Managing late season 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.      

With this hot summer, berry moth development is moving along quite rapidly. Recent observations of grape clusters have revealed increasing levels of infestation by grape berry moth larvae, and this reflects infestation by the second generation that layed eggs during mid-late July. The grape berry moth degree day model is also predicting the start of egglaying by the third generation (1,620 GDD) of this pest this week for many regions of southwest Michigan. This is almost two weeks earlier than during 2009, but with this warm season, the start of the third generation at this timing just in advance of veraison suggests that moth phenology is tracking vine development quite closely. Overall, berry moth pressure is higher than last year, and management of this pest during the next month before harvest is critical to ensure that fruit infestation by insects and the associated diseases are not a problem facing growers, processors and wineries. With many Niagara vineyards being harvested after Concords, plantings of this variety will also require more attention than is typical.

Scouting is critical at this time of the season to identify vineyards that require protection from grape berry moth through the rest of the season. Growers with vineyard blocks that have a history of infestation by this pest should take the time to walk through those blocks and assess the level of infestation before deciding whether a spray is required. In vineyards with very low infestation or where early-season frost damage removed most of the crop, the time and expense of a pesticide application is not warranted at this time of the season. Insecticide applications should be limited to vineyard borders where berry moth pressure is highest, to blocks where infestation is developing and a crop will be harvested. With regular vineyard scouting, growers can continue to monitor this pest and make decisions on whether pest populations warrant insecticide control as we go through August and into September.

Maintaining control of grape berry moth requires a combination of good timing, high insecticide activity and excellent cluster coverage. This update will cover each of these issues.

Timing

The late summer generation of grape berry moth typically starts laying eggs in the period just before veraison with increasing egglaying through August and into September. This late-season generation can lead to infestation of harvested clusters and can expose clusters to fruit rots, making it important to reduce injury from this generation. Using crop growth stages provides some adjustment for variation between the seasons, but we also now have the degree day model at www.enviroweather.msu.edu to help refine these timings and ensure optimal timings for control sprays. The model is predicting the start of third generation grape berry moth this week in southwest Michigan. With fewer degree days up north, this point in the insect’s development is expected to be reached by the middle to end of next week. This is the predicted start of the third generation egglaying, a timing that is appropriate for growth regulator insecticides such as Intrepid. Growers planning to use a broad-spectrum insecticide should wait for 100-200 growing degree days before applying insecticide to ensure that applications target eggs as they hatch, and so the residual doesn’t decline before egglaying peaks. Follow-up application may be needed to maintain control in areas with very high pressure.

In 2010, our degree day accumulations are so far ahead of normal that there is potential for some late-season activity of grape berry moth (a partial fourth generation). The risk of this is lessened by the insect population naturally being triggered by shorter day lengths to enter diapause, where the larvae develop to a pupa, but do not emerge again as adults. Instead, they prepare for the cold winter months and drop to the vineyard floor. During very warm years, some of these larvae may use the warm conditions to bypass the diapause and attempt a fourth generation. We will continue monitoring this pest through August and September to determine whether a fourth generation is possible.

Insecticide activity

When selecting an insecticide, there are many options for berry moth control. Some of these are selective for this pest, while others will also provide control of leafhoppers, Japanese beetles and other insects that can occur at the same time. For details of registered pesticide options, consult MSU Extension publication E-154.

The selective insecticide Intrepid has shown good effectiveness against berry moth in small plot and vineyard-scale trials, and we have tested it in the mid-season timings in July and August at the 12 oz rate and at 8 oz/acre. Although this is more expensive than many standard insecticides, the product lasts a long time (two to three weeks depending on the rate) and is resistant to wash-off. This helps make it an effective tool to use against the high pressure of egglaying by berry moth seen late in the season, when maintaining control would otherwise require multiple sprays. This works on the molting system of the moth larvae and therefore allows biological control to remain active. However, because it is selective, Intrepid will not control leafhoppers or beetles. It also has a 30-day PHI, so many growers have been using this in their programs a month or more before harvest to protect clusters while they get ready for the harvest activities. Use of Intrepid has also reduced the number of infested berries and the number of diseased berries in samples taken at harvest. Intrepid is quite stable under hot conditions and resistant to wash-off once sprayed providing good residual control.

There are many broad-spectrum insecticides available for berry moth control, including a number of pyrethroids that provide inexpensive control and that have broad insect activity. These provide effective control of moths, eggs, and larvae of grape berry moths. They have relatively short residual control in the hotter summer weather when growers might be spraying for the third generation of grape berry moths. In our trials with Danitol, Baythroid, and Capture, the lower rates of these products declined in activity against grape berry moth after nine days. If using a pyrethroid to control grape berry moth along with Japanese beetle in the hot sunny conditions of August, using the full rate will provide the best residual control, but no more than 10-14 days control should be expected. Despite the temptation to look only at the price per acre when making decisions, be sure to rotate this class with other chemical classes to avoid resistance developing. This means that growers should rotate out of this group of insecticides (Baythroid, Danitol, Capture, Mustang Max, or any generic pyrethroids) and use an alternative chemical class the next time an insecticide is used. Sevin or Imidan (buffered to pH 6) are both in different chemical classes. Be aware that Imidan now has a 14 day re-entry interval in grapes. There are also many effective reduced-risk insecticide options. These include Intrepid that was mentioned earlier, and also Altacor and Belt that have high activity on moth larvae. Altacor has also demonstrated activity on Japanese beetles in recent trials this summer, reducing feeding damage to leaves, and it provides control of grape berry moths with a 3-4 oz rate with a 14-day pre-harvest interval. Assail is a neonicotinoid insecticide that has moderate activity on grape berry moth and will also provide control of leafhoppers and Japanese beetles.

Coverage

Getting cluster coverage with your spray material is essential for berry moth control. This is important for getting full activity from broad-spectrum insecticides and even more important if applying any of the newer chemistries that must be eaten to be effective. As the canopy becomes denser after bloom, increase the water volume and slow down to ensure the pesticide has a chance to contact the pest. Juice grape canopies have many layers of leaves during the late summer, making it hard to penetrate to the clusters, but this is essential if the insecticide is to work against grape berry moth. If the spray doesn’t hit the cluster, a significant investment of time and money is being wasted. Spraying every row is another important component of ensuring that your clusters are well covered.

To illustrate this, our research in a mature Niagara vineyard found that an airblast sprayer operated at 20 gallons of water per acre gave only half the control of grape berry moths in August compared with one running at 50 GPA. We have also seen that vineyards treated using alternate row spraying at 20-30 gallons of water per acre have poor control of berry moth, likely due to the spray material not reaching both sides of the clusters.

One way to test your coverage is to spray water or SURROUND WP kaolin clay through the sprayer in a test run. Immediately after spraying (with water) or after the spray has dried (for the kaolin), lift the canopy of the sprayed and adjacent rows to see where the material hits the cluster. If there are untreated berries, these are sites where a berry moth larva could avoid the treatment and survive. These results emphasize the need to calibrate your sprayer and adjust through the season to ensure it is getting good cluster coverage, because it can make a big difference for control.

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