Aug 5, 2020
Weather conditions favorable for SWD spray applications in Michigan

Managing spotted wing drosophila in blueberries and raspberries is critical during harvest. Lower daily temperatures are an opportunity for using a wider range of insecticide products recommended for control. Currently, pyrethroids are very effective.

As of Aug. 4, blueberry harvest of mid-season varieties continues in west central Michigan. Dry weather conditions coupled with temperatures below the mid-80s provided excellent conditions for harvesting high quality fruit. Daily minimum temperatures during the past seven days were on average 60˚ F and the daily maximum temperatures averaged 80˚ F. There were some scattered rain showers in the area, but accumulation was less than 1 inch. Although this accumulation was not enough to satisfy the blueberry water requirements, these rain showers provided some relief from the high temperatures observed during the previous weeks.

The Michigan State University Enviroweather extended weather forecast for the next six days indicates that low daily temperatures and dry conditions will continue unchanged. Dry conditions and temperatures below the mid-80s maximize the effectiveness of most insecticides recommended against spotted wing drosophila (SWD) as demonstrated by Steve VanTimmeren and Rufus Isaacs from Michigan State University. However, the efficiency of the insecticide application could be affected by the low relative humidity that prevails during those dry conditions.

According to Ozkan and Zhu from Ohio State University, ambient air temperature and relative humidity influence the evaporation rate of spray droplets. As temperature increases and relative humidity decreases, droplets will evaporate more rapidly. However, ambient temperature and relative humidity have little influence on the longevity of droplets larger than 200-micron diameter.

Accordingly, insecticide applications under low relative humidity (rH) require increasing the volume of the water used for the application to compensate for the potential evaporation of insecticide droplets before reaching the SWD habitat. Therefore, to obtain the maximum benefit from the insecticide applications, we recommend using the maximum volume of water per acre, 40 gallons, to full cover the bush canopy. This will also ensure the insecticide will reach the inside of the bush where SWD looks for shelter.

Under low relative humidity (rH) conditions, aerial applications may also reduce insecticide losses by depositing the spray right on top of the bush canopy (see photo). Insecticide selection also needs to be according to the prevailing weather conditions. Check the 2020 Michigan Fruit Management Guide (MSU Extension bulletin E154) for the best options and doses.

If you have questions or require help developing your insecticide application program against SWD, please call your local MSU Extension office for assistance.


Aerial application reduces insecticide loss to evaporation by reducing the distance between the spray equipment and the target below. Photo: Carlos Garcia/MSU Extension.

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