Michigan spotted wing Drosophila update – July 24, 2018

Spotted wing Drosophila catches are continuing. Keep protecting fruit through harvest season.

This week, the number of spotted wing Drosophila (SWD) caught in traps remained steady from last week overall. This likely reflects a combination of cooler and rainy weather that is more suitable for SWD, combined with the sprays for this pest being used at many of the sites we monitor during this harvest season.

Seventy-two sites across the fruit production regions of Michigan were monitored for SWD last week, with an average of 24.6 flies per trap overall, a little than the 22.2 flies per trap during the previous week. We have been finding many more SWD in habitats adjacent to crop fields in recent weeks, perhaps because these areas include blackberry, honeysuckle and buckthorn hosts, so the risk of infestation is greater at field borders. More female flies were trapped than males in our monitoring traps, and a low level of egglaying and larvae were found in fruit at unmanaged sites using fruit sampling with a salt flotation test method. However, at most farms where management actions have been implemented, the fruit sampled in the past week were un-infested, indicating management programs are working well.

This week’s cooler weather and rain showers are making conditions much more suitable for SWD activity than the recent hot weather. Therefore, we expect the activity of SWD to increase in the coming weeks. For growers still harvesting summer raspberries or moving into later-season blueberries or late-harvested cherry plantings, some fruit sampling to determine the status of fruit infestation is worthwhile to determine fruit quality before it is harvested. Placing fruit in a concentrated salt solution for an hour and then inspecting the fruit can be very informative as larvae in the fruit will float and can be counted.

All thin-skinned berries and cherries become vulnerable when they start to color, including blueberries, cherries, raspberries, gooseberries, currents and saskatoons. Pressure from this pest can be reduced through sanitation techniques such as exclusion, rapid harvest and removing and disposing of infested fruit. These cultural controls will likely not be sufficient for effective control in most farms, so for best results in controlling this pest, use insecticides rated excellent against SWD to protect fruit. As always, be sure to calibrate your sprayer to maximize coverage and rotate insecticide chemistries to prevent the risk of developing insecticide-resistant populations of SWD on your farm.

We will continue to report on our SWD monitoring program for a few more weeks. Our general reports provide an overview of the situation, but monitoring flies on your own farm provides a more relevant source of information on SWD activity. Check fruit for infestation through the season.

For the recently released guide to SWD management in organic systems, see “Management Recommendations for Spotted Wing Drosophila in Organic Berry Crops.”

For more information on effective insecticides registered for use to control SWD, refer to Michigan State University Extension’s Michigan Fruit Management Guide for 2018 (E154).

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