Post-harvest spraying for spotted wing Drosophila: pros, cons, and a list of issues

While research is underway, here is what we know about late-season management of spotted wing Drosophila for blueberry and raspberry growers.

In recent weeks, monitoring traps have detected increasing activity of spotted wing Drosophila (SWD) flies at many Michigan blueberry and raspberry farms. This is partly because the populations are growing over time, and partly because many blueberry fields have completed harvest and management intensity has been relaxed. For fields that are still being harvested, it is imperative that growers remain vigilant and protect their fruit from this pest.

A detailed article on SWD management in blueberries, Managing spotted wing Drosophila update, was posted online at MSU Extension News in early July 2012, and these recommendations are still relevant for fields that are still being harvested. For raspberries, growers should read the management guide posted online at the MSU IPM Spotted Wing Drosophila website.

While these recommendations provide guidance for management in-season, they provide little information on what to do once harvest ends. There have been increasing questions about the effectiveness of post-harvest sprays to reduce the population of SWD for next year, but there is no research-based information yet that has measured whether post-harvest spraying reduces pest pressure next year. Because of this, we cannot make any recommendations about the effectiveness of this approach.

My research team at the Isaacs Lab is currently investigating this question at four pairs of fields, with one receiving insecticide sprays post-harvest and the other not getting treated. But, our results won’t be available until next season when we get to test whether the treatment in fall 2012 results in any measurable change in the timing of first fly activity or the fly numbers caught in traps next season.

In the meantime, we can think about this pest’s biology and the potential effects of post-harvest spraying. As we learn more about this challenging pest and how to control it, it seems that the short-term emphasis needs to be on protecting fruit from first fly activity until they are harvested, and not trying to control every SWD until they stop flying. 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.

  1. 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. If resistance develops, having a period without sprays would allow time for recovery of susceptibility so that insecticides will still work.
  2. Biological control of SWD is less likely to become established in fields if they are being sprayed.
  3. SWD is active through fall until the first severe frost. In our last fall (2011), they were trapped until late December and early January, which would be a very long time to try and achieve control.
  4. The money spent on this spraying might be better saved until 2013 for doing an excellent job of protecting next year’s crop against this pest.
  5. SWD will experience very high winter mortality in our climate.
  6. This pest is well-established in our natural areas with many widespread host plants such as autumn olive, blackberry, pokeweed, honeysuckle, etc., and eradicating that background population seems like a very challenging task.

There are too 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 their management of 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. Our 2012 research on SWD is still underway, but we plan to report on these studies during the Great Lakes Expo and other regional Extension meetings this winter.

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

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