Vigilance needed against spotted wing Drosophila for berry growers still harvesting

Use traps and salt testing of berries to determine how well your spotted wing Drosophila management program is working.

As reported in the most recent weekly scouting report for spotted wing Drosophila (SWD), since early July there has been a steady increase in the average number of flies caught per week in Michigan State University Extension’s statewide monitoring program for this pest, and also in the proportion of traps catching SWD. For each of the last two weeks, the average catch has doubled, indicating that fly populations are building. This is particularly evident in the southwest and west central regions of the state where catches of SWD are highest. At some fields in this region, we are also finding low levels of larvae in fruit assessed with the salt test, further indicating that pest pressure is building.

The trend in SWD captures and the levels of pest activity mirror what we have seen in previous years, where pressure from SWD increases rapidly during August. This may be a reflection of the cooler temperatures experienced recently, since the optimal temperature for SWD development is around 70 degrees Fahrenheit. It may also reflect the challenge of maintaining control of SWD when there are repeated rain events. So far this season there have been 14 days with rain recorded since early July in some parts of southwest Michigan.

In addition to monitoring crop fields, traps placed in woods have also seen a rapid increase in SWD captures, with some sites showing very high counts of more than 50 per trap in the past week. This indicates that the active population of SWD is building on wild hosts that are fruiting through this part of the summer, such as Lonicera (honeysuckle), and this is expected to keep growing as wild fruit continue to develop and ripen.

For growers of susceptible fruit crops in regions where SWD has been detected, these recent observations highlight the need to maintain control of this pest until the 2014 crop is in. It is important to be sampling your fields using traps and the salt water test to determine whether larvae are present and also to assess the performance of your management program. Doing this sampling from different parts of a field can also help you make more refined control decisions, for example by indicating whether the rows adjacent to woods require additional control.

If flies are detected or larvae are found in berries, we recommend growers use the most effective insecticides and maintain coverage with a weekly schedule of effective insecticides until harvest is complete. This is the most conservative approach and is most likely to provide berries without infestation. However, there is also a need to be concerned about development of resistance and the potential for loss of natural enemies, so we are continuing to study how reliable the traps are for being the sole basis of a pest control decision, or whether a fruit sample and trap information are both needed to make a decision to spray or not.

Until we have a reliable system, there remains some risk of a “false negative,” and with the potential for rapid population growth at this time of the season, we consider it prudent to take a conservative approach. As more is learned about this pest’s management and more effective baits are developed, we expect this situation to change.

If treating a field to protect berries from SWD, check on the pre-harvest interval restrictions and seasonal limits and be sure to rotate chemical classes to avoid development of resistance. For large, dense crop canopies, it is also critical to achieve good coverage of the fruit, so sprayers should be set up to reach the inner parts of the canopy. For more details on SWD management, refer to the SWD crop recommendation guides posted at the MSU Spotted Wing Drosophila website.

As a final note about SWD management, this summer we have been evaluating physical exclusion of SWD in small plantings using netting, and the preliminary results are very promising. While this approach will not be suitable for everyone, it could provide an alternative to repeated insecticide applications. Further details of the approach and our results will be reported this winter at MSU Extension meetings.

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

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