Spotted wing Drosophila
Spotted wing Drosophila (SWD) was first found in Minnesota in 2012. There are still many questions as to how SWD will impact strawberries and other fruit in Minnesota. Damaging populations so far have impacted fruits ripening in August and later. This suggests that the most vulnerable strawberries are ever-bearing (day-neutral) cultivars that begin ripening in July and continue to frost. June-bearing strawberries are much less likely to be infested.
SWD can be difficult to distinguish from similar flies. SWD is a small fly, only 2 - 3 mm (1/12 - 1/8 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.
Close up of female's ovipositer showing large dark teeth.
Hannah Burrack, North Carolina State University
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 has large, dark- colored teeth which permit her to puncture immature fruit. This can only be viewed under high magnification, (e.g. a dissecting microscope).
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.
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; there is increasing evidence that SWD is able to survive Minnesota winters to some extent.
SWD larvae feed on healthy, intact, ripening strawberries. SWD can also attack other soft-skinned fruit, such as raspberries, blackberries, cherries, blueberries, plums, and grapes. SWD larvae feed within the strawberries causing brown, sunken areas. Eventually, the fruit becomes soft and decomposes. 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 home gardeners 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 strawberries 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 hold the bait. 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 capture flies, place a small yellow sticky card inside. Yellow sticky cards can be purchased from local garden supply companies and from Gempler's. The traps will also work without the yellow sticky insert, but then add a drop of unscented dish soap to the vinegar so the flies remain trapped in the liquid.
Place traps on a stake just above the canopy in the row, and begin monitoring before the strawberries begin to ripen. Make sure the trap is clear of vegetation with holes exposed so that SWD can easily enter the trap.
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. Insecticides are used to prevent the females from laying eggs. It does not affect the larvae once they have infested the fruit. Effective chemicals are permethrin, spinosad and malathion. Be sure to read and follow all pesticide label directions.
For more information, see Spotted wing Drosophila in home gardens, http://www.extension.umn.edu/garden/insects/find/spotted-wing-drosophila-in-home-gardens/