2013 Spotted Wing Drosophila Summary for Maine Berry Growers
David Handley, Vegetable & Small Fruit Specialist
James Dill, Pest Management Specialist
The spotted wing drosophila (Drosophila suzukii) is an insect invasive to Maine that was first captured here in the fall of 2011. Based on crop damage in other parts of the country and our own experience in 2012, we know that this insect poses a serious threat to most of the soft fruit crops we grow here, including raspberry, blackberry, blueberry and strawberry. During the summer of 2013 the University of Maine Cooperative Extension Pest Management Program set up drosophila traps in berry fields around the southern, central and coastal regions of the state to monitor the presence and activity of this insect.
The traps were 16 oz. red plastic cups with 30, 1/8” holes punched under the rim to allow the flies access. A 1/2” wide band of black was painted just under the rim of the cup to increase its visual attractiveness. The cups were topped with a tight fitting plastic lid and mounted on 4’ tomato stakes fitted with 4” hose clamps to act as a cup holder. Four to six ounces of bait/killing solution (a mixture of cider vinegar and alcohol) was poured into each trap. A 60 ml plastic specimen cup containing a second bait consisting of water, sugar, flour and yeast was then placed within the trap to further increase its attractiveness. We placed traps either within the crops or in a wooded area near the crops, knowing the insect prefers humid, shaded areas. We emptied the traps weekly and restocked them with fresh bait. The insects captured in the traps were brought back to our lab at Highmoor Farm in Monmouth to be identified. Many different species are attracted to these traps and proper identification, while time consuming, is essential. As populations increased we informed growers through our IPM newsletter, blog and webpage, hoping to keep the pest as well managed as possible throughout the summer.
In 2013, the first spotted wing drosophila were caught in Warren and Wells on July 19. With the exception of trapping sites in Warren and Bowdoinham, captures were very low, just a few flies per trap, and scattered, most sites having no flies, until the third week of August. At that point we began catching low numbers of flies at nearly all locations, including Wells, Limington, Limerick, Springvale, Cape Elizabeth, New Gloucester, Poland Spring, Mechanic Falls, Wales, Livermore, Bowdoinham, and Dresden. Traps in wild blueberry fields in Hancock and Washington counties were also catching flies at this point, but also in low numbers. By the end of August, our Monmouth, Farmington and Oxford sites had also captured flies. Trap captures generally remained low (0 to 20 flies/trap) with occasional small flare ups (20 to 100 flies/trap) until the first week of September. At that point numbers rose fairly consistently in nearly all locations, with weekly trap counts ranging from just a few flies to nearly 1,000. Raspberry and blueberry fruit infested with the small white larvae were being reported. The highest numbers of flies continued to be found in the most southern and coastal sites. By the end of October many sites were catching flies well into the thousands (14,000 during one week in Limerick) while some caught only a few flies. At this point flies were readily visible around ripe fruit in many fields and larvae were found infesting most of the fruit in any plantings that had not been sprayed. At the end of the season we found that a trap maintained for us by a grower in Caribou had caught three flies.
Similar to the 2012 season, growers using insecticides to control spotted wing drosophila found that weekly sprays appeared to provide adequate control when populations remained relatively low (0-10 flies/per week). However, as fly populations expanded, growers found that twice weekly sprays were needed to keep larvae out of the fruit. Growers used Entrust®, Delegate®, Brigade®, Bifenthrin®, Hero®, Mustang Max®, and/or malathion insecticides, and most found that these products usually offered adequate control if applied on a frequent basis. Growers who did not apply pesticides saw near total crop loss, following the arrival of spotted wing drosophila in their fields.
Spotted wing drosophila trap catches remained relatively high throughout the remainder of the season, with dips in late September and early November, which may correlate with dry periods. As in 2012, the highest trap catches occurred late in the season, well after most of the crops had been harvested or lost to frost. The lack of food likely make the traps more attractive, at least partially accounting for the increased catch, but this reiterates that high numbers of flies survive long after killing frosts have occurred. By the end of November, populations finally dropped significantly, suggesting that the flies were now entering the over-wintering stage.
Spotted wing drosophila overwinter as adults (flies). Any time the air temperatures exceed 45ºF for more than a few hours, it is likely that some adults will start becoming active. The winter of 2013-14 has been one of the coldest in recent memory, and it will be interesting to see how well the flies come through it. However, it would be unwise to assume that we will not be seeing many flies this summer because of the cold winter. In its native Asia, it survives cold winters well. Additionally, any storm fronts moving into Maine from the south could carry with it flies from southern states where the winter has not been so harsh. Thus, berry growers should anticipate needing to manage drosophila for the 2014 season. Based on our 2012 and 2013 experience, we believe it will be unlikely to significantly infest crops until relatively late in the season when populations reach damaging levels (late August in 2013 at some sites). Therefore, earlier ripening crops such as June-bearing strawberries should not be significantly impacted; but later ripening crops such as fall fruiting raspberries, late ripening varieties of blueberries and fall strawberries will need to be protected as soon as fruit begin to ripen. We plan to monitor drosophila populations in Maine again in 2014, and carry out research on improving our trapping strategies to provide an early warning system in the future.
Based on what we know so far about this pest, here are six rules for managing spotted wing drosophila.
- Monitor for the flies with traps, and for the larvae in fruit.
- Spray regularly and often once flies have been found in the field (1-2/week).
- Harvest fruit regularly and often; do not leave any ripe/rotten fruit in the field.
- Sort fruit at harvest; do not leave any soft fruit in the container to be sold.
- Chill all fruit immediately after harvest to 38ºF (or as close as you can) for at least 12 hours to slow development of any eggs or larvae.
- Prune the planting to open up the canopy and create dry, light conditions.
Please follow our blog providing regular updates of spotted wing drosophila trapping data and management strategies during the growing season, where you can sign up for notifications of updates.
David T. Handley
Vegetable & Small Fruit Specialist
Highmoor Farm Pest Management Office
P.O. Box 179 491 College Ave
Monmouth, ME 04259 Orono, ME 04473
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