Mark the date of wild grape bloom to help predict spray timings for grape berry moth

Know the date of wild grape bloom to manage grape berry moth later in the season.

The date of wild grape bloom is used as the biofix for running the MSU Enviro-weather grape berry moth degree day model for timing sprays against the second and third generation of grape berry moth. This is typically about a week before the primary buds of Concord vines bloom, and well before secondary buds bloom. The biofix date is when 50 percent of the wild grape clusters have 50 percent of their flowers in bloom. Samples of wild grape from different vineyards are likely to show different biofix dates, allowing you to adjust the timing of sprays later in the season for the predicted phenology of this damaging grape pest, according to the different conditions at different vineyard sites. Using this model can help take some of the guesswork out of knowing when best to make applications to protect the crop from grape berry moth.

Wild grape vines have started blooming in some areas of Berrien and Van Buren counties already. The warm weather this week is expected to bring bloom along quickly around vineyard sites with later wild grape bloom in Allegan County, and then the rest of the state in the coming days and weeks.

Once the date of wild grape bloom is set, record this date in a notebook, on the calendar or anywhere else for later retrieval. The start of the second and third generations of this pest are then predicted to start at 810 and 1,620 degree days later, respectively, using the grape berry moth degree day model that can be found online at the Enviro-weather website. These typically occur in mid-July and mid-August in southwest Michigan, but with this year’s warm weather, these events are likely to be early.

The predicted dates of early egglaying would be the appropriate timing for insecticides that target egglaying and young larvae such as the insect growth regulator Intrepid or the new diamide insecticides Altacor or Belt. For broad-spectrum insecticides (Imidan, Sevin, Baythroid, etc.) with the best activity on young larvae, delaying the timing of applications to 100 degree days later is recommended.

A final point on this unusual spring: As secondary clusters develop in many southwest Michigan vineyards, there are sites that have primary and secondary clusters and others with only secondary clusters. Whatever has happened to the timing of the developing grapes, the timing of grape berry moth development this year will still be based on the degree days from wild grape bloom described above. This may result in predicted egglaying at times when cluster development is out of synchrony with the crop, but the insects are not expected to change their timing for these later clusters.

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

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