Is Spring finally here?! This has been a Wisconsin Spring, rainy, cloudy, cool weather with some (few so far!) days of sunshine and colder than the last few years but closer in degree day accumulations to spring 2018. Nonetheless, grape buds are starting to swell and now is the time to scout for flea beetle and cutworms that may hollow out grape buds. The injury observed from both species look very similar: they both burrow into developing buds and feed on the young tissues inside, hollowing out buds from bud swell until bud break. Each bud fed upon can destroy 1 to 2 clusters of grapes. However, it is important to determine which is causing damage, as control measures differ depending on which pest is present in your vineyard.
Cutworm larvae climb up the grape stem each evening and feed on the buds at night, then return by morning to hide in the leaf litter or in the soil under the vine during the day. These caterpillars will feed not only on grape buds, but also on weeds in the vineyard. Because they return to the ground during the day, and feed on alternate hosts, cutworm larvae are most prevalent in weedy vineyards on sandy soil. To determine if cutworms are the culprit causing your bud damage, go into the vineyard at night with a flashlight, into an area with a lot of damage, and look for the thick brown caterpillars feeding on the buds (picture in table below).
Flea Beetle adults feed on swelling grape buds during the day in the spring, and then the larvae and summer-generation adults feed on grape leaves during the summer. However, the spring generation feeding on the buds is the only that can cause economic damage. These adults can sometimes be found sunning themselves on the vines and buds, and are easy to spot since they are an iridescent-black/blue.
Cutworm cultural control measures include removing weeds from the vineyard. This can be somewhat effective, since it removes alternate hosts and hiding places for cutworms. However, if 2-4% or more buds have damage, and you are able to go out at night with a flashlight to determine that cutworms are the culprit, we recommend you consider an insecticide application. Cutworm damage is often spotty and highly localized because female moths lay eggs in clutches, so it is recommended to spot treat areas of the vineyard showing higher damage levels, instead of spraying the entire vineyard.
Flea Beetle cultural control can include the removal of brushy overwintering sites on the edges of the vineyards. Additionally, discing the aisles in June or July, during pupation, can damage or desiccate the delicate pupae. However, early season damage to 2-5% of buds warrants an insecticide application, especially during a cool spring such as this one.
At this time, it is important to remember that cooler temperatures in the spring tend to prolong the bud swell stage which provides a longer period of feeding activity for both flea beetle and cutworms.
For more detailed information about these insects, please refer to the Bud Swell Webinar recording.
Below is a summary table of the main differences between these groups of insects.
For more chemical control recommendations, please refer to the Midwest Spray Guide. Please note some overlap in insecticide options between insect groups. The label is the law, please follow the label!This article was posted in Insects and tagged Christelle Guédot, cutworm, Grape Flea Beetle, Grapes, insects.