Monitoring for spotted wing Drosophila to detect early activity

Updated spotted wing Drosophila management plan released for blueberry growers.

We have not detected any spotted wing Drosophila flies this spring, but with blueberry fruit approaching early ripening, we are recommending trap deployment this week. Regular checking of the traps will allow detection of spotted wing Drosophila activity and appropriate fruit protection practices. To prepare for activity later this summer, MSU Extension has released the updated version of its Spotted Wing Drosophila Management Recommendations for Michigan Blueberry Growers, available online at MSU’s spotted wing Drosophila website in the “Control recommendations by crop” section.

The most important step in managing spotted wing Drosophila is to determine whether they are present in your fields and when they may become active. Monitor until the end of harvest, as fruit are susceptible from coloring until the last harvest.

Spotted wing Drosophila can be trapped using a simple monitoring trap consisting of a plastic, 32 oz cup with 10 3/16- to 3/8-inch holes around the upper side of the cup, leaving a 3- to 4-inch section without holes to facilitate pouring out of the liquid attractant, or bait. The holes can be drilled in sturdy containers or burned with a hot wire or soldering iron. The small holes allow access to vinegar flies, but keep out larger flies, moths, etc. The traps can be baited with 1 inch of apple cider vinegar. To help ensure that trapped flies do not escape, a small, yellow, sticky trap can be placed inside, and hung on a paper clip. This can be attached with hot glue to the inside of the lid. The traps will also work without the yellow sticky insert, but then a drop of unscented dish soap should be added to the vinegar to ensure flies remain trapped in the liquid.

trap
Trap for capturing spotted wing Drosophila
adult flies.

In 2011, we found that spotted wing Drosophila traps were more attractive to spotted wing Drosophila when they were baited with a yeast-sugar mix compared with the apple cider vinegar. This mix is made by combining 1 tablespoon of active dry yeast (we use Red Star brand) with 4 tablespoons of sugar and 12 oz of water. This ratio produces a solution that ferments and the flies are attracted to the odors.

Using this mix, spotted wing Drosophila were trapped earlier, more of the traps caught spotted wing Drosophila and spotted wing Drosophila were trapped in greater numbers than with the apple cider vinegar. Although these traps are harder and messier to service, the yeast bait is less expensive than the apple cider vinegar traps and the benefits of earlier detection are obvious when needing to protect crops from infestation. Traps baited with yeast will collect many flies, so sorting through these traps will take more time. For blueberry growers in Michigan, we recommend a minimum of one yeast-baited trap for spotted wing Drosophila every five to 10 acres.

Any traps for spotted wing Drosophila should be hung in a shaded area of the bush canopy in the fruit zone, using a wire attached to the top of the trap. Make sure the trap is clear of vegetation with the holes exposed so that spotted wing Drosophila can easily fly in.

Traps should be checked for spotted wing Drosophila flies once a week at a minimum by looking on the yellow sticky trap and in the liquid. If you use a yeast trap, checking only the sticky trap can be used as a way to reduce the amount of time needed to service the trap. However, checking the trap insert and liquid will provide the best ability to detect early fly activity. At each check, fresh bait should be swapped out and disposed of away from the trap location. Spotted wing drosophila captures should be recorded each week in a log book.

Identification of flies

Vinegar flies are small (2 to 3 mm) with rounded abdomens. Traps catch both male and female spotted wing Drosophila flies and native species of vinegar flies. This means that spotted wing Drosophila need to be distinguished from the others when checking the traps. Identification of spotted wing Drosophila flies becomes easier with practice, especially when using a hand lens to examine the wings of trapped flies. Some native flies have dark patches on the wings, but will not have the distinctive dark dot that is present on the wings of spotted wing Drosophila males.

male fly
Male spotted wing Drosophila fly with distinctive wing markings
(3 mm long). Photo credit: Kurt Stepnitz.

Female spotted wing Drosophila do not have dots on the wing, so their ovipositor needs to be examined closely in search of its serrated characteristics. Use of a 30X magnification hand lens or microscope is needed to detect the distinctive saw-toothed ovipositor on female spotted wing Drosophila. This is challenging to detect, so we recommend that growers and scouts focus on checking more traps for males than spending lots of time identifying females.

If you check a trap that has flies matching these descriptions, but are unsure of their identification, contact your local MSU Extension office or a trained scout or crop consultant for assistance.

For flies suspected of being spotted wing Drosophila that are trapped in counties where this insect has not yet been reported (see Spotted Wing Drosophila Management Recommendations for Michigan Blueberry Growers), we encourage growers, scouts and consultants to place flies trapped on the sticky traps into another container (or pull those floating in the vinegar out of the liquid and place in a small vial) then send them for identification to:

Howard Russell
SWD Monitoring, MSU Diagnostic Services
578 Wilson Rd., Room 107
East Lansing, MI 48824-6469

Include the location and date of collection along with your contact information.

A photographic guide to identifying spotted wing Drosophila is provided at the MSU spotted wing Drosophila website.

Dr. Isaacs’ work is funded in part by MSU’s AgBioResearch.

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