Spotted wing Drosophila in home gardens
Spotted Wing Drosophila (SWD), Drosophila suzukii, is an invasive insect pest mainly of raspberries, blackberries (and other cane berries), blueberries, grapes, and stone fruit. Native to Asia, SWD was first found in North America in 2008 in California, and is currently found in Florida, Michigan, Wisconsin, and most of the primary fruit growing regions of the U.S. In August of 2012, the first confirmation of SWD was made in Minnesota in Ramsey, Hennepin, Anoka, and Olmsted Counties and as of December had been found in 29 counties (Figure 6). To date the primary crop attacked has been raspberry but wild blackberries were also found to be infested.
Photo: Martin Hauser, California Dept. of Food and Agriculture
Figure 1. Male SWD with distinctive wing markings
SWD can be difficult to distinguish from similar flies. SWD is a small fly, only 2 - 3 mm (1/8 - 1/12 inch) long, with yellowish-brown coloration and prominent red eyes. The males are fairly easy to identify – they have clear wings and a dark spot along the first vein near the tip of each of wing (Figure 1).
Female SWD also have clear wings, but lack any spots on them which makes them difficult to identify. The ovipositor (used by the female fly to insert eggs into berries) needs to be examined under high magnification, (e.g. a 30X magnification hand lens or dissecting microscope) to see the distinctive large, dark-colored teeth on it which identifies it as a female SWD.
SWD larvae (also called maggots) are white with a cylindrical body that gradually tapers on one end. This is a small insect, only reaching 1/8th inch long (Figure 2).
Adult flies insert eggs into soft fruit where the larvae develop. The larvae will then leave the fruits to pupate and later emerge as adults. SWD can complete its life cycle in as little as seven days. Multiple generations of SWD can occur in a year, with populations building throughout the summer. SWD overwinters as an adult; however, its ability to survive Minnesota winters is unknown at this time.
SWD larvae feed on healthy, intact, ripening fruits. In particular, the SWD will feed on thin-skinned, soft fruits such as raspberries, blackberries, blueberries, strawberries, grapes, plums and cherries. SWD larvae feed within the fruits causing brown, sunken areas. It is possible these symptoms won’t appear until after the crops are harvested. In addition to the damage caused directly by the larvae, the feeding makes the fruits susceptible to infestation by other insects, rot fungi, and bacteria.
Monitoring should occur from fruit set until the end of harvest. This allows the home gardener to identify the start and end of fly activity, although the most critical time period to monitor is when fruit color first starts to develop until the crop is harvested. This is when the fruit are susceptible to SWD infestation.
Adult SWD flies can be trapped using a plastic 32oz cup with several 3/16”-3/8” holes around the upper side of the cup, leaving a 3-4 inch section without holes to allow you to pour the bait (Figure 5). Holes can be made using a drill in sturdy containers or, if in softer material, burned with a hot wire or soldering iron. The small holes allow access to SWD, but keep out larger flies and other insects.
Pour one inch of apple cider vinegar into the trap as bait. To help capture the flies, place a small yellow sticky card inside. Yellow sticky cards can be purchased from local garden supply companies and from Gempler’s (http://www.gemplers.com/). The traps will also work without the yellow sticky insert, but then a drop of unscented dish soap should be added to the vinegar to ensure flies remain trapped in the liquid. Traps should be hung in the fruit zone, in a shaded area of the canopy, using a wire attached to the top of the trap (Figure 5). Make sure the trap is clear of vegetation with holes exposed so that SWD can easily enter the trap.
If flies that are suspected of being SWD are found in counties where this insect has not yet been reported, contact the Minnesota Department of Agriculture’s “Arrest the Pest” hotline at 1-888-545-6684 or Arrest.the.Pest@state.mn.us. Place flies stuck on sticky traps in another container (or if they are floating in the vinegar, remove and place in a small container). Please note the location and date of collection for the specimen.
Sanitation is important to reduce the local buildup of SWD populations. The best sanitation practice is to frequently harvest crops to ensure ripe fruits are not in gardens for extended periods of time. It is also important to remove and destroy any old fruit that remains on stems or that has fallen to the ground.
It is important to remember that SWD females can start laying eggs one day after adult emergence. This makes it very important to monitor to detect whether SWD is present and when they first appear. The sooner the flies are discovered, then the quicker management decisions can be made. SWD will complete multiple, overlapping generations so there will be continuous activity once the flies become active. Effective insecticides are listed in the table below.
|*Long residual can persist as long as four weeks. Medium-long residual can persist as long as 10-14 days.|
CAUTION: Read all insecticide label directions very carefully before buying, and again before using, to ensure proper application. Be sure that the label specifies that it can be used on the specific crop you wish to treat. The label is the final authority on how you may legally use any pesticide. Whenever using any pesticide, follow all label directions including the storage, mixing, application and disposal of pesticides.