region. Spotted wing Drosophila, Drosophila suzukii, Matsumura (Diptera: Drosophilae) (SWD) is an invasive vinegar fly of East Asian origin, that was recently introduced into the United States. It was first found in California in 2008 and is now found in all major fruit-growing regions of the country including Pennsylvania. It was first discovered in Pennsylvania’s Lake Erie grape growing region in the late fall of 2011. 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 SWDs’ high potential for fruit infestation and their potential for spreading rapidly through a field or a vineyard. Eggs hatch in two hours to three days with the larvae feeding in the fruit for about 3 to 13 days before pupating into adults. Thus, multiple generations occur per year. Drosophila suzukii is now one of the most serious pests of thin-skinned fruits including blueberry, raspberry, cherry, grape, and strawberry. 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. A good YouTube video on how to identify SWD damage is: https://youtu.be/DLNDnMMfWfs
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 attact the 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 also found numerous SWD in the traps we placed by cherries, strawberries, raspberries, and corn. Females SWD were caught in traps before males and males were caught in the fall after the females. It is believed that the overwintering populations are mostly female.
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’s (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 only laid a few eggs on the wild grapes. SWD do attack injured grapes before non-injured.
These vinegar 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 a concern of juice grape growers, it should be of decided concern to wine grape growers. Besides the problem of late season rots this insect can impart, there is the problem of wine taint. The beneficial aspects of fruit fly infestation of grapes are stated in a TreeHugger article. It states that the fruit fly also helps the wine by carrying yeast on its body on imparting this to the grapes. Saying that the new knowledge about the interaction between fruit flies and yeast “helps us understand why yeasts make interesting aromas, and opens up the possibility of using flies to help find new yeasts.” Matthew Goddard, from the School of Biological Sciences at the University of Auckland, studied the matter. Goddard’s studies found that when fruit flies had a choice of yeasts, they carried the aromatic wine yeast 100 times more – showing that the smell did have an effect. According to LiveScience, they can find wine or fermenting juice from half a mile away. These beneficial effects are decidedly outweighed by the negative effects from fruit fly infestation. The alcohol in the wine softens the fly’s body and it releases a nasty-smelling enzyme into the wine, they can also transmit large Acetobacter populations. Acetobacter can cause of host of other problem with wine making such as acetic acid. 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.