End of season spotted wing Drosophila management in raspberries

Do post-harvest applications of insecticides or early mowing of fall red raspberries reduce spotted wing Drosophila populations for next season?

Fall red raspberry harvest has wrapped up at almost all farms across the state. Trapping that has continued for spotted wing Drosophila (SWD) has seen populations of the pest increase dramatically over the last few weeks. These high trap catches have prompted a number of questions from fall red raspberry growers concerning the management of SWD. The two most frequently asked questions in the last week are concerning a possible post-harvest application of an insecticide to help reduce SWD pressure for next season and whether cane mowing now instead of mid- to late winter would reduce SWD pressure for next season.

While there are examples in the fruit arena of using a post-harvest insecticide application to reduce overwintering pest populations the following season, such as with cherry fruit fly, this is not known for SWD. This same question was asked by blueberry growers at the end of blueberry harvest, and for fall red raspberries the answer is the same. At this time, there is no research-based information that has measured whether post-harvest spraying reduces pest pressure next year. Because of this, Michigan State University Extension cannot make any recommendations about the effectiveness of this approach.

An experiment was started at the end of this season’s blueberry harvest to answer this very question in blueberries, but the will not be available until the end of the 2013 blueberry harvest. While the desire to control SWD after harvest is understandable given the threat it poses, there are some potential issues inherent in post-harvest spraying for SWD that should be considered. The major concern is related to the possibility that repeated applications of insecticides against pest populations increase the chance for development of resistance. This is even more likely in a pest that has a short generation time like a vinegar fly, and this group of insects has been shown previously to have an inherent capacity for developing resistance to insecticides.

There are many unanswered questions related to SWD management that need to be resolved, and research entomologists throughout Europe and North America are actively pursuing some of the highest priority questions to aid growers in managing SWD. Many of these questions take time and resources to answer, however, and we will aim to get information out to the grower community as soon as we have answers. In the meantime, the latest pest control strategies for SWD are contained in a guide titled Spotted Wing Drosophila Management Recommendations for Michigan Raspberry and Blackberry Growers, posted at the MSU Spotted Wing Drosophila website.

The second question fall red raspberry growers have raised relates to mowing of canes now in an attempt to reduce the overwintering SWD population, thus reducing pest numbers for next season. Fall red raspberries have traditionally been mowed in February or early March. All perennial plants, including fall red raspberries, go through a long process of physiological changes as they prepare for winter. These steps are commonly referred to as hardening. First, carbohydrates and nitrogen move from the senescing leaves into the canes. Movement continues into the crown of the plant throughout the fall and early winter. If canes are mowed before movement is complete, these important resources are removed and not available to the plants the next years. Plant vigor and perhaps yields may be reduced the next year as a result. Early mowing may also reduce the winter hardiness of the plants crowns.

The best time to mow canes is in February or early March (before buds begin to swell). While mowing plants now may reduce the overwintering populations slightly, the negative impacts of early mowing are much greater than the benefits of a slight reduction of the overwintering SWD population. Thus, early mowing is not considered to be a wise management decision.

Look for more information on SWD management at many winter fruit conferences and meetings, such as the Great Lakes Fruit, Vegetable and Farm Market Conference in early December in Grand Rapids, Mich.

Dr. Isaacs’ and Dr. Hanson’s work is funded in part by MSU’s AgBioResearch.

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