As the brix levels in the grapes continue to rise, the attractiveness of the fruit to spotted wing drosophila (SWD) also increases. We are currently only catching a few SWD in our monitoring traps, but this is expected to drastically increase in the next few weeks. And since SWD is associated with sour rot and I have already received reports of sour rot issues, I wanted to send out some information on monitoring and management of SWD.
If you are unsure whether or not SWD is a problem at your vineyard, it is relatively easy to monitor for the flies. Traps can be made from plastic cups with lids.
- Approximately 1 inch from the top and 2/3 around the cup, cut or punch six to twelve evenly-spaced 3/16-inch-diameter holes.
- Fill the cup with 1-2 inches deep with bait.
- There are several bait recipes out there, but one of the easiest is using either straight apple cider vinegar or a mixture of red wine + apple cider vinegar (60:40, wine:vinegar), both with a drop of unscented soap to break the surface tension.
- Additionally, sticky card can be hung inside trap to catch flies
- Place traps on the north side of rows at fruit level.
- Females may be caught first, but are difficult to identify. Males have the characteristic spot on their wings (see image).
- For more information, check out: https://extension.psu.edu/plants/vegetable-fruit/fact-sheets/spotted-wing-drosophila/spotted-wing-drosophila-part-3-monitoring
Unfortunately capturing SWD flies in traps does not correlate with potential infestation of the fruit. In order to determine whether infestation has occurred and/or whether your management program is effective, you can monitor the fruit for SWD larvae.
- Collect intact, ripening or ripe grapes
- Place fruit in a flat, dark pan or zip-lock bag
- Add a salt solution (1⁄4 cup salt to 4 cups water)
- Wait ~15 minutes for larvae to exit fruit
- Larvae found in recently ripened fruit are most likely SWD (see ima
- If larvae are found, begin or continue management program.
- The full life cycle from egg to adult for SWD can be as quick as 9 days, so continue to monitor and manage until harvest.
- Note that flies will infest fallen fruit and discarded fruit, so waste disposal and sanitation are important
Sanitation of the vineyard can help reduce SWD populations. Remove unmarketable fruit and Harvesting grapes frequently and completely will prevent the buildup of ripe and rotting fruit, reducing potential breeding areas and sour rot inoculum for the flies.
While too late for this year, canopy management will make the vines less favorable for SWD. Prune and remove leaves to maintain an open canopy, which will increase sunlight and reduce humidity, decreasing the suitability of the vines for SWD. This will also expose the grapes, improving spray coverage, and hopefully increasing control of the flies.
If you have a known history of SWD in your vineyard or have issues with sour rot, insecticides are effective at killing the adult flies. Insecticide treatments should begin when the grapes are near 15 degrees brix. Treatments should be applied at least every seven to ten days. Rotating insecticides with different modes of action (IRAC codes) is very important. Below are a list of recommended insecticides. Make sure to read the label and check the pre-harvest intervals before applying any of these chemicals.
Management recommendations can be found at: https://www.smallfruits.org/SmallFruitsRegGuide/Guides/2016/BunchGrapeSprayGuide2016.pdf
Additionally, Cain, Phil, and I are currently running a trial looking at the combination of insecticide and Oxidate 2.0 along with leaf pulling to manage spotted wing and sour rot. We don’t have any data yet, but results out of Cornell showed that the combination of insecticide and Oxidate 2.0 significantly reduced the spotted wing and sour rot prevalence in treated plots.