Michigan spotted wing Drosophila update – June 19, 2018
It’s a much later season for spotted wing Drosophila in 2018. Recent monitoring shows how this pest can vary from year to year.
In 2017, spotted wing Drosophila (SWD) catches in monitoring traps started earlier than ever before and increased rapidly to create high risk for Michigan fruit crops including cherries, blueberries and raspberries. That early season caught some growers by surprise and led to concerns that SWD would continue to be early and have high populations each year. However, this 2018 season we are seeing a much slower start to this pest’s development, which is good news for growers working to harvest insect-free fruit.
We have learned SWD develops into an adult “winter form” in the fall that prepares it to survive winter conditions, but these adults hiding in the soil and leaf litter are still sensitive to extreme cold. The past winter had some significant cold periods in late December, and it also included some events that were very cold without snow cover. This spring, we had a rapid warmup followed by a very cold period in April and then mid-90 degree conditions around Memorial Day. From our limited experience of different winter/spring conditions, it seems this pest is affected by how suitable that part of the yearly cycle is, and that affects how quickly SWD builds populations in the spring.
As an example of the difference, so far in 2018 the monitoring network has trapped an average of 0.01 SWD per trap (that’s one fly in 80 traps) compared with 1.04 per trap by this date last year (June 19, 2018). There was a single female SWD trapped one month ago in an Allegan County blueberry field, but since then no new catches in blueberry fields being monitored by Michigan State University.
In cherries, a single fly has been caught each week in different parts of the state starting with one male caught in Manistee County just after Memorial Day, one female in Ionia County during the week of June 4 and then a single female in Berrien County during the week of June 11—all from traps set up in tart cherries.
This type of variation from year to year highlights the value of monitoring for SWD so growers know whether this will be a high or low pressure year by the time the fruit start to ripen.
That said, given how quickly this pest can reproduce and how devastating infestation can be, if your crop is at a susceptible stage, apply a cover spray of an insecticide that is rated excellent against this pest to protect fruit and maintain populations at current low levels. Also, rotate insecticide chemistries once you begin your spray program. A slow start likely means this pest will be easier to manage this year than last year.
There may be some fields and orchards that are able to have fruit harvested before SWD populations build to levels where sprays specifically targeting this pest are needed. By trapping for flies and sampling fruit using the salt test in the days prior to harvest, growers can determine how well their program is working.
For the recently released guide to SWD management in organic systems, see “Management Recommendations for Spotted Wing Drosophila in Organic Berry Crops.”
For more information on this pest, including how to identify, monitor and manage this pest in fruit crops, please visit the MSU Spotted Wing Drosophila webpage.