A late-season flight of grape berry moth

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Monitoring traps for grape berry moth checked in the past few weeks across southwest Michigan have indicated an upswing in activity from grape berry moth at high pressure sites, with associated egg laying on berries. This pest pressure seems mainly in traditional hot spots, but growers are advised to check their vineyards (especially on wooded borders) to look and see whether they are getting new infestations developing at the vineyard edges. With the cooler nights and windy days this week, the suitability of the weather for berry moth mating and reproduction is not ideal. But, this pest has apparently been able to provide some late-season pest pressure by trying to fit in another generation.

If vineyards are being harvested this week or next they are unlikely to benefit from attempts to control berry moth, because larvae are either already inside berries, or the eggs laid in the next week will grow slowly under these cool temperatures, making them less likely to be detected. For those hot spots where additional activity is being seen in vineyards that are being harvested later in September or early October, growers will need to decide whether additional expense is worthwhile at this point in the season. This decision will obviously need to take into account the level of infestation, expenses to date in the vineyard, and the level of crop present.

Why are we seeing this late season berry moth activity? With the very warm 2010 season, we have accumulated sufficient degree days for a fourth generation of this pest, exceeding the 2,430 growing degree days from wild grape bloom that is required to start another generation. This is much more than usual, and the insects are responding to this heat. For comparison with last season, we had accumulated 2,660 grape berry moth degree days in Berrien Springs yesterday, September 8, whereas only about 2,100 had been accumulated at this time last year. In a typical season, as the days get shorter in August grape berry moth enters a resting state or “diapauses” so that larvae develop to pupae and then stop at the pupal stage to make it through the winter. With this season’s hot summer, they apparently could detect the signal from the environment that it might be worth trying another generation, and so the heat counterbalanced the usual effect of the shorter days. This resulted in a significant portion of the larvae developing through to adult moths that are now flying, mating and looking for egg-laying sites on clusters. As a result, we are now seeing some higher late-season activity from berry moth.  

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

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