Michigan spotted wing Drosophila update – July 7, 2020

Julianna Wilson<jkwilson@msu.edu>, Rufus Isaacs<isaacsr@msu.edu> and Larry Gut, Michigan State University Department of Entomology - July 07, 2020

SWD populations are active and growers should be protecting susceptible crops.

Figure 1. Counties where SWD populations are being monitored during the 2020 season are shaded in yellow, blue or green, corresponding to regions where fruit production is concentrated in Michigan. Counties in green are referred to as the northern region. Counties in blue are referred to as the central region. Counties in yellow are referred to as the southern region. Photo by Julianna Wilson, MSU Entomology.

This year, Michigan State University Extension is monitoring for spotted wing Drosophila (SWD) activity in 17 counties (Figure 1) at 90-plus sites that include vineyards, sweet and tart cherry orchards, and blueberry, raspberry and strawberry plantings. This report is the third of the season, and week five of our monitoring efforts.

As we mentioned last week, weather is an important driver of SWD activity. This past week continued to be hot and dry across the state and we saw a corresponding drop in total number of SWD caught in traps (Figure 2). That said, every region (Figure 1) is still reporting greater than 20% of the traps catching SWD, and the highest per site catches continue to be from Allegan County.

Although hot, dry conditions may slow SWD activity, in mid-season we know SWD are going to be most active in the morning between 6-8 a.m., and then again in the evening from 5-8 p.m. on cooler, humid evenings. Sites that tend to be cooler, such as those close to Lake Michigan, or that are surrounded by ample wild hosts, are likely to see higher local numbers of this pest.

Figure 2. Current SWD population levels. Comparison of SWD population levels at this time in the season (blue line) compared with the last few years.

Berries

June-bearing strawberry harvest is finished in southern Michigan and wrapping up this week in northern Michigan. Early varieties of blueberries and summer raspberries in southern Michigan are becoming ripe enough to be susceptible.

Cherries

Cherry harvest is underway in southern Michigan and is expected in the next five to 10 days in other regions. This is the first season that we are exploring the use of fruit phenology to help predict when cherries may be at low, medium or high risk of infestation by SWD. The following model output, which we are working towards validating during the 2020 field season, is based on the assumption that cherry ripening will occur between 954 to 1,170 growing degree days (GDD) base 39.2 degrees Fahrenheit post bloom, and that SWD flies have been caught in the area.

* indicates low risk, ** indicates medium risk, and *** indicates high risk of fruit infestation by SWD. The first table reports an expectation that sweet cherries are at high risk of infestation almost everywhere they are being grown if not protected this week. The second table reports that tart cherries are at moderate risk in northwest Michigan and at high risk of infestation everywhere else this week.

Sweet cherry
Weather station	Bloom (biofix)	Current risk for July 6 (GDD base 39.2 F since bloom)	Forecasted risk for July 10 (GDD base 39.2 F since bloom)
SWMREC	1-May-20	1747***	1915***
Commerce TWP	4-May-20	1568***	1740***
Romeo	4-May-20	1563***	1735***
Sparta	7-May-20	1528***	1690***
Benzonia	20-May-20	1257***	1411***
NWMHRC	22-May-20	1257***	1414***
Elk Rapids	22-May-20	1193***	1348***
Old Mission	22-May-20	1204***	1358***
Williamsburg	22-May-20	1221***	1377***
East Leland	23-May-20	1139**	1291***

Tart cherry
Weather station	Bloom (biofix)	Current risk for July 6 (GDD base 39.2 F since bloom)	Forecasted risk for July 10 (GDD base 39.2 F since bloom)
Commerce TWP	10-May-20	1532***	1704***
Romeo	10-May-20	1531***	1703***
SWMREC	12-May-20	1654***	1822***
Hart	17-May-20	1381**	1540***
Benzonia	24-May-20	1157**	1314***
East Leland	25-May-20	1093**	1245***
Eastport	25-May-20	1121**	1278***
Old Mission	25-May-20	1133**	1287***
NWMHRC	26-May-20	1152**	1309***

Grapes

Grapes are not at a susceptible stage anywhere in the state at this time. Details on how to manage SWD and other vinegar flies to minimize their role in sour rot in grapes will be presented during the SWD Friday webinar series on Aug. 14.

Managing SWD

Given how quickly this pest can reproduce and how devastating infestation can be, if your crop is at a susceptible stage and you have begun catching SWD flies in traps monitored on your farm, or if at least 20% of traps in your region have caught SWD flies in the past week, which they have in all regions we are monitoring, a cover spray of an insecticide that is rated excellent against this pest should be applied to protect fruit. Remember to rotate insecticide classes once you begin your spray program. Also, keep in mind that some classes of insecticides wash off after rain events or when using overhead irrigation more readily than others or will be less effective under high heat conditions (e.g., some pyrethroids).

Most of the crop-specific management guides have been updated for the 2020 season and are available as free downloadable PDFs, including a guide for home gardens. For more information on effective insecticides registered for use to control SWD, refer to the MSU Extension Michigan Fruit Management Guide (E154).

Sampling fruit for SWD infestation

You can determine how well your program is working by sampling fruit and using the salt test in the days prior to harvest. Details on fruit sampling methods and identifying SWD and other insects that can be found in berries and cherries will be presented during the SWD Friday webinar series on July 10 from 12-1 p.m.

Monitoring for SWD on your farm

The deli cup style trap is still the standard and can be used with either a commercial lure or a homemade yeast and sugar bait. For more information on how to monitor for SWD, refer to the crop management guides on the MSU Spotted Wing Drosophila website. For help with distinguishing SWD from all the other small flies that may be caught in SWD traps, refresh your identification skills with the Spotted Wing Drosophila Identification Guide from MSU.

Counties being monitored in 2020: Allegan, Berrien, Genesee, Ingham, Livingston, Macomb and Van Buren in the south, Ionia, Kent, Mason, Oceana and Ottawa in central Michigan, and Antrim, Benzie, Grand Traverse, Leelanau and Manistee in the north.

This article was published by Michigan State University Extension. For more information, visit https://extension.msu.edu. To have a digest of information delivered straight to your email inbox, visit https://extension.msu.edu/newsletters. To contact an expert in your area, visit https://extension.msu.edu/experts, or call 888-MSUE4MI (888-678-3464).