Michigan spotted wing Drosophila update – June 30, 2020

Julianna Wilson<jkwilson@msu.edu>, Rufus Isaacs<isaacsr@msu.edu> and Larry Gut, Michigan State University, Department of Entomology - June 30, 2020

Hot weather is here and SWD populations are rising; growers should begin protecting susceptible crops.

Summer raspberries are beginning to ripen and become vulnerable to infestation by SWD in southern Michigan. Photo by Julianna Wilson, MSU Entomology.

This year, Michigan State University Extension is monitoring for spotted wing Drosophila (SWD) activity in 17 counties at over 90 sites that include vineyards, sweet and tart cherry orchards, and blueberry, raspberry and strawberry plantings. This report is the second of the season and week four of our monitoring efforts. Of the 73 sites reporting this week, 80% of the traps have caught SWD, and in every county in which we are currently monitoring. The highest per site catches are being reported from Allegan, Berrien and Macomb counties.

As we mentioned last week, weather is an important driver of SWD activity. With warmer and drier than usual conditions in most of the state, some sites caught fewer SWD this past week. 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. Overall, there was a slight increase in average trap catch compared with last week (Figure 1), driven mainly by sites in Allegan County, close to Lake Michigan where conditions are a little cooler and where there are ample wild hosts in the surrounding habitat.

Figure 1. Current SWD population levels. Comparison of SWD population levels at this time in the season (blue line, 2020) compared with the last few years.

Berries

Strawberries are finished with harvest in southern Michigan but are in the middle of the second week of harvest in northern Michigan. Early varieties of blueberries and summer raspberries in southern Michigan are becoming ripe enough to be susceptible. More on managing SWD in berry crops will be presented this week during the SWD Friday webinar series on July 3.

Cherries

Crop loads in cherries are highly variable across the state. With fewer cherries on the trees, the expectation is that remaining cherries will be extremely vulnerable to attack. 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 1170 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.

Sweet cherry
Weather station	Bloom (biofix)	Current risk for June 29 (GDD base 39.2 F since bloom)	Forecasted risk for July 3 (GDD base 39.2 F since bloom)
SWMREC	1-May-20	1484***	1640***
Commerce TWP	4-May-20	1306***	1456***
Romeo	4-May-20	1302***	1445***
Sparta	7-May-20	1264***	1418***
Benzonia	20-May-20	1006**	1147**
NWMHRC	22-May-20	995**	1138**
Elk Rapids	22-May-20	949*	1042**
Old Mission	22-May-20	954**	1092**
Williamsburg	22-May-20	969**	1114**
East Leland	23-May-20	895*	1032**

Tart cherry
Weather station	Bloom (biofix)	Current risk for June 29 (GDD base 39.2 F since bloom)	Forecasted risk for July 3 (GDD base 39.2 F since bloom)
Commerce TWP	10-May-20	1271***	1421***
Romeo	10-May-20	1270***	1413***
SWMREC	12-May-20	1391***	1546***
Hart	17-May-20	1043**	1186***
Benzonia	24-May-20	908*	1050**
East Leland	25-May-20	849*	986**
Eastport	25-May-20	870*	1014**
Old Mission	25-May-20	883*	1021**
NWMHRC	26-May-20	890*	1034**

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 to catch 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.

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).