Michigan spotted wing Drosophila update – July 17, 2018

Levels of SWD in traps continue to rise in southern Michigan, are leveling off mid-state, and dipping a bit in the north.

Adult SWD.
Adult SWD. Photo by MSU AgBioResearch.

This week the number of spotted wing drosophila (SWD) caught in traps doubled overall, continuing to rise at southern sites, leveling off at central sites, and dipping a bit at northern sites being monitored in Michigan’s fruit production areas. Ninety-five sites across 20 counties were monitored for SWD with an average of 36 flies per trap at southern sites, 13 flies per trap at central sites, and two flies per trap at northern sites.

By now, if you grow a susceptible crop at a vulnerable stage, you should have begun a management program for this pest. All thin-skinned berries and cherries become vulnerable when they start to color, including blueberries, cherries, raspberries, gooseberries, currents and saskatoons. Wild hosts include blackberries, honeysuckle and buckthorn – so susceptible crop plantings next to these are expected to have higher pressure from SWD.

Both cherry and blueberry farms in southwest and west central Michigan being monitored this past week had similar or declining SWD catches in some plantings, while others increased sharply. This variation likely reflects the management program being used on individual farms and the amount of ripe and ripening fruit in the vicinity. This is a good time to check the patterns of fly captures to see where the fly hot-spots are and to use this information to ensure those areas are well protected.

For best results in controlling this pest, use materials and methods that are 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.

Berry crops

Typically we see faster increase in SWD fly captures in the southern edge of blueberry plantings next to woods where the crop remains in shade through the afternoon heat. We also see this pattern in field edges adjacent to ditches with moist soils where wild host plants such as honeysuckle are common. Currently the early ripening varieties such as Bluecrop are at highest risk, but fields we are monitoring using the salt test are remaining free of larvae if the grower is following recommendations to use effective insecticides, ensure good berry coverage and rotate insecticide classes. As Jersey fields move into ripening, we typically see greater pressure from SWD and so those will need protection too, particularly as the temperatures become less extreme over the coming seven-to-ten days.

Tree fruit

In cherry orchards, SWD can take cover from the heat of the day in the crop canopy, so the distribution of flies may be a bit more random. Good coverage with effective insecticides is critical, including fruit in the tops of trees. There is now a 24C label for Mustang Maxx to be able to use this product in tart cherries up to three days prior to harvest. Contact your local Extension educator if you need a copy of this special use label. For more information on managing SWD in cherries, download the free guide Managing SWD in Michigan Cherry available as a PDF. Since plums and other larger stone fruit can be fully ripened off the tree, simply not letting them get to the 3.5 pounds of pressure stage before harvest will prevent SWD infestation.

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. We are also recommending that growers check their fruit for infestation through the season.

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