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Control of Gall-Making Mites

Source: NDSU Extension Service Horticulturist

Homeowners who have noticed small unsightly growths on the leaves of their shade trees and certain shrubs can do something about this problem.

These small growths on the leaves of maple, elm, ash, boxelder and plum trees are galls caused by the feeding of very small mites. These mites become active early in the spring just as leaf buds are beginning to open. At this time mites begin feeding on the newly developing leaves.

The evidence of mite feeding damage does not appear until late spring and on into the summer. As warm weather continues, the gall formations become more noticeable. Many times affected leaves will be heavily covered with unsightly growths or galls which are often mistaken for a disease condition. The growths assume different shapes, sizes and colors depending upon the type of tree affected. One of the most common gall problems in North Dakota occurs on maple. Maple bladder galls are at first green, later changing to a bright red and in mid-to-late summer the galls turn black. With ash flower gall, the flowers develop abnormally and form very irregular galls up to one-half inch in diameter. These galls dry, forming clusters which are conspicuous on the trees during winter.

You can control the mites which cause gall problems on various trees most effectively in the spring. If tree buds haven't opened yet, a dormant oil can be applied according to label directions. However, do not apply dormant oils after buds break and foliage begins to appear as injury may occur. If it is too late to apply a dormant spray, apply malathion or Kelthane according to label directions as buds are beginning to open and follow with a repeat spray in 7 to 10 days.

For future reference, you may want to obtain a copy of Extension bulletin E-296, "Insect Pests of Trees and Shrubs," which is available at your county office of the NDSU Extension Service.

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