Maple leaf bladder galls

Have you noticed bumps appearing on your maple tree leaves and wondered what caused them? This spring several inquiries from homeowners wondering as to whether this may affect the health of their tree.

First appearing in the spring, these bumps are the result of tiny mites that are feasting on the tree foliage. Depending on the insect, galls appear as different irregular shapes and sizes on the upper surface of the leaf. Galls first appear green in color, but then change from to red to black as they mature. Some leaves may become so covered with galls that they completely twist out of shape and may even drop early.

Silver maple trees are common hosts for the bladder gall mites, which cause a warty type growth. According to Dr. Donald Lewis, of ISU Extension Entomology, these galls are caused by extremely small mites only about 1/125 inch long. The mites spend winters under the bark and other protective places on the tree. In early spring the adult mites emerge to the developing, unfolding leaves and begin feeding.

Hormones in new leaves respond to mite feeding by rapidly producing extra cells to enclose the insects by forming an abnormal growth at the feeding site. This extra cell growth covers the insect to form a gall which covers the mite as it continues to feed and lay numerous eggs within the gall site. Reproduction is prolific and as new mites mature, they leave the gall and move on to other newly emerging leaves to repeat the process. Only new leaves are capable of producing galls. Mite activity will continue until mid-summer. Adult mites leave the foliage in the fall and move to overwintering sites in the tree.

Other leaf galls include maple spindle gall, which appears as a spindle shape, and is commonly found on sugar maple trees. Hackberry trees are also greatly affected by mite galls and form what is called hackberry nipple gall on the underside of leaves.

As gall infected leaves may drop prematurely, the cleaning up of dropped leaves and pruning out affected branches and twigs may help to reduce mite populations. Disposal of infested plant tissue in plastic bags may to help minimize spread of adult mites.

Did you know? Although detracting from a tree’s beauty, leaf galls seldom cause permanent damage to trees. Leaf galls cannot be “cured” once they have formed and will not be eliminated by sprays or systemic insecticides. Therefore, chemical controls are not recommended as they are not effective, as leaf galls are considered to be only an aesthetic problem.

Questions? Contact McCormick at yvonne@iastate.edu for information or advice.