By Emily Popp, U of M Extension Intern
With raspberries ripening, gardeners become excited to eat their favorite jam, dessert, or raspberries by the handful. Of course, with such an amazing tasting fruit we aren’t the only ones who enjoy raspberries. Spotted wing drosophila (SWD) (Drosophila Suzuki) also enjoy feeding on such fruit.
Spotted wing drosophila are in the same family as small fruit flies. They are similar because they are seen feeding near fruit. However, a major difference is that fruit flies develop on the yeast, and spotted wing drosophila develop inside the raspberry.
These drosophila are quite small, around 2-3 mm. To help you identify them, use a magnifying glass or microscope. The male drosophila have a black spot at the tip of their wing, and the only way to identify the females is for their ovipositor with their teeth-like structure that digs into the flesh of the fruit in order to lay her eggs. Once the eggs are laid, they will hatch in 12-72 hours, and the maggots will feed on the fruit and cause them to rapidly soften. After becoming full-grown in 5-7 days, they transform into their pupae. In 4-15 days of being in their pupae stage, they emerge as adults and begin mating right away, repeating the cycle, and often the number of spotted wing drosophila increases as the summer moves forward.
Now that I have talked about what they look like and their cycle of life, we need to figure out how to control them. One of the best practices would be to pick berries often, at least every 3 to 4 days, and do not let overripe berries fall to the ground because that will provide the perfect breeding ground for the spotted wing drosophila. Making your own trap is a good way to monitor insect pressure. Use a cup with a clear plastic lid that has small holes (3/16”) poked into it. Then fill the cup with one inch of apple cider vinegar. Hang a small yellow sticky trap in the cup. Hang traps in the shade near the growing fruit. Check and change out apple cider at least once a week. Make sure to dispose of the old apple cider vinegar away from the raspberries. If monitoring your traps and there seems to be high SWD pressure, there are natural predators that love to feed on spotted wing drosophila. Hummingbirds benefit from the protein that SWD provides. In order to attract hummingbirds, a feeder with sugar solution that is filled twice a week should be hung near the raspberry patch. Another way to reduce SWD populations would be to prune your raspberry plants. SWD are often found near the canopy floor because they like it cool and moist. By pruning the branches, more light penetrates the canopy making it a less desirable environment for the SWD.
If you are finding spotted wing drosophila in other fruits, it is because they like more than just raspberries. They also feed on blackberries, blueberries, strawberries, grapes, and cherries.