Mid-Summer Scourge
Spotted Wing Drosophila (Drosphila suzukii) or SWD is an invasive vinegar fly which lays eggs in immature or ripe fruit. This can result in larvae being present in otherwise saleable fruit. These fruit flies are usually active mid to late summer and damage is most often found on raspberries, blackberries and blueberries. However, almost any soft-skinned fruit is at risk including cherries, plums, peaches and grapes.
First found in California in 2008, these flies have dispersed throughout most of the country, reaching the northeast in 2011. Within our region SWD is most problematic on blackberries and fall raspberries, blueberries that ripen from mid to late season and late summer raspberries. Although some crops are more severely affected by damage secondary to actual infestation, such as increased susceptibility and incidence of fruit rot.
This pest prefers a temperate climate with high humidity. Adults can live up to 2 months during the growing season and adult females will lay hundreds of eggs during this time which hatch rapidly into larvae. A generation of SWD can be completed in 10 to 20 days resulting in the occurrence of multiple generations each season. Spotted wing Drosophila appear to overwinter as adults in the northeast, but populations suffer and decrease during the long brutal winters. Spring population size will be low allowing species with early fruit production, such as cherry and strawberry varieties that fruit in June, to escape damage. As the season progresses SWD population size increases exponentially peaking in the fall.
Insect Identification
The basis for the name “spotted wing Drosophila” is one obvious spot on each of the males’ wings, although at times the wing spot can be faint or missing. When this is the case, the front legs need to be carefully inspected for the presence of “sex combs” which appear as two black patches of hairs on each front leg.
SWD females have a larger saw-like ovipositor (egg-laying device than females of other fruit fly species, but they have no other distinguishing markings or features. Unfortunately, identification of female SWD is not as straightforward as with males.
Monitoring and Management
Inexpensive traps can be created from 16 to 32 ounce drinking cups, with lids, deli containers or wide mouth, screw-top plastic jars. The solution in the trap should 1–2 inches deep and contain one drop of unscented dish detergent, which breaks down the solution’s surface tension so the flies sink rather than escape.
Monitoring for both larvae and adults can help fruit growers, landscapers and homeowners determine whether action is necessary. Bait traps have proven useful for the trapping and identification of vinegar flies, specifically males. SWD can be lured to a trap by a variety of volatile chemicals such as those contained in vinegar, wine, yeast and, obviously, fruit. Apple cider vinegar was very commonly used in bait traps due to its translucency, preservation of flies and inhibition of mold. However, it did not provide a lure stronger than the fruit itself and so was deemed an insufficient warning system. Yeast bait appears to be most effective early in the season and a Merlot wine/apple cider vinegar lure is effective later in the growing season. There are some commercially available traps but a simple, home-made trap is equally effective. Place traps on the north side of fruit fields or landscape trees, at fruit level, approximately 2 weeks before fruit begins to color. Check traps as frequently as possible, preferably every day after
fruit starts to color and at least twice per week. Traps are not adequate for control or management of SWD.
There are several cultural management techniques you can employ to protect your fruit trees from infestation damage:
- Harvest thoroughly– remove all ripe fruit from trees and plants, cull any overripe or dropped fruit from fields.
- Dispose of unwanted fruit properly– discourage flies from feeding, laying eggs and hatching on old fruit. SWD and other fruit fly species will multiply in cull fruit, so remove and destroy it, or bury it at least 2 feet deep.
- Field or property management– wild fruit can cultivate spotted wing Drosophila (and other vinegar fly species), try to avoid planting cultivated fruit near wild fruit, if this is not possible removal of wild species may prove necessary.
- Exclusion– screening with very fine mesh may protect crop plants, keep in mind this method may present ventilation and pollinator issues.
There has also been some success managing spotted wing Drosophila populations with biocontrol; several predatory insects feed on SWD adults and larvae, 2 known species exist in the mid-atlantic region. But, predatory insect populations may not multiply as quickly nor do they feed exclusively on SWD, making sole use of this method less than sufficient. Chemical controls have been successful where populations and damage made it necessary. Consult with your arborist if you suspect or have found spotted wing Drosophila on your property, they can help you develop a monitoring and management plan suited to your needs.
