If your raspberries had small maggot-like larvae in the fruit like the ones pictured below you are not alone. It is very likely you had a close encounter with Iowa’s newest invasive insect, the spotted wind drosophila (Drosophila suzukii). The spotted wing drosophila (SWD) (named for the distinctive dark spots in the males' wings) feeds on a variety of thin skinned fruit. Although it has a wide host range, reports in Iowa have been primarily from raspberry and blackberry growers. We expect that other fruit crops such as grapes, strawberries, cherries, and aronia to also be impacted.
Spotted wing drosophila is a similar size to our ‘regular fruit fly’ but the female's ovipositor (egg layer) is capable of cutting through the skin of intact, ripening fruit to lay eggs. Eggs are laid in fruit that is nearly ripe. Larvae feed and grow in the fruit after harvest and severely limit usefulness, marketability and storability.
The spotted wing drosophila is native to Asia and was first reported in the west coast of the US in 2008. It has spread rapidly and is now reported throughout the Midwest. The first report in Iowa was in 2010. SWD is so new there are as many questions as answers and it is still unknown what the long term impact of this species will be. However, it is clear that it has quickly spread and established itself across the U.S. and will be a serious pest to contend with for the foreseeable future.
For now control measures are limited to sanitation and monitoring combined with insecticide application. Sanitation can greatly reduce problems with SWD and should include cleanly picking all ripe berries, as well as removing and disposing of any damaged or dropped berries. Dispose of berries in a manner that prevents flies from emerging and reinfesting good fruit.
Monitoring for SWD can help determine if they are present on your property. There are currently no thresholds, so if they are found the recommendation is to treat with an insecticide. Please see table below for a list of products and be sure to follow all labeled instructions. Pay particular attention to the pre-harvest interval as you cannot harvest before that time has elapsed after a treatment.
Monitoring is straightforward and easy to do but remember that apple cider vinegar traps will attract numerous fly species, so you need to check to see if any of them are SWD. For more information on identifying SWD please see the Pest Alert.
You also can submit samples to the ISU Plant & Insect Diagnostic Clinic. If you are sending a yellow sticky card we appreciate them wrapped in plastic wrap first so they don’t stick to everything!
SWD Larvae in raspberry fruit crawling out of the berries a few minutes after the berries were submerged in a salt solution (monitoring).