Learn About The Impact Of Cold Weather On Survival Of The Spotted Wing Drosophila

Posted: March 29, 2014

The Spotted Wing Drosophila (SWD), Drosophila suzukii, is a small vinegar fly which has done major damage to many fruit crops. This insect is a pest of most berry crops, cherries, grapes and other tree fruits, with a preference for softer-fleshed fruit.

Spotted Wing Drosophila was first discovered in the western United States in 2008 and moved quickly through the Pacific Northwest into Canada. In the spring of 2010, SWD was discovered in Florida on strawberries and detected later in the summer in The Carolinas. The SWD became a pest in the Northeast in 2011. It has also been detected in Europe. Because the flies are only a few millimeters long and cannot fly very far, human-assisted transportation rather than natural dispersion is the most likely cause of the rapid spread.

Female SWD can cut into intact fruit using their serrated ovipositor to inject eggs under the skin. By being able to insert eggs into intact fruit, the larvae of SWD can be present during ripening, leading to a risk of detection of larvae in ripe fruit after harvest. There is a greater risk of fruit contamination at harvest from SWD compared with native species that lay eggs only in already-damaged and rotting fruit. The adult SWD lives for about two weeks, and can lay more than 300 eggs. This demonstrates their high potential for fruit infestation and distribution through a field if not controlled. Infested fruit do not show obvious symptoms of infestation at first, with only a small pin-prick visible from egg-laying. Within a few days, the fruit flesh will start to break down, leading to discolored regions and eventual collapse of the tissues. By this point, the white larvae can be relatively easy to detect.

Can we expect reduced SWD pressure this season due to our extremely cold weather?  Dr. Greg Loeb, grape and small fruit entomologist at Cornell spearheading work on SWD in NY and the NE region reports that we don’t have an answer to this yet. The SWD does go into a non-reproductive phase (diapause) later in the fall and it’s likely the adults have improved capacity to tolerate cold temps, at least to some extent. Probably not enough to handle the kinds of temperatures we have seen this winter in unprotected places. Of course, we would expect adults to seek out protected places such as in the soil litter or in rotting wood, etc. But we really don't have any data on this yet. Loeb did set up some cages without bottoms outdoors this winter (with leaf litter and with or without logs) and released flies reared from late season fruit. Around January 1 he pulled the cages off and allowed snow, etc. to accumulate. The plan is to put cages back on in March and monitor for emergence. Lobe expects survival will not be very good but we shall see.”

Penn State Horticulture Specialist Kathy Demchak is tentatively hopeful that few SWD survived, however protected locations such as compost piles, brush piles, or crawl spaces  may still have afforded them some protection.  We expect that Northern June-bearing strawberry crops will not be affected, or will be only minimally affected by SWD once again.  Numbers are still likely to increase sometime next summer, so stay tuned throughout the next growing season for updates.