Two years ago Corrine and I attended a seminar in Fairmont for small fruit growers facilitated by the University of Minnesota staff.
He mentioned a new fruit fly that was already confirmed in Rock County. Drosophila has moved east from California since it was discovered in 2008 and is now a threat to all of our soft-skinned fruits, namely strawberries, raspberries, blueberries, plums, grapes and cherries.
The speaker asked if I had seen any evidence in my raspberries, and when I said no, his response was “consider yourself lucky.” Well, my luck ran out! Early this summer I lost a good share of my cherries and early raspberries to this new pest. We went to Detroit over the Fourth of July weekend, and while I was gone, the bugs attacked!
Spotted Wing Drosophila is a tiny fruit fly about 1/8 of an inch long. They are easily recognizable because they have large red eyes. The adult female pokes a hole in the skin of ripening fruit and deposits one to three eggs, doing this in multiple fruits. The eggs hatch into small white larvae. Cherries rot almost immediately. Raspberries and strawberries continue to mature in spite of the puncture wound. The area where the larvae are present becomes soft and mushy. The larvae do their feeding and pupate in the fruit and emerge as an adult fly. That adult matures and begins to lay eggs within 24 hours. Consequently, any spray program must be ongoing while the fruit is ripening through harvest. The adult does not feed, so any spray to control them has to kill the insect by contact; larvae are not affected by the spray.
I have not found any information on effective trapping methods. With such a short life cycle, there can be in excess of ten generations per season. We’re talking a major problem for fruit growers here! I have found an organic spinosad spray with the trade name of Entrust … cost is over $400 per quart. I’m not expecting many backyard gardeners will find their fruit crop worth that price. Thus, without control, the populations will produce enough flies to share with everyone … not good news!
The first step in control is to pick fruit immediately as it ripens. The scent of ripening fruit is what attracts the fly. Refrigerating the fruit at 35 degrees for three days will kill any eggs or larvae that may be present. Immediately pick up any damaged fruit or ripe fruit that has fallen on the ground, seal it in a garbage bag and dispose of the bag in the garbage … do NOT compost the infected fruit.
Sorry for the news. But if you’re informed, you can be prepared. I was informed and thought I was prepared, but I was not present when the drosophila attacked, so now I am engaged in the battle.