Michigan berry growers face a challenging time managing spotted wing Drosophila

The month of August is becoming the most problematic time period for berry growers in the middle of harvest under difficult conditions due to the presence of spotted wing Drosophila in their fields.

Weather conditions that created the largest blueberry crop in the last three year have also provided ideal environmental conditions for growth and development of large populations of spotted wing Drosophila (SWD) that in a matter of days has increased dramatically in the fruit growing region.

To make matters worse, the most effective insecticides available to growers for SWD control have been substantially affected by intense rains occurring in the area over the past weeks. Particularly problematic has been the effects of rains that occurred all over the southwest and central region on Aug. 12, 2013 when the accumulated precipitation reached up to 2.5 inches in southwest Michigan and 1.5 inches around Ottawa County in the central region.

The results of these events have been that growers following a strict integrated pest management (IPM) program to prevent SWD infestations at harvest time, especially in blueberries, are finding that gains obtained in their battle against this pest were partially lost when they find SWD larvae in fruit being harvested. The immediate question has been “What is wrong with my IPM program?”

The answer seems to be nothing is wrong with your program except that the weather is not helping to keep applied insecticides working long enough to maintain the crop free of SWD. The explanation is found in research conducted by John Wise at the Trevor Nichols Research Center related to the rainfast characteristics of fruit crop insecticides.

Research has shown that the amount of rain that occurs right after the application of some of the main insecticides used against SWD affects the permanence of the insecticide on the fruit. That in turn affects the performance of the insecticide that in many instances loses its effectiveness. Accordingly, Imidan is severely affected when the amount of rain exceeds more than 0.5 inches one day after the application.

In a similar manner, MSU entomologist Rufus Isaacs found that Malathion effectiveness decreases immediately one day after a rain grated than 0.5 inches and SWD control was only around 20 percent of the control in another filed not exposed to the rain. Mustang Max does not escape similar fate. Although more persistent than the OPs, a 2-inch rain one day after the application will substantially reduce the effectiveness of the product. The take-home message is that if rainfall occurs after insecticide application, reapplication is needed to maintain fruit protection.

Also, before harvesting and taking the fruit for processing, growers need to evaluate the conditions of the fruit using the Michigan State University Extension-recommended salt test method. If SWD are found, try to “rescue” the crops applying Imidan or Malathion Aqua at the recommended rates, and use the highest volume of water (40 gallons/acre). A complete cover of the plant is needed to have a good control of an insect like SWD.

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