Using the grape berry moth degree day model
Editor’s note: This article is from the archives of the MSU Crop Advisory Team
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MSU has developed and released a degree day model to predict the start of generation two and three of grape berry moth. To use this, the date of wild grape bloom should be recorded near vineyards where grape berry moth control is needed later in the season.
Grape berry moth typically has three generations per season in Michigan, and predicting when these occur can help growers target management at the right time to reduce infestation by this pest. The first generation is usually at a low level, but many growers target this generation with a 10-day post bloom insecticide. In vineyards with this pest, berry moth populations build through each generation and can reach high abundance immediately before harvest causing yield loss, disease, and the risk of crop contamination. Prevention of damage by generation two and three is the most economically important for growers, and accurate timing of controls for these generations is an essential aspect of effective management of grape berry moth.
The MSU degree day model for grape berry moth has been developed to predict the start of egg laying in the second and third generations in southwest Michigan vineyards. It uses growing degree days (GDD) accumulated after wild grape bloom, so it is important to record the date of wild grape bloom near vineyards to run this model. This insect takes 810 GDD (base 47°F) to complete a generation, and we have found that egg laying starts to increase at around 810 and 1620 GDD after wild grape bloom for the second and third generations, respectively.
If vineyard pest history and cluster scouting indicate that protection from berry moth is needed, this model can be used to predict when egglaying by the second and third generation are starting. For insecticides that work best when applied just before egg-hatch such as insect growth regulators, application at 810 and 1620 GDD are expected to provide good control of this pest. For example, MSU research trials in high pressure vineyards during 2008 found excellent control of berry moth using Intrepid (8 oz/acre) applied at these GDD timings. If using this insecticide or any other product that requires excellent coverage of the fruit to achieve control, applications should be made with increased water volume as the season progresses. For broad spectrum insecticides that target larvae and with shorter residual control application of these products should be delayed to after the start of egg laying. Based on models for other similar pests, this would likely be about 200 GDD after the predicted start of egg laying.
Running the degree day model
Step 1. Record when wild grape blooms near your
vineyard, typically in late May or early June. The date to record is
when approximately 50 percent of the flowers are open on approximately
50 percent of the wild grape clusters. This date will be needed later
for running the model.
Step 2. Go to www.enviroweather.msu.edu and select the nearest weather station to your farm. Select “Fruit Pages” and then select “Grape Berry Moth model” in the “Insects” section. Once wild grape bloom is approaching, a new page will appear with a table that has dates and daily degree day totals on the left, and wild grape bloom date across the top.
Step 3. Look across the top of the table for the date(s) when wild grape bloomed on your farm. Look down the table for the row where the table cell turns red, indicating 810 (and later 1620) degree days after wild grape bloom (base 47°F). These red shaded boxes indicate the timing of the start of egg laying by the second and third generations of grape berry moth.
In this example, if wild grape bloom was recorded on June 10, more than 810 degree days have passed as of July 20. The red cell indicates that egg laying of the second generation has begun. If wild grape bloom was recorded on June 16, however, less than 700 degree days have passed since wild grape bloom, and egg laying of the second generation has likely not yet started.
Step 4. Make management decisions. The model provides information on timing for the start of mid- and late-season berry moth generations, but not on the need for treatment. Based on pest scouting and vineyard history, make decisions about the need for an insecticide application.