Galls are plant structures that form as a result of arthropod feeding. The abnormal plant growth is stimulated by a combination of plant hormones and hormones introduced into the plant during arthropod feeding. The plant responds by increasing the production of leaf tissue, thereby enclosing the individual mite or insect. The enveloped arthropod feeds and reproduces from within the gall.
The upper surface of silver and red maple leaves often become covered with small, red, round wartlike structures about 1/16- to 1/8-inch in diameter. These are maple bladder galls caused by a small eriophyid mite, Vasates quadripedes (Shimer). The structures are generally noticed first in May, about the time the leaves have become fully expanded. At first the galls are green, but they quickly turn pink to red and eventually black. Leaves frequently become so covered with the galls that they completely twist out of shape and may even drop early.
On sugar maple, another leaf gall is commonly found, the maple spindle gall. This gall is caused by the mite, Vasates aceriscrumena (Riley). The gall appears as thin, elongate bladders arising from the upper leaf surface. These galls rarely distort the leaf, but considerable numbers of galls can make the leaves unsightly.
|Maple bladder gall.||Maple spindle gall.|
The maple velvet erineum gall is another gall commonly found on maples, primarly silver, Norway and boxelder maples. This colorful gall is caused by another eriophyid mite, Eriopyhes spp. Crimson to yellow, velvet-like patches form on the upper and lower leaf surfaces as a result of mite feeding activity. These conspicuous, hairlike patches may resemble a fungus or a disease.
The maple gouty vein gall midge attacks only sugar maple and causes thickened pouches along the major veins. These galls can completely crumple the early leaves of maples, often making them look like herbicide damage. The galls are caused by the larvae of a small gnatlike midge, Dasineura communis (Felt).
|Maple velvet erineum gall.||Gouty vein gall.|
Homeowners often become quite alarmed when they discover that their maple tree leaves are infested with leaf bladder, spindle, erineum or gouty vein galls. They fear that the tree is going to die unless something is done quickly. This is not the case. Maple leaf galls seldom, if ever, cause permanent injury to a tree, but they do detract from the beauty of the leaves.
Other than the mere aesthetic damage and possible early leaf drop, no significant damage is done to the health of maple trees. Following a mild winter, damage from these leaf galls can be excessive, but affected trees often send out new leaves to replace the damaged ones.
Description and Life Cycle
Both the maple bladder, maple spindle and erineum gall mites overwinter as free-living mites under loosened bark and around the callous growth of wounds, scars and pruned branches. These overwintered forms produce the gall-forming stage in early spring. When the maple leaves first appear, the mites migrate to expanding buds and begin to feed on the undersurface of leaf buds. This causes the formation of a blister, which expands into a hollow bladder or spindle as the leaf expands. The mites enter the cavity and continue to feed within its protective walls. This stage reproduces asexually within the galls, and the new mites mature by late June to mid-July. At this time, the galls dry out and the adult mites emerge. These mites then seek out overwintering sites.
The maple gouty vein gall midge spends the winter as full grown larvae in the ground and leaf litter under their host sugar maples. In late January into early March, these larvae spin small white cocoons in which to pupate. The pupae rest until April and early May when the gnatlike adults emerge. These midges have black wings and heads, but the body appears reddish from the eggs inside. Each female may lay up to 100 eggs among the leaf hairs on the lower leaf surfaces of expanding leaves. The tiny, maggotlike larvae hatch in a couple of days, and they migrate to the upper leaf surface. Here they line up in small groups along the major leaf veins. At these congregation points, the leaves swell and the vein edges fold over to form the galls. Within a few days the galls are fully formed. The larvae feed within the protection of the gall until October. At this time the galls dry and a slitlike opening is formed. The mature larvae drop to the soil to seek shelter.
Since these leaf galls of maple are typically only aesthetic and do not cause any real harm to the trees, control measures are generally not recommended. Tree owners and tree managers are encouraged to learn about the life cycles of these pests and learn that no lasting damage will result.
Strategy 1: Resistant Varieties
Norway maples and some of the named cultivars of maples with outstanding red or yellow leaf color appear to be resistant to these gall mites and midges. Talk to the plant supplier to see if the tree cultivar has a history of leaf gall problems.
Strategy 2: Cultural Control
Encourage plant health with proper fertility and adequate moisture. Prune out affected branches and twigs to reduce populations. Dispose of infested plant tissue in plastic bags to minimize spread of adult mites.
Strategy 3: Dormant Oils
Use of dormant oils on maples is discouraged because leaf and twig damage can result unless the tree is truly dormant. However, some reports of success have been made where the trunk has been drenched with dormant oil to kill the overwintering stages of the bladder gall and spindle gall mites.
Strategy 4: Standard Insecticide/Miticide Sprays
Several insecticides and miticides are registered for control of gall mites (eriophyid mites) and gall midges. If these products are to be used, they have to be applied precisely when the new leaf buds are opening. Most sprays have little, if any, effect because the window of opportunity is very short. Once the gall has formed, it is too late to make an application as the pest is protected by the gall and does not come into contact with the pesticide.
Strategy 5: Systemic Insecticides/Miticides
Several systemic pesticides (sprayed, soil-injected or trunk-injected) have been recommended as useful in controlling these gall-forming pests. However, little evidence of success has been found in the current literature. Once the gall has formed, it is too late to make an application. Always read and follow the pesticide label.
This fact sheet is a revision of HYG-2004.