I had an email come in tonight that I imagine includes a question on many people’s minds. That is, as we near veraision (I saw my first Marquette changing color today), what do we need to worry about as far as spotted wing drosophila (SWD) is concerned? SWD (Drosophila suzukii) is an invasive species of fruit fly with the unique ability to lay legs in intact (i.e., undamaged) fruit, thanks to its serrated ovipositor. Mated female adults use that tool to lay eggs into developing fruit. The ovipositor puncture is a great site for rots to move in; the eggs in the fruit hatch into maggots that eat out the insides of fruit and lend to more rot. This isn’t a fun pest, read more about it at: http://fruit.cornell.edu/spottedwing/.
In general, cultural measures are the first and best line of defense against SWD. There are many practices that can reduce SWD populations and reduce susceptibility in grapes. One really nice thing is that the practices that make for high-quality (ripe) grapes also help to reduce SWD infestation.
The best thing is that grapes are not a ‘top’ preferred host for SWD- they’ll go for many other crops first. Generally, they come in after working over other more desirable crops, including wild choke cherry, elderberries, and similar soft fruit. Of course, raspberries and blueberries are the most preferred cultivated crops, and netting is becoming a more common practice used on those crops. Less so on grapes- the tight weave needed to keep out SWD would reduce the critical sunlight that is needed to fully ripen winegrapes, and as you get to larger acreages, the cost becomes prohibitive. Where a grower is not limited by organic certification, there are more tools available to manage SWD and other pests, but pesticides should not be the first defense against this pest. You’ll be playing a losing game and find yourself doubling down with increasing regularity
The scent of fermenting/rotting fruit will bring SWD into the vineyard more quickly, so do everything to maintain fruit quality and to prevent damage. That includes managing for disease, maintaining an open canopy, shoot thinning / positioning, cluster thinning, leaf removal, you know the drill. The higher quality (and the more defect-free) the crop, the less likely they will be to draw in SWD.
Sanitation should be a first line of defense against SWD. Any fruit that are decaying need to go, not on the vineyard floor, but buried into a compost pile. If you do feel a need to treat (spray) for them, I would recommend doing so as a secondary pest when spraying for grape berry moth (GBM). That is, treat for GBM (more on that shortly), but instead of using a lepidopteran-specific material like Intrepid or Bt, use something with activity against diptera as well. Delegate (spinetoram) is the preferred material, Entrust if you’re certified organic. Do Not get into the game of spraying either of these materials on a 7-10 day basis until harvest, it’s a losing game. Not only will you likely build up resistance in the SWD population to your pesticide of choice, but you’ll kill any potential predators that may fee on SWD in the vineyard. One, maybe two shots are plenty, and remember to time them against grape berry moth. Notice I’m not suggesting using pyrethroids (e.g., Danitol) or carbamates (e.g., Sevin) against these insects, as they are generally too harsh on predator insects.
GBM are starting to show up in Vermont vineyards- some sharp-eyed students of mine scouted our vineyard and found that we were over threshold and need to treat for that insect. We’re also running up against the bunch closure window after which it gets much more difficult to manage GBM (or SWD, for that matter) that are tucked up in the cluster happy. Protected, and fat on your ripening and rotting grapes.
So this is it- time to get out there and optimize the conditions for what looks like another fabulous vintage. Manage that canopy, expose those clusters, treat these couple if insects, and get your netting ready. Harvest will soon be upon us.
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