Trees and Galls - Part 1

Kathleen Cue, Nebraska Extension Horticulture Educator

Galls are structures made up of plant tissue, forming in response to the saliva of mites or small insects as they feed. The number and variety of galls found on trees in our landscapes are closely associated with the weather and how conducive it is to gall-producing arthropod populations. Galls happen every year, it’s just some years the number may be higher because that insect population is higher. For the most part, gall formation on leaves is of little concern, while those affecting the twigs, branches and stems merit closer monitoring.

Maple Bladder Gall

Bright green nubs, just 1/8” across, form on the upper surface of silver and red maple trees. The galls turn from green to red, eventually darkening to black. In some years, the galls are so numerous on a leaf that the extra weight causes leaf drop. While the galls look weird, they are actually made up of leaf tissue, with one mite for each gall. The gall serves as the mite’s home and food source. Once tree owners notice galls, spraying a miticide is ineffective because of the protection the leaf tissue provides to the mite. Maple trees exhibit no stress from the presence of mites. In fact, galls photosynthesize, providing needed sugars for the tree.

Linden Spindle Gall

Of all the potential problems lindens can have, their tubular-shaped leaf gall is the least of them. Spindle galls are caused by the eriophyid mite. Like the maple bladder gall, the mite’s saliva initiates a response in plants where they increase the size and number of leaf cells, which then grow over and encapsulate the mite. Since no real harm comes to linden trees from the spindle gall mite, treatment is not necessary.

Willow Pine Cone Gall

The pine cone gall is more of a curiosity than a problem. Cone-like growths form on the ends of willow twigs. Formation stems from the activity of the gall gnat midge. Masquerading as a pine cone on a willow, the galls are green and scaly, developing fuzziness as they age. Treatment for the gall gnat midge is not necessary. Galls may be removed if tree owners find them offensive or they can be left on the tree to show off their quirkiness.