Fourth generation grape berry moth is coming to southwest Michigan

The long, hot 2012 season provided sufficient heat accumulation for fourth generation grape berry moth before harvest in southwest Michigan. Grape growers across Michigan should be aware of this potential as they make management decisions before harvest.

The MSU Enviro-weather grape berry moth model predicts that egglaying by the fourth generation of grape berry moth will start at 2,430 growing degree days after wild grape bloom, and this point in the season will be reached this weekend (August 25-26) or early next week in Berrien County and Van Buren County vineyards. For the Fennville, Mich., region, this point is still more than a week away. In the Traverse City, Mich., area, the model predicts that the third generation has only recently started, and so a fourth generation is unlikely before harvest.

This variation around the state emphasizes the importance of tracking pest development and responding to the situation that develops each season, and the grape berry moth degree day model can help with this. For vineyard managers in southern Michigan with a crop to harvest in 2012, late-season berry moth control is something to consider as harvest approaches, and management can be focused in areas of the farm where this expense is warranted.

Why is this happening?

Grape berry moth has flexibility in its life cycle that allows it to adapt to changing environmental conditions. The current hot year provides a good example of how it can make use of warmer conditions to fit an additional generation into a growing season. This is possible because in southwest Michigan, the third generation egglaying started in late July. When this happens, the eggs are laid when the days are still long, and this provides a signal that the larvae can develop through to a moth and continue another generation before it will get too cold. This is the fourth generation we predict will start over the next week in southwest Michigan.

If eggs of the third generation are laid in August, as happens in a typical year (whatever that is!), then the larvae develop only to a pupa stage and they prepare for winter. This flexibility allows berry moth to make the most of long, hot seasons, but it also can create some headaches for grape growers when the fourth generation is timed before grape harvest.

Will the fourth generation affect me?

How much effect this fourth generation of grape berry moth has on grape yield and quality will depend on a number of factors. If you are growing grapes in southwest Michigan and can answer no to any of these questions, then it is not something to be too concerned with this season.

  1. Is there a harvestable crop?
  2. Does the vineyard have berry moth infestation currently?
  3. Will the vineyard be harvested after mid-September?

Conversely, if you can answer yes to these questions and you do have a vineyard with sufficient grape yield to harvest with some history of berry moth infestation, and the harvest will be on the later side of the harvest schedule, it is important to scout the vineyard now and determine the level of damage from the third generation. Some vineyards we have scouted in recent weeks are already at levels where the border rows require protection from this fourth generation to minimize the yield loss and contamination risk during harvest. At a few of these sites we are also seeing infestation moving toward the center of the blocks, so it is worth the time to scout through vineyards and determine whether a border spray is sufficient or if the whole block requires control.

What can I do to protect clusters against the fourth generation?

If you decide to protect clusters from this last generation, there are various insecticide options, but it is very important to observe the pre-harvest intervals (PHI). For example, Intrepid, Hero, Brigadier, Bifenture and Capture have a 30-day PHI, Danitol has a 21-day PHI and Altacor, Imidan, Lannate and Voliam Flexi have a 14-day PHI. Check the PHI of any product before using it in this pre-harvest period. It may also be a good idea to check with your winemaker or processor regarding their policy on late-season sprays.

Both Altacor (14-day PHI) and Belt (seven-day PHI) are newer insecticide options with long-lasting efficacy against grape berry moth eggs and larvae, and these should be timed for application at the predicted start of the egglaying (i.e., this weekend or early next week in southwest Michigan). For these products, excellent cluster coverage is essential, but once it is on the clusters, long residual control of grape berry moth (two to three weeks) is achieved, so these would protect the clusters through until harvest.

For products that are broad-spectrum that are best timed for egg hatch, applications should be delayed to be timed 100 degree days after 2,340 (i.e., at 2,440 GDD from wild grape bloom). Insecticides with a seven- day PHI include Entrust, Delegate, Sevin and Avaunt, and there are various, inexpensive pyrethroid insecticides with shorter PHIs that can provide excellent control for a week, such as Baythroid (three days) and Mustang Max (one day). These have broad-spectrum activity on grape insect pests. Both Venom and Scorpion have a one-day PHI. Dipel and Javelin are B.t.–containing insecticides with high activity on grape berry moths and no side-effects on natural enemies, but they also have short residual control so they would require repeated application to protect berries from the fourth generation of grape berry moth.

For any late-season application targeting berry moth, cluster coverage is essential, so be sure to have the sprayer delivering excellent coverage of the fruit, and don’t waste spray out on the leaves.

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

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