By Laura Jesse
Iowa State University Extension
Do your maple leaves look like they might need a good shave? Then you may be lucky enough to have a maple with maple spindle galls. I have seen these so thick on leaves this year that it looks like a beard. Galls are one of the more beautiful and fascinating things that can happen to tree leaves, although this is the opinion of an entomologist. Galls that are formed by insects or mites are basically outgrowths of the plant's tissue. The insects or mites stimulate the plant to form these strange formations around them. This provides food and protections for the insect or mite inside the gall.
Two common galls on maple leaves are the maple bladder gall and the maple spindle gall. These galls are caused by two different species of mites. Galls are present every year, though the abundance varies greatly from year to year and from tree to tree. Maple bladder gall is a common and well-known leaf gall found on the upper surface of the foliage on silver and red maple trees. Maple spindle gall is often found on sugar maples.
Maple bladder galls are a “pouch”gall. They typically appear as a rounded or elongate pouch on a slender, short stem or neck (though highly variable). Total height of the gall is approximately 1/8th inch. At first, the galls are light green in color. They quickly turn bright red and finally black by the end of the summer.
Maple spindle galls are yellowish-green and have a spindle-like shape. The galls usually stand out from the leaf about 1/5 of an inch.
Maple bladder galls and maple spindle galls are caused by extremely small mites only 1/125 inch long. The adult mites spend the winter under the bark and other protective places on the trees. In the early spring the adults move to the developing, unfolding leaves and begin feeding. The leaf responds to the small irritation by rapidly producing extra cells that form the abnormal growth at the feeding site. The gall encloses the mite which continues to feed and lay numerous eggs within the gall.
Reproduction is prolific and as the new mites mature, they leave the gall and move to other newly emerging leaves to repeat the process. Only new leaves are capable of producing galls. Mite activity continues until mid-summer when it starts to decline. Adult mites leave the foliage in the fall and move to the overwintering sites.
Heavy infestations of galls cause leaves to be disfigured. At the worst, leaves become curled or rolled up, and may change color and drop prematurely. These effects are not detrimental to the overall health of healthy, well-established trees. The galls are unsightly and may appear to be severe, but the effect on the tree is not significant. Maple bladder galls are not the cause of the sparse foliage and other symptoms observed on many maple trees (especially silver maples) around Iowa.
Galls cannot be “cured” after they have formed. Neither sprays nor systemic insecticides will eliminate the galls nor improve the condition of the trees. Preventive treatments applied at the time of bud break in early spring and again at regular intervals throughout the first half of the summer could prevent galls, but these are not practical. As mentioned, the galls are not significant to the trees and massive pesticide use with little returned benefit is not justified.