Bob Koch, Minnesota Department of Agriculture
Jeff Hahn and Eric Burkness, University of Minnesota
A new fruit pest, the spotted wing drosophila (SWD) (Drosophila suzukii), has arrived in Minnesota. This pest feeds on small fruits and stone fruits. The SWD is an invasive pest of Asian origin that was first detected in the continental United States in California in 2008 and has since spread to several western and eastern states. It was found in Minnesota in August, 2012.
The first two detections of this pest were made by members of the public who reported the flies to the Minnesota Department of Agriculture (MDA). First, a homeowner from Hennepin County contacted the MDA about some flies she found in a yeast-baited trap she placed near a raspberry patch. Days later, the MDA was contacted by a citizen who found an abundance of maggots in some wild raspberries picked in Ramsey County. The MDA quickly followed up on both of these reports to visit the sites, collect specimens and confirm the identity of this new pest. It is impressive that people noticed such a small fly (or maggot), realized that it could be a new invader, and knew to contact agricultural authorities regarding the finds. It goes to show how much people care about protecting our resources.
Bob Koch, Minn. Dept. of Ag.
Photo 1: Close up of a male spotted winged drosophila. Note the spot on the wing
The SWD looks very similar to the small fruit flies you might occasionally see flying around overripe fruit on your kitchen counter. However, unlike these other flies, which typically feed on overripe or deteriorating fruits, the SWD feeds 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.
The SWD is difficult to distinguish from other species of small flies. The SWD is a small fly, only 2 - 3 mm (1/12 - 1/8 inch) long, with yellowish-brown coloration and prominent red eyes. Male SWD have dark spots near the tips of the clear wings. Several other species of small flies with spots on their wings can easily be confused for SWD. Female SWD have few distinguishing characters and are even more difficult to identify. Larvae of SWD are white with a cylindrical body that tapers on both ends. To date, SWD is known to be an outdoor pest; fruit flies found indoors are likely to be a different species.
Female SWD use a saw-like egg laying structure to lay their eggs in ripening fruits. The larvae of the SWD then feed within the fruits causing brown, sunken areas. Sometimes the symptoms will not show until after the crops are harvested and sometimes not until the fruits are in possession of the consumers. In addition to the damage caused directly by the larvae, the feeding makes the fruits susceptible to infestation by other insects and rot fungi and bacteria. The larvae will then leave the fruits to pupate and later emerge as adults. Multiple generations of SWD can occur in a year, with populations building throughout the summer. The overwintering stage of the SWD is the adult; however, its ability to survive Minnesota winters remains unknown.
Hannah Burrack, North Carolina State University, Bugwood.org
Photo 2: SWD damage to raspberry. Note the two larvae that just visible.
With this pest being so new to Minnesota and the United States, little is known about how big of an impact it will have and what management tactics will be most effective. The MDA will be working with the University of Minnesota (Extension and the Department of Entomology) to determine how widespread this pest is in Minnesota and to alert farmers and gardeners of its presence and potential impacts. The University of Minnesota will also be developing recommendations for management of this pest on Minnesota fruit crops. SWD could be particularly devastating to blueberry, raspberry and grape growers, but we will need more information on when the pest is active in Minnesota and how well it can survive our winter weather.
Extension programs from other states have suggested several items for consideration in management of this pest. Sanitation is an important consideration to lessen local buildup of SWD populations. Sanitation practices include frequent harvest of crop to ensure ripe fruits are not in field for extended period of time and removal and destruction of old fruit remaining on stems and fallen fruit. Furthermore, crops can be monitored with traps baited with yeast or vinegar; however, yeast-baited traps appear more effective. Traps should be checked frequently (at least weekly) to determine the presence and abundance of SWD males and females. Monitoring for activity of SWD adults is also important, because once eggs are laid in the fruits it will be too late for other management tactics (for example, insecticides) to be effective. If SWD are found in the traps, an insecticide that is registered for use in the specific crop and effective against the pest should be applied. University of Minnesota Extension is evaluating what insecticide options will be effective in Minnesota. After treatment, monitoring of SWD should continue, with additional timely treatments applied as needed.
The adult flies are difficult to distinguish from other small flies; however, if you find an abundance of small, white maggots in what were apparently healthy fruits at the time of harvest, contact the MDA's "Arrest the Pest" hotline at 1-888-545-6684 or at Arrest.the.Pest@state.mn.us. For more information, and SWD updates, please see the University of Minnesota SWD Web page.