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There has been a wide range in the timing of wild grape bloom in southwest Michigan this spring, even within the same farm and at farms across local regions. The range of these bloom dates is much more than usual. This reflects similar variation in development seen across fruit crops this spring. While the exact causes aren’t clear, the variable wild grape bloom timing has implications for how we plan for managing grape berry moth this summer, especially in vineyards in Berrien and Van Buren counties.

Because wild grape bloom is used as the biofix for timing sprays for second and third generation grape berry moth, this situation raises questions on how to plan for grape berry moth control treatments this summer. With wild grape bloom timing being so variable, there are two options: use an average of the 2026 bloom dates in your area, or base spray timings on what has worked previously.

Average bloom date

With this approach, growers can set the bloom date biofix based on observed dates in their region. Although some very early sites near Benton Harbor and Lawton, Michigan, showed bloom in early–mid-May after a burst of hot weather, at other sites wild grape bloom was delayed until the last week of May, which is more in line with the typical timing for this region.

Using a May 25 biofix in the grape berry moth model on Enviroweather for the Benton Harbor (SWMREC) station, the model returns a calculation of 782 growing degree days (GDD) for today, Tuesday, June 30 accumulated since the biofix using the base temperature of 47 degrees Fahrenheit. This is only 30 degree days away from the target of 810 degree days that predicts the start of second-generation egg laying by grape berry moth.

With such a hot week, the degree days are accumulating quickly, so the prediction is for 810 GDD to be reached tomorrow, July 1, at the Southwest Michigan Research and Extension Center (SWMREC). If growers in Berrien and Van Buren counties are targeting the start of second-generation egg laying with an insect growth regulator such as Intrepid 2F,  it should be applied before the July 4 weekend. If using a broad-spectrum insecticide or a B.t. like Javelin or Dipel that targets the larvae when they hatch from the eggs (100 degree days later), it’s probably best for that treatment to be timed for immediately after the July 4 weekend, as 910 GDD (start of egg hatch) is predicted at SWMREC for July 4.

The 810 and 910 GDD since wild grape bloom are both markers for the START of the egg laying and egg hatch periods, but there will be a period of increasing activity for the second generation through July. It is important through this phase of the season to maintain control of grape berry moth using follow-up applications depending on the longevity of the materials being applied.

For regions north of the southwest grape production region discussed above, timings will be delayed based on slower degree day accumulations. Adjust for local conditions accordingly.

Crop and pest phenology

If the wild grape biofix is variable, then other clues to the timing of crop protection can come from the insects developing on the crop back in early June, and from the crop development stage. In recent weeks, grape berry moth larvae has been feeding on the young developing grape clusters at sites with a history of grape berry moth infestation. This is the typical timing for first generation larvae, and the sites observed last week with an effective bloom-time insecticide spray had already stopped those larvae from developing further. This suggests that second generation egg laying and larval development is running on a normal timing in relation to crop development.

As the primary clusters develop and get toward berry touch, growers are often targeting a fungicide application to get into the tight spaces of the cluster before they close. In sites with grape berry moth infestation, this is also a critical timing for adding an insecticide to protect the inner sections of the clusters from second generation grape berry moth. Ensure you have enough water volume and use a spreader sticker adjuvant to increase the coverage on clusters for this critical spray.

Product selection

There is a wide range of insecticides labeled for grape berry moth control in vineyards. They are listed in the E-154 “Michigan Fruit Pest Management Guide” from Michigan State University Extension.

As previously mentioned, at this timing in the season, applying a long residual insecticide such as the growth regulator Intrepid 2F can help carry protection through into July. These types of products should be timed for early egg laying at 810 GDD after biofix. Their application needs to be with good cluster coverage, using plenty of water and a spreader-sticker. If targeting the larvae to prevent their entry into berries, time those broad-spectrum products and B.t. sprays for 910 GDD after biofix, which is usually about a week later (see above for timing guidance).

Since egg laying continues through July, one spray will not protect through the generation. A follow-up application is needed 10–14 days later to protect against the continued activity of second generation grape berry moth. Using products with some rainfastness and resistance to breakdown by UV is a good choice for late June or early July treatments.

Short crop sites

The spring frost damage experienced in some locations this spring means growers must decide on whether there is a sufficient crop to plan for a harvest, protect a small crop, or save sprays where there are no clusters. In years with light crops, there can be a concentrating effect of the similar insect population on a lower number of clusters, so continued scouting and adjustment through the rest of the season is worthwhile for vineyards where growers still plan to harvest even if the crop is light.

For vineyards without clusters, maintaining the canopy is the focus, and that can be done with a much lighter spray program. Scout for leafhoppers and Japanese beetles to make sure the leaves remain healthy. These can be controlled with systemic insecticides such as Assail or Admire Pro that will have long residual control and be protected from washoff.

For a discussion of the decisions related to insect and disease pest management in sites affected by frost damage, see this MSU Extension article from 2024 with a longer write-up of the decision making on what to skip and what to keep focused on from now until September.

Our current work on vineyard pest management is supported by Project GREEEN, MSU Extension, AgBioResearch, and by the Michigan Department of Agriculture and Rural Development.

This work is supported by the Crop Protection and Pest Management Program (grant no 2024-70006-43569) from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.