Tree Facts: Insect and mite galls of trees

Farm Forum

What are those swellings or galls on trees and what causes them? Galls are abnormal growths on plants caused by a wound, infection, or feeding by insects and mites. The most common types are leaf, stem and flower galls. Galls are unusual in shape and people frequently become very concerned about them, but are not a serious plant health issue.

Most galls start in late spring and early summer when adult insects become active and lay eggs. In general, gall makers have only one generation per year. The gall-making insect or mite develops within the plant gall. Gall makers feed within the gall as it grows and then exit the gall after it is fully developed.

Many insects and mites make plant galls on the growing green parts of trees. There are over 700 types of galls that occur on oaks. Gall mites cause a wide variety of galls, such as small finger-like galls, pocket galls, felty masses, curled up leaves and deformed flowers or buds on ash and cottonwoods trees. Psyllids or plant lice are notorious for producing the common nipple gall on undersides of leaves of hackberry trees. Other psyllids form blister galls on leaves and developing buds of hackberrys. Aphids cause various stem and petiole galls and distorted leaves, on cottonwood, aspen and ash trees. Other aphids cause cankers on branches and trunks of apple and crabapple trees.

Some insects and aphids form galls on the woody parts of trees. Wooly aphids primarily affect evergreens causing cone-like galls on spruce. Gall wasps are the single largest group of gall-making insects. Gall wasps produce a wide range of galls from woody, rounded galls on stems or leaves to woolly or mossy galls. Essentially all insect galls found on oak or roses are produced by gall wasps. Gall flies cause spherical gall swellings to form on shoots of aspen and poplars. The galls caused by twig gall flies continue to grow for years, producing large knot-like growths.

Although galls are conspicuous and ugly, they really do not hurt trees. Gall-making goes in cycles and often subsides over time. Once you notice galls there is really nothing you can do to get rid of them. Rarely heavy infestations occur over several years that slow the growth of trees.

Mites and few gall-making insects that overwinter on the plant may be controlled with dormant oils. However, most galls are produced by insects that move to the trees as new growth develops in the spring. They can be controlled only with sprays that cover the leaves during the egg-laying period Repeat applications often are needed. There are no effective controls for some gall-making insects.

If insecticides or miticides are applied, the following treatments are recommended for some gall-making insects and mites: spruce gall aphid – Sevin, permethrin, imidacloprid & horticultural oil, mites – Sevin & Kelthane, Gall wasps – Sevin, Hackberry nipplegall maker – Orthene & imidacloprid, petiole gall aphid – Horticultural oil (dormant application) and Poplar twig gall fly – None recommended.

Remember galls are caused by the feeding and egg-laying of insects and mites. Even though galls are very noticeable and ugly they rarely cause any harm to the tree. Once galls start, you cannot get rid of them. Under most circumstances, control of the gall making insects, aphids and mites is not recommended.

My source for this news release was the Colorado State University Cooperative Extension Service. If you would like more information about “Insect and Mite Galls of Trees,” contact Bob Drown @605-244-5222 Extension 109 or by e-mail at