Spotted wing Drosophila catches increase: monitor and protect ripening, susceptible fruit

The recent detections of this pest are of greatest concern for growers with raspberries, blackberries or blueberrues yet to harvest.

Traps for spotted wing Drosophila (SWDF) are increasing in their frequency and the level of catches of SWD, especially at blueberry and raspberry sites where growers have completed harvest. Fields that are still being managed actively generally have lower catches than those where harvest and insect pest control sprays are complete for the year.

More SWD flies are being caught in traps baited with yeast-sugar solution compared with those baited with apple cider vinegar and in some cases the yeast traps are catching at sites where the vinegar traps have not caught any flies. Although we have recommended the apple cider vinegar bait because of its relative ease of use and lower cost to operate, scouts checking fields with ripe fruit still to harvest might want to also deploy a trap with an inch of yeast bait for comparison. The yeast bait recipe we are using is 4 Tbsp. sugar: 1 Tbsp. active dry yeast: 12 oz. water.

The increase in captures raises the question of whether pest control activities are warranted. If the crop has been harvested and there are no susceptible fruit nearby, we do not consider it worth the time and expense to apply insecticides for SWD control. This is based on the fact that this pest has very high levels of winter mortality and so the vast majority of the flies will be killed during the winter, plus this insect is present in natural areas around fields where sprays will have little effect. However, if fields still have some harvest remaining and flies are trapped in that field or if flies are trapped in post-harvest fields immediately adjacent to such fields, applications to control SWD and protect fruit from this pest should be made.

We have recently completed a study in which blueberry clusters were treated with different insecticides to test their performance against SWD. Treatments were made in a field of Jersey variety. Treated shoots were picked the next day, then placed in containers with 10 SWD (five male flies: five female flies). After one week, the fruit were collected and assessed for the number of Drosophila larvae. The results are presented below.

Average number of Drosophila larvae in blueberries exposed to SWD flies 

Based on this research of testing insecticides for SWD control in Michigan blueberries, we recommend the following insecticides at the rates per acre shown here: Delegate (6 oz), Imidan (1.33 lb), Lannate (1 lb), Malathion 8F (32 oz), or Mustang Max (4 oz). Entrust (2 oz) is the available and effective option for organic blueberry growers that has residual control. Pyganic (64 oz) had low activity in these trials, but may provide some control if applied on a shorter interval. We also emphasize that the neonicotinoid class (Provado, Actara or Assail) have shown low performance against this pest in our trials and in trials in other states, so these should not be used for SWD control.

Growers should use this information plus the pre-harvest interval (PHI) restrictions, and any potential MRL considerations, when making decisions regarding which insecticide to use to protect fruit against SWD. For example, Delegate and Imidan have three-day PHIs in blueberry, whereas Malathion has a one-day PHI.

This article has focused on blueberries because that is where much of our research is focused. There have also been detections of SWD in raspberry and blackberry fields, so the points made above are also relevant to those crops. However, pesticide registrations and restrictions may be different, so please check MSU Extension bulletin E-154, 2011 Fruit Management Guide, to make sure that control options are registered. There is also a bramble-specific SWD management guide, Spotted Wing Drosophila Management Recommendations for Michigan Raspberry and Blackberry Growers, available online at MSU IPM Program’s spotted wing Drosophila website.

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