It’s that time of year again. The trees are fully leafed out, and I am inundated with calls and emails about trees with weird gnarly leaves covered with all sorts of misshapen lumps and bumps in a variety of colors and textures. The questions range from the curious to the panicky: What is this alien growth? Is it killing my tree? What should I do about it?
Most of the abnormal growths I am seeing are galls created by egg laying or feeding insects and mites. Galls generally form in early spring on new leaves and emerging shoots, and often go unnoticed by tree owners until the tree is covered with galls. By this time, the insects themselves may have left or perished, and all that is left is the gall structure.
The great news is that although galls cause leaves to curl and drop, very few of them have an adverse effect on the health of your tree. The damage they cause is merely cosmetic. Although there are some chemical options available to control galls, by the time you realize there is a problem, it is too late for treatment. In most cases, treatment is unnecessary and galls should be tolerated.
Maple bladder galls, those little red lumps on the top of silver maple leaves, are caused by eriophyd mites that overwinter in the tree and get an early start in the spring.
Spindle, finger or needle galls are commonly found on plum, cherry, linden and maple trees. These galls are also caused by eriophyd mites but appear as slender projections on the leaf surface.
Erineum galls, sometimes called velvet galls, form a red velvety carpet on the underside of leaves, and are very common on compact cranberry viburnum shrubs and maple trees.
Most ash trees are dioecious – that is, the tree has either all male flowers (staminate) or all female (pistillate) flowers. Early in spring, a tiny eriophyid mite, Eriophyes fraxinivorus, feeds on the tiny male ash flowers. The ash tree, in response to the insect feeding, grows tissue around the insect. As a result, the flowers enlarge and stay on the tree for up to two years, instead of dropping off the plant during the first spring. The galls are green at first, but become brown later in the growing season. Ash flower galls do not damage the tree, although they can be unsightly, and if the clusters are numerous, their weight can strain branches.
There are over 700 different kinds of galls found on oak trees. A fuzzy hot pink or yellow growth on the underside of white oak leaves is caused by a cynipid wasp. These tiny wasps are the most common gall producing insects. They produce a variety of oak galls, including: oak hedgehog gall, oak apple gall, jumping oak gall, and oak bullet gall.
Coniferous galls are generally caused by aphid-like insects called adelgids.
The eastern spruce gall occurs in Norway and white spruce trees. These galls are a half-inch to 1-inch long green, pineapple shaped structures at the base of new shoots. These galls may weaken stems and cause them to break in heavy snow or high winds.
The Cooley spruce gall occurs in Colorado blue spruce and white spruce. The galls are 2”-4” long, cone-shaped, and start out as green-purple, turning brown as they dry in the late summer. Tree health is not affected with either of these adelgid galls unless they are abundant and occur over several consecutive years. In most cases, galls are not plentiful enough to injure a plant.
For more information about galls, visit the University of Minnesota Extension website at: www.extension.umn.edu/yard-and-garden-insects/insect-and-mite-galls
Until next time, happy gardening!
“Even if I knew that tomorrow the world would go to pieces, I would still plant my apple tree.” ~ Martin Luther
Robin Trott is a horticulture educator with University of Minnesota Extension. Contact her at 320-762-3890, or at trot0053@umn.