With veraison upon us, it is time to start planning your spotted wing drosophila (SWD) management to help suppress sour rot in your vulnerable grapes. The SWD flies are active nearly year round in these parts, but near veraison as the brix levels in wine grapes reach approximately 15 degrees Brix, the grapes become attractive (and thus susceptible) to SWD. While infestation of wine grapes by SWD is not directly problematic, SWD and other drosophila flies are can spread and exacerbate sour rot. As such, I wanted to send out some information on monitoring and management of SWD and sour rot.
Sanitation of the vineyard can help reduce SWD and other drosophila fly populations. Harvesting grapes frequently and removal of fallen/unmarketable fruit will prevent the buildup of ripe and rotting fruit, reducing sour rot inoculum and potential breeding areas for the flies.
While too late for this year, canopy management will make the vines less favorable for SWD. Prune and remove leaves after bloom 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.
Note that the combination of an insecticide and Oxidate 2.0 significantly reduced SWD and sour rot prevalence in treated plots.
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 above)
- 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 SWD larvae in a grape.
- 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
For more information about sour rot, please see UGA’s plubication here: https://secure.caes.uga.edu/extension/publications/files/pdf/C%201212_3.PDF
And as always, management recommendations can be found at: https://smallfruits.org/files/2019/06/BunchGrapeSprayGuide.pdf