Raisin Moth Is Not Easy to Control
(2003 Unpublished)
by Devin Carroll
In recent years, grape growers have become more concerned about raison moth. This pest is difficult to monitor and hard to control, but it may not be as harmful as many fear.
Growers routinely control omnivorous leafroller (OLR) and grape leaf skeletonizer with a treatment of cryolite, Confirm, or B.t. in May, at or near bloom. A second treatment may be applied in July. Unfortunately, both these sprays miss raisin moth. The overwintering larvae remain under the bark in their loose cocoons until June, protected from the bloom sprays. By July, most of the larvae are tunneling inside grape berries, exposed only by a pinhole entrance, again protected from sprays.
Field scouts can get a rough idea of the raisin moth population by noting the number of cocoons under the bark in spring, while also checking for mealybug eggs and skeletonizer cocoons.
The only effective spray window might be in late June. We have observed that when the first larvae hatch, many feed first on the grape pedicels, where they eat into the “warts” on the expanded portion next to the berry. At this time, they should be susceptible to stomach poisons such as cryolite. Indeed, some tests have suggested that a late June application of cryolite is helpful. Confirm, which poisons the young larvae when they leave the eggs, should also work if coverage is good. As it happens, this late June timing also is good for second generation OLR hatch.
Researchers are working on a pheromone disruption system for raisin moth. In theory, that should work better than sprays.
It is not clear to me that raisin moth often needs control measures. It may be a secondary pest induced by other problems. In my experience, the larvae are common only in bunches already starting to rot, or bunches infested by mealybugs. They seem to be attracted by mealybug honeydew. We have even observed them hanging around mealybugs under the bark, apparently feeding on the honeydew. Bunches heavy with mealybugs often have many raisin moth strikes, whereas clean bunches only occasionally have any. Raisin moth strikes also seem more common where a berry is touching bark.
I am convinced that raisin moth strikes are less likely to cause rot than OLR strikes. OLR larvae feed on the surface, where they create a large wound exposed to rot organisms. Most raisin moth strikes have only a pinhole entrance, where fungus spores have less chance of getting in to cause infection. I have not observed any rot developing around these pinholes. A few times, I have seen a raisin moth larvae feeding between two berries, causing surface wounds similar to OLR feeding. Sometimes rot develops around this kind of raisin moth strike.
Although the raisin moth pinholes strikes rarely cause rot, they do show an ugly black spot where the worm has excavated under the surface around the entrance. This means extra clipping during table grape harvest, but it is a minor problem.
The best way to reduce raisin moth may be sanitation. The overwintering population comes from rotten and rejected bunches left hanging on the vines, so dropping these during or after harvest will help. During pruning and thinning, shape the vine to keep bunches away from the bark. Use chemicals or biological control for mealybugs. Lastly, consider a double-duty spray for OLR and raisin moth in late June.