After a week of rains in west central Michigan, we are passing through a short drought period. No substantial precipitation has occurred during the past seven days, except for some localized rains that in some areas left an accumulation of up to 1 inch of precipitation. Temperatures, on the other hand, have remained around the 80s. For the past seven days, daily maximum temperatures averaged 80 degrees Fahrenheit and the minimum temperatures averaged 60 F. These weather conditions are demanding supplemental irrigation in small fruit crops, which raspberries and blueberries being harvested now.

Tempered weather conditions are a relief for small fruit growers. As a result, harvested raspberries and blueberries are of excellent quality, thanks in part to rains from the previous weeks. However, the same conditions are also helping spotted wing Drosophila (SWD) populations. For the past two weeks, we have observed a large peak of SWD in all monitoring traps, especially at monitoring sites in Allegan and Van Buren counties, and at a lesser degree in counties north of Allegan (Ottawa, Kent, Iona, etc.).

It is critical to maintain a tight SWD monitoring program with short time periods between trap inspections. At times of high pest pressure, checking traps twice a week provides precise information on SWD population for implementing a timely and swift response.

Follow these recommendations for managing SWD under heavy population pressure:

• Keep SWD traps in place, especially at locations on the farm in proximity to wooded areas or neglected blueberry fields.
• Service the trap twice a week. Traps are accumulating so many non-SWD flies that it is necessary to remove all the filth created by dead insects to ensure the effectiveness of the trap.
• Spray as soon as you detect flies in monitoring traps. Do not wait!
• Keep a tight spray application schedule. Remember, insecticide labels may indicate that a product can protect the crop up to seven days after the application. That information is under perfect weather conditions.
• Select the best insecticide according to present and forecasted temperatures and provability of rain.
  • Pyrethroid insecticides are excellent products at temperatures below 85 F. They are rainfast but degraded by high temperatures. Brigade is the exception.
  • Imidan and Lannate are excellent for controlling SWD. They are not affected by high daily temperatures, but Imidan is susceptible to wash off from the plant when 0.5 inches of rain occurs after the application.

Overall, if you need to irrigate your blueberry fields and control SWD at the same time, apply the irrigation first and then carry on the insecticide application.

Under current conditions of temperature and probability of rain in the area, Mustang Maxx, Brigade and Assail are good alternatives. These insecticides have a preharvest interval of 24 hours. Between harvests, you may use Imidan, Lannate, Danitol, etc. Their preharvest interval is three days. For more insecticide options, consult the “Michigan Fruit Management Guide,” MSU Extension bulletin E154.

Following this advice is important for managing SWD in fruit crops where the industry has established a “zero”-tolerance for SWD larvae in fruit delivered for processing.