Bud Mites Cause Real Damage in Grapes
(2003 Unpublished)
by Devin Carroll
Part of my job is to dissect and examine dormant grape buds to estimate the number of primordia that will develop into bunches. During the process, I also look for grape bud mites and assess the amount of damage they cause. Sometimes the damage is quite serious. If a vineyard has unexplained low fruitfulness, bud mites should be investigated as a possible reason.
Bud mite is a strain of the grape erineum mite. The other strains, which either curl leaves or form hairy leaf galls, are easily controlled with sulfur. But the bud mite is better protected and not so easily controlled.
Bud mites spend most of the year feeding inside the bud scales. As long as the damage is limited to the scales, it is not a problem. But with high numbers, hundreds per bud, they may reach the center and feed on the fruit primordia. Damage ranges from slight scarring to complete destruction of the future bunch.
To estimate the damage to the crop, I rate bunches as either “damaged” or “destroyed”. Many of the “damaged” bunches will still produce fruit, but the size or quality of the bunch may be reduced. The grower will not notice the damage. Also, the mites will continue to feed after my inspection, typically between November and January, so the number of destroyed bunches may increase.
Some cultivars are more susceptible than others. Damage to Flames is often 10-20%, ranging to over 50% in extreme cases. I have seen Crimsons and Black Seedless in the 20-40% range, and Autumn Royals and Thompsons up to 10%. Red Globes appear to be almost immune. The number of destroyed bunches is usually less than 10%, but sometimes reaches close to 20%.
Bud mites continue to feed on warm days during the winter. Just before budbreak, the adults appear more robust, and I have seen eggs at that time. As the new shoot grows, and before new buds form, the mites stay protected beneath the flaps at the base of the leaf petioles.
Erineum mites are a favorite food of predatory mites common in vineyards, but it is not clear how much impact the predators have on bud mites squeezed inside the buds. Predators may partially explain why bud mite numbers vary widely from year to year in the same vineyard. Weather and pesticides probably also contribute to this variation.
Few if any formal studies have been made on how to control bud mites with chemicals, but growers have tried various treatments. I worked with a grower who applied Vendex or Agrimek for several years to Flame vineyards in mid-April, and numbers declined. However, when we tried side-by-side tests, we found no difference between treated and untreated rows. Another grower told me that MPEDE soap applied in a high volume of water after bloom was an effective treatment. I speculate that this treatment might be effective any time from early shoot growth to formation of new buds. Wettable sulfur in high volume might also work.
We accidentally found one apparently effective treatment one year when we applied Lorsban before budbreak to kill mealybugs. Some sections of the blocks were too muddy and received no treatment. When I dissected the buds the following winter, those sections consistently had about twice or more as many infested buds. Our experience in subsequent years generally confirms that Lorsban provides about 50% control or more. Unfortunately, Lorsban is an environmental hazard and is harmful to many beneficial insects, so I hope we can find other good treatments for bud mites.