Maple bladder gall: what extension services say

This page describes what the named extension publications say. It does not identify a gall on any particular tree, say whether a tree is healthy, or tell anyone to treat or leave it. Pesticide advice in a source is mentioned only in general terms, and the guide on who may apply pesticides covers licensing.

What it looks like

University of Missouri Extension (publication G7272, reviewed August 2014 and September 2017, Bruce Barrett) says that in spring small bladder-like growths often appear on the leaves of red and silver maples. They are about 1/8 inch across, light green at first, then red, and finally black by the end of summer. Cornell University's fact sheet on galls on plants (Klass and Johnson, July 1972) gives the same size, says most of the swellings are on the upper leaf surface, and says the new galls are yellowish-green, turning reddish toward the end of June and black late in the season. Nebraska Extension in Lancaster County (Sarah Browning, May 14, 2024) calls it wart-like and one of the most common galls on landscape trees, and says leaves can become so covered that they twist out of shape.

What causes it

All three sources name a mite. Missouri says an extremely small mite, 1/125 inch long, makes it: adults overwinter under the bark and in other protected places, and may rest in the buds. In early spring the adults move to the unfolding leaves and feed, and the leaf quickly grows a gall at the feeding site around the mite. The mite keeps feeding and lays numerous eggs inside. As they mature, the mites leave the gall and infest new foliage until about July, when their activity falls off, and in fall the adults move to winter sites. Nebraska says the mite overwinters on the bark, particularly of silver maple, and that the leaf forms the gall in response to feeding as it comes out of the bud. Nebraska describes the general mechanism for galls: the damage or saliva of the feeding insect or mite makes the plant raise its production of hormones, which causes abnormal growth in a small area.

Whether the sources call it harmful

Missouri says that in heavy infestations the leaves are disfigured and often change colour early and drop, but that the galls are rarely detrimental to the overall health of large trees. Nebraska says maple leaf galls seldom, if ever, cause permanent damage, although they are ugly, and that leaf galls in general detract from a tree's beauty rather than harming it. Cornell says most leaf galls are not harmful, at most causing some leaves to fall early. These are descriptions of what the sources say about the galls in general, not a judgment on any one tree.

Where the sources differ on management

Missouri lists spray options for the mite, and then says that controlling galls with chemicals is not recommended in most situations, that sprays are of little use once a gall has formed and that the best spray times are not well known for many gall makers. Nebraska says nothing removes galls after they have formed, that preventing them is very difficult, and that control is neither recommended nor necessary. Cornell says a few galls seldom warrant control.

Look-alikes the sources name

Missouri names maple gouty vein gall, with swollen veins on the underside of sugar and red maple leaves, caused by midges. Nebraska and Cornell name maple spindle gall on sugar and soft maples, thin erect structures on the upper leaf surface caused by mites. The wider group is on the tree galls page.