I have had a couple people ask me about lots of small red growths on the leaves of silver maple this summer. If I have identified them correctly they are maple bladder galls which are caused by a mite. The galls may be unsightly but pose no significant threat to the tree.

Plant galls have been described as a plant tumor, but not a cancer. They can form in several plant parts, such as stems, leaves (including the leaf stalk or petiole), flowers, twigs, buds, and even roots. I have read that over 2,000 types of galls have been described with causes ranging from insects and mites to nematodes, fungi, and bacteria. As you would expect, they come in all sorts of shapes, sizes, and colors.

Most people may not be aware of it, but gall formation in plants is a defense mechanism. The mass of plant cells forming the gall are in response to a mite, insect, or pathogen in an attempt to isolate it from the rest of the plant. But there is a lot more to it than simple cell division.

Let’s use insect galls as an example. Gall formation is often initiated when an insect oviposits an egg or eggs inside the tissue of the host plant. The gall itself will be composed of plant tissue, but the insect apparently controls the form. The characteristics of the gall are unique to the insect species. The female injects some chemical or chemicals into the plant which stimulates gall formation and when the larva develops it may also continue to produce the substances. The chemistry of all this, however, is not well documented. In some cases sugars even accumulate in the gall in response to enzymes produced by the insect, assumedly to ensure energy for development.


No doubt many of you are familiar with the galls on Canada goldenrod. Two types of galls may be found in our region. One type of gall is spherical and is caused by a fly which has laid its eggs in the stem of the goldenrod. The other type of gall is spindle-shaped gall (football shaped if you prefer) and is caused by a moth. This coming fall and winter look for a small hole in the gall where the adult emerged.

Chuck Lura

Natural North Dakota is supported by NDSU Central Grasslands Research Extension Center and Minot State University-Bottineau, and by the members of Prairie Public. Thanks to Sunny 101.9 in Bottineau for their recording services.


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