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Spotted wing drosophila fly activity starting but still low this spring

EAST LANSING, MI. — Monitoring traps set by Michigan State University researchers in southwest Michigan unmanaged and commercial farms have started to catch spotted wing drosophila (SWD) adult flies this season. These traps were baited with Scentry lures and checked weekly. The timing of these detections is in line with the typical timing of SWD activity. We usually catch the first SWD in traps in late May or early June in southwest Michigan farms, and that has been the case this season with the first flies caught in a few traps in the last few days of May. This was followed by continued low catch until last week when there was an increase of activity from an average of 0.2 flies per trap to 17.5 flies per trap. With a few of the earliest ripening blueberries and cherries starting to color in southern Michigan, growers of these crops should be paying attention to this pest.

High heat slows - but does not stop - SWD development

With fruit starting to ripen under humid conditions, it is not surprising that we are seeing SWD activity getting started. However, the continued blazing hot temperatures this week are expected to limit activity of SWD flies. These conditions also inhibit egg laying and reproduction, and a study from University of Georgia showed that temperatures over 83 degrees Fahrenheit inhibited this insect. We also know that it can hide from the heat during the daytime and shift its activity to the early morning or late evening when conditions are more suitable for them. So, the heat will likely delay activity this season but its not likely to stop it. High heat conditions will also affect the performance of some insecticides registered for use in fruit crops against this pest.

Growers are encouraged to consult the Michigan Fruit Management Guide (E-154) or their local Michigan State University Extension fruit educator for guidance on this issue.

Monitoring traps and fruit sampling

Traps are useful for indicating when fly activity starts and when it spikes up. The best information will come from your own farm, with greatest relevance to your own farm management. Although the traps catch flies, relevance of fly catches to management decisions is not always clear and so we also recommend that growers check their fruit regularly for larvae. This can provide you with confidence that your management program is working, or it can indicate a need to adjust control programs.