Spotted Wing Drosophila Alert — July 14, 2023

David Handley, Vegetable and Small Fruit Specialist; James Dill, Pest Management Specialist, Phil Fanning, Assistant Professor of Agricultural Entomology, Christina Howard, Maine New Farmer Project Manager

We have found spotted wing drosophila fruit flies in raspberry plantings and blueberry fields in Maine over the past week. This compliments reports from other New England states indicating that SWD is now active, and numbers are increasing early this season, likely encouraged by wet, warm conditions this spring and summer.

Fly counts this week were mostly at low levels, but traps in Wells and Mechanic Falls captured numbers above what are considered potentially damaging to ripening berry crops, especially raspberries and blueberries. Research suggests that when 6 to 10 flies are caught in a yeast-baited trap in a week, larvae will start appearing in the fruit. This week counts have been ranging from 0 to 66 in our traps, indicating that ripening fruit should be protected to prevent infestation.

Spotted wing drosophila populations are likely to build rapidly in the coming weeks as more food (fruit) becomes available for the flies, especially if conditions remain warm and wet as predicted. We now have traps set up at 11 raspberry, highbush blueberry and strawberry sites around the state, and Dr. Phil Fanning and Ben Johnson are monitoring traps in wild blueberry fields Down east. Now is the time to set out your own traps, if you haven’t already. Start protective sprays on any berries that have begun to ripen when more than four spotted wing drosophila flies are caught in a trap, or any larvae are noticed in the fruit. Look for fruit flies hovering around fruit and symptoms of premature fruit decay.

Research has shown that dry, open conditions reduce the number of eggs these insects will lay in the fruit. This supports recommendations to open up your berry plantings by pruning, especially low growth, as these insects favor dark, moist conditions, close to the ground.

Important points for managing spotted wing drosophila include:

1. Monitor for the flies with traps, and for the larvae in fruit.
2. Spray regularly and often once flies or larvae have been found in the field (every 5 to 7 days).
3. Harvest fruit regularly and often; do not leave any ripe/rotten fruit in the field.
4. Sort fruit at harvest; do not leave any soft fruit in the container to be sold.
5. Chill all fruit immediately after harvest to 38ºF (or as close as you can) for a minimum of 12 hours to slow or stop development of any eggs or larvae.
6. Prune the planting, especially the lower region, to open up the canopy and create dry, light conditions.

Products that provide good control of drosophila on berry crops include spinosad (Radiant® for strawberries, Delegate® for raspberries and blueberries), Asana®, Brigade®, Danitol®, Exirel® and Assail®. Research suggests that adding table sugar to group 4A insecticides, such as Assail®, may improve their effectiveness. The recommended rate would be 1-2 lbs. sugar per 100 gallons of spray. Also, it is recommended to add 4-16 oz Nu Film P®/100 gal with all materials to improve SWD efficacy and, if it rains after you spray, re-apply a pesticide material. (Read the label for any re-application restrictions of the same material.) Please check product labels for rates, post-harvest intervals and safety precautions.

Characteristics of Insecticides for Spotted Wing Drosophila Control

Trade Name Days to Harvest Blueberry Days of Residual IRAC Group
Assail® 1 5-7 4A
Mustang Max® 1 7 3
*Bifenture® 1 (3 raspberry) 7 3
*Brigade® 1 (3 raspberry) 7 3
Danitol® 3 7 3
Delegate® 3 (1 raspberry) 7 5
Entrust® 3 (1 raspberry) 3-5 5
Success® 1 (raspberry only) 3-5 5
Exirel® 3 (1 raspberry) 5-7 28
Imidan® 3 (not for raspberry) 5-7 1B
Verdepryn 1 5-7 28

*contains PFAS chemicals

A Simple Monitoring Trap for Spotted Wing Drosophila:
The trap body is made from a 16-ounce red plastic cup (we use Solo Brand P16RLR). You’ll need one that has a tight-fitting lid (we use Solo Brand 626TS). Using a 1/8” hole punch (available through art suppliers), punch about 15 holes in a row around the cup just under the lip about 1/2” apart. Leave about 2” of the diameter of the rim with no holes so that liquid can be poured in and out. Punch a second row of holes just under the first row, to give you a total of 30, 1/8” holes. Use a black permanent marker to paint a 1/2” wide black strip around the cup under the rim, right over the holes you punched. To support the trap, cut a wooden tomato stake down to about 30”. Attach a 4” or larger hose clamp near the top of the stake to act as a cup holder for the trap. (We punched a hole in the metal band of the hose clamp and attached it to the stake with a flat-headed wood screw.) Place the trap holder in a shady, moist place in or near the fruit planting, with the cup height 12” to 18” off the ground. Fill the trap with 4 to 6 ounces of apple cider vinegar, water + sugar + yeast, or whatever bait you prefer. It is best to add a few drops of unscented soap to break the surface tension of the liquid. Place the lid on the cup to keep rain and critters from getting in, and place the trap in the holder. Adjust the hose clamp so that the trap fits in snugly but the trap holes are not covered up. Empty and re-bait the trap every week. Do not pour out the old bait on the ground near the trap, as this will draw flies away from it. Check the traps and change the bait at least weekly.

An effective commercial trap and bait is now available from Scentry. The trap is reusable and the bait lasts 4-6weeks. Cost for both is about $20 plus shipping, it is available from Great Lakes IPM Company.

For more information on identifying spotted wing drosophila (SWD) and updates on populations around the state, visit our SWD blog.

Other IPM Web Pages
Michigan State University Spotted Wing Drosophila
Penn State University Spotted Wing Drosophila Overview
University of New Hampshire SWD Weekly IPM Reports


David T. Handley
Vegetable and Small Fruit Specialist

Highmoor Farm
P.O. Box 179
Monmouth, ME 04259

Pest Management Office
491 College Avenue
Orono, ME 04473

Where brand names or company names are used it is for the reader’s information. No endorsement is implied nor is any discrimination intended against other products with similar ingredients. Always consult product labels for rates, application instructions and safety precautions. Users of these products assume all associated risks.

The University of Maine is an equal opportunity/affirmative action institution.

Weekly Spotted Wing Drosophila Trap Captures

Town Spotted Wing Drosophila weekly trap catch 7/14/23
Wells 66
Bowdoinham 3
Mechanic Falls 10
Monmouth 0