Monitoring and managing grape berry moth in July and August
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.
The third week of July is a critical time to scout vineyards for the effectiveness of post-bloom insecticides and to decide whether another insecticide application is needed. For vineyards where grape berry moth is usually a problem late in the season, and particularly where the crop was heavily infested with berry moth last year, this middle-season generation will require management to keep the population under control.
What stage is the population at?
At this time of the year, the best way to determine the stage your own grape berry moth population is to look directly on the clusters and try to find a sample of larvae. Mature larvae are dark purple, and 1 cm long and will soon pupate into an adult moth. From mature larva to an adult laying eggs takes approximately 8 to 10 days. Within a few days of eggs being laid, the young larvae will hatch from the eggs and bore into the berries.
Why scout vines?
By scouting now, an informed decision can be made as to whether to spend time and money on berry moth control, or wait until later in the year. At the same time, looking closely at the vines can detect the level of leafhopper and beetle infestation. If grape berry moth infestation is detected near harvest, it is often too late to get it under control because of pre-harvest interval restrictions. Because of this, proactive scouting and management now are key to protecting the fruit. Berry moth can often be a problem only at the vineyard edges, and scouting also allows growers to decide whether a border spray would be sufficient to control the infestation. This strategy can help reduce costs while preventing development of berry moth populations. Take a close look at grape clusters on the vineyard borders and also in the vineyard interior to determine how much cluster infestation is present. Sampling 25 clusters at two positions at the border and two in the inside is recommended, for a total of 100 clusters.
What to spray?
If damage is above the threshold of 6% infested clusters in mid- to late June (1 to 2 clusters in each 25 cluster sample), the decision to treat with an effective insecticide should be made. About one week of activity can be provided by broad-spectrum insecticides Imidan (use pH 6.0 spray water) and Sevin. A little longer activity can be provided by the pyrethroids Danitol, Capture and Baythroid, but at the current high temperatures and UV conditions, these pyrethroids are likely to break down more quickly. SpinTor is a new class of insecticide that has some contact activity and works best when eaten by the insect. It has about a week of activity but is susceptible to wash-off. The previously-listed chemicals can also provide varying levels of control of leafhoppers and Japanese beetles. One highly selective insecticide that has shown good performance against grape berry moth is Intrepid. This acts by disrupting molting of the larvae, and so it has to be applied in enough water to get excellent coverage of the grape clusters. Once on the clusters, it has 14 days of activity and is highly resistant to being washed off. A good time to consider this insecticide is just before veraison when there is still time until harvest (it has a 30 day PHI) and when we typically see a long period of berry moth egglaying. A spreader-sticker can help get cluster coverage with this product. B.t.(Dipel, Deliver, Javelin) is highly specific to berry moth larvae, and active for only three to four days.
Coverage, coverage, coverage
Achieving control of grape berry moth with any product requires good cluster coverage, so it’s a good idea to focus sprays on the fruiting zone if targeting cluster pests only. The insecticide must get past the leaves to the clusters, and cover them, to be effective. Use of more water, driving more slowly, and spraying every row will all help improve efficacy. Pesticide longevity is also very important because recent research has shown that egglaying by second generation grape berry moth is very spread out. Choose an effective insecticide with enough residual activity that eggs and young larvae are controlled as they develop on the cluster. Finally, remember resistance management: Change the class of insecticide from whichever was used earlier in the season. This will help prevent grape berry moth becoming tolerant to the currently effective tools that are available.
Taking the time to scout vineyards during the next month to determine infestation and see how well the spray program is working will reduce the chance of late season-surprises. Because vineyards can vary greatly in their level of infestation, this scouting should be done in as many different vineyards as possible. Most growers know the hot-spots on their farm, and can focus this sampling there to determine the need for sprays against second and third generation grape berry moth.