Michigan spotted wing Drosophila update – June 23, 2020

Julianna Wilson<jkwilson@msu.edu>, Rufus Isaacs<isaacsr@msu.edu> and Larry Gut, Michigan State University Department of Entomology - June 23, 2020

Warm weather is here and SWD populations are rising. Expect to begin protecting susceptible crops.

Tart cherry tree — In a year with a short crop, ripening fruit that remain, like these tart cherries shown here, will be at higher risk of infestation by SWD. Photo by Juan Huang, MSU Entomology.

For 2020, Michigan State University Extension will be monitoring for spotted wing Drosophila (SWD) in 17 counties at 91 sites that include vineyards, sweet and tart cherry orchards, and blueberry, raspberry, and strawberry plantings. This report is the first of the season and week three of our monitoring efforts. Of the 47 sites reporting this week, 71% of the traps have caught SWD. SWD has been caught in all of the following counties this season: Allegan, Antrim, Benzie, Berrien, Genesee, Grand Traverse, Kent, Leelanau, Livingston, Macomb, Manistee, Mason, Oceana, Ottawa and Van Buren.

Weather interactions

Weather is an important driver of SWD activity. Over the past few weeks, we have seen some flip flopping from week to week in terms of temperature and rainfall. Last week was warmer and drier than normal; over the weekend and Monday night, June 22, some areas got some rain. The increased rainfall and resulting humidity resulted in recent rapid SWD population growth compared with previous seasons (Figure 1), but it is mainly southern Michigan sites that are driving this trend.

Figure 1. Current SWD population levels. Comparison of SWD population levels at this time in the season compared with the last few years. This season, the SWD populations have already begun to surge.

Berries

Strawberries are being harvested across northwest, central and southern Michigan, and there have been some reports of SWD being active in and infesting strawberries in some areas. Blueberries and raspberries are not at a susceptible stage at this time. More on managing SWD in berry crops will be presented in two weeks 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. More about how to use this model will be presented during the SWD Friday webinar series on June 26.

Sweet cherry
Weather station	Bloom (biofix)	Current risk for June 22 (GDD base 39.2 F since bloom)	Forecasted risk for June 26 (GDD base 39.2 F since bloom)
SWMREC	1-May-20	1255***	1377***
Commerce TWP	4-May-20	1102**	1222***
Sparta	7-May-20	1058**	1168**
Benzonia	20-May-20	819*	915*
NWMHRC	22-May-20	802*	900*
Elk Rapids	22-May-20	762*	858*
Old Mission	22-May-20	764*	860*
Williamsburg	22-May-20	776*	873*
East Leland	23-May-20	712*	809*

* = low risk
** = medium risk
*** = high risk

Tart cherry
Weather station	Bloom (biofix)	Current risk for June 22 (GDD Base 39.2 F since bloom)	Forecasted risk for June 26 (GDD Base 39.2 F since bloom)
Commerce TWP	10-May-20	1066**	1187***
SWMREC	12-May-20	1162**	1284***
Hart	17-May-20	929*	1032**
Benzonia	24-May-20	721*	817*
East Leland	25-May-20	666*	763*
Eastport	25-May-20	684*	784*
Old Mission	25-May-20	693*	790*
NWMHRC	26-May-20	697*	795*

* = low risk
** = medium risk
*** = high risk

Grapes

Grapes are not at a susceptible stage anywhere in the state at this time. Details on how to manage SWD and other drosophilids 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 (Figure 2) – a cover spray of an insecticide that is rated excellent against this pest should be applied to protect fruit and maintain populations at current low levels. Remember to rotate insecticide classes once you begin your spray program. Most of the crop-specific management guides have been updated for the 2020 season and are available as free downloadable PDFs here.

For more information on effective insecticides registered for use to control SWD, refer to the MSU Extension Michigan Fruit Management Guide for 2020 (E-154).

Figure 2. Percent traps that caught SWD overall and by region over the past three weeks. Southern counties include Allegan, Berrien, Genesee, Ingham, Livingston, Macomb and Van Buren. Central counties include Ionia, Kent, Mason, Oceana and Ottawa. Northern counties include Antrim, Benzie, Grand Traverse, Leelanau and Manistee. The threshold if using the regional trapping network to guide management decisions is when 20% of traps in your region have captured SWD in the last week (indicated by dashed line).

Sampling fruit for SWD infestation

Growers can determine how well their 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 IPM SWD webpage. For help with distinguishing SWD from all the other small flies that may be caught in SWD traps, refresh your ID skills with this factsheet: Spotted Wing Drosophila Identification Guide.

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).