Be prepared for a late season generation of grape berry moth

Growers are advised to check vineyard borders for new grape berry moth infestations.

Although the 2011 growing season started slowly and growing degree day accumulations were sluggish early in the season, multiple periods of hot, summer weather have provided enough heat for a late-season burst of grape berry moth flight and egglaying in southwest Michigan. During recent scouting in southwest Michigan, we have seen an increase in moths in traps and freshly laid eggs are readily found on clusters. Both of these observations suggest we will have a late-season grape berry moth generation, and there is increased risk of fruit infestation in the pre-harvest period. Pest pressure will likely be highest in traditional hot spots such as vineyard borders, and growers and scouts are advised to check their vineyards to look and see whether they are getting new infestations developing at the vineyard edges.

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 the cooler temperatures that are forecast. The slow growth caused by cooler weather will make recently hatched larvae 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. Additionally, vineyards that have received Intrepid to protect against grape berry moth for the final 30 days before harvest should not need an additional application.

Why are we seeing this late season berry moth activity? With the warm 2011 season, we have accumulated sufficient degree days for a fourth generation of this pest, exceeding the 2,430 growing degree days since wild grape bloom that is required to start this late season generation. In a typical season, we generally do not see this fourth generation because as the days get shorter in August, grape berry moth enters a resting state or “diapause” so that larvae develop into pupae and then stop developing in order to make it through the winter.

With this season’s periods of hot weather, grape berry moth apparently detected this as a signal from the environment that it might be worth “trying for” another generation, and heat may counterbalance the usual effect of the shorter days. This can result 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.

If a decision to apply further crop protectants is made, then pre-harvest intervals need to be closely watched, plus sprayer setups should be optimized for good coverage of clusters.

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