Publication 444-218, August 1996
Galls come in an endless variety of forms. Many are strikingly colored or curiously shaped. Each gall-making species causes a gall structurally different from all others. By noting the type of host plant and the structure of the gall, one can identify the gall-making organism without actually seeing it.
The most important groups of gall-producers are gall mites, aphids, adelgids, phylloxerans, psyllids, gall midges, and gall wasps.
Erineum Galls are dense, felt-like patches of plant hairs. On maple, erineum galls are sometimes red, while on alder, beech, and poplar, they are buff-colored.
Blister Galls are irregular, blister-like swellings on leaves. Walnut and butternut are especially susceptible hosts.
Leaf Roll Galls are often seen on pecan. Gall mites live within the narrow infoldings of the leaf edges.
Spindle and Bladder Galls occur on maple and other hosts. Each gall can contain up to a hundred mites.
Slippery Elm Pouch Galls are elongated pouches on the upper surface of elm leave. The galls appear in May and are soon filled by female aphids and their offspring. Usually, only one gall occurs per leaf.
Elm Cockscomb Galls are named for their resemblance to the comb of a rooster. One of several of the galls may appear on the upper surface of leaves on American and slippery elm.
Cooley Spruce Galls are elongated cone-like galls on the new growing trips of Colorado blue spruce. The Cooley adelgid has an interesting and complicated life history. Immature females overwinter on spruce. In the spring they mature and lay eggs on the twigs. Feeding by young nymphs causes abnormal tissue growth which soon surrounds them and forms the gall. By midsummer, openings appear on the galls, and the adelgids, which then molt and develop wings, migrate to Douglas fir. A generation of "woolly aphids" occurs on this host. Their feeding causes yellowish spots and bent needles, but no galls. A year later, another generation returns to spruce.
Eastern Spruce Galls are similar to the above gall, but occur at the base of new shoots. They are found primarily on Norway spruce, but also on white, red, and Colorado blue spruce.
Several other adelgids attack spruce, such as Spruce Gall Adelgids. The resulting galls are usually cone-like, but many appear as scraggly, deformed twigs.
Hickory Leaf Stem Galls appear as irregular, globular growths on the leaves, petioles, and twigs. This phylloxeran overwinters in bark crevices as eggs which hatch when the buds begin to open. All the young develop into females which, through their feeding, cause the formation of galls. Eggs are then laid inside the galls, and a new generation of winged phylloxerans develops. By the end of June the galls open, and the insects fly off to summer hosts.
Other phylloxeran galls on hickory usually occur on the leaves and may be disk-, button-, or bead-shaped.
Psyllids that feed on hackberry cause the Hackberry Button Gall, Hackberry Flask Gall, Hackberry Nipple Gall, Hackberry Star Gall and the Hackberry Melon Gall. In the Gulf states, several galls on bay are caused by psyllids.
Dogwood Club Galls are elongate swellings at the tips of small twigs of flowering dogwood. Female midges deposit their eggs in the tiny terminal leaves just as the buds begin to open. Larvae work their way into the tips of the new twigs. The first symptom of their presence is a wilted, gnarled leaf. Soon after, the adjacent tissue begins to swell, and the gall forms. By late summer, the maggots chew exit holes in the gall and drop to the ground where they overwinter.
Vein Pocket Galls are hard, tan-colored swellings along the midrib and major veins of pin oak.
Other galls caused by gall midges include: Beaked Willow Gall, Willow Pine Cone Gall, Woolly Fold Gall on oak, Maple Leaf Spot Gall, Gouty Vein Gall on maple, Grape Tomato Gall, Ash Midrib Gall, Pine Needle Gall, Gouty Pine Gall, and many others.
These insects have complicated life cycles, and the galls they produce occur in an endless variety of shapes and colors. In some species, alternate generations produce distinctly different galls. The insects that emerge resemble their grandparents, but are physically different from their parents.
Oak Apple Galls are golfball-sized growths with thin shells and spongy cores. A single larva develops inside each gall.
Wool Sower Galls are white, spongy fibrous masses with yellow, seed-like capsules throughout. A gall-wasp larva develops within each capsule.
Oak Spangles resemble small buttons on the surface of oak leaves. Other wasp-induced leaf galls on oak resemble blisters, beads, or fuzzy balls. Many times these galls, also called Oak Button Galls, will fall from the tree and land on cars, decks, and sidewalks below, making a sticky mess.
Horned Oak Galls are hard, woody swellings on oak twigs. Small horns protrude through the surface of the gall, and a single gall wasp develops in each horn. Gouty Oak Galls are similar to the horned oak gall, but lack the protruding horns. These two galls are the only wasp galls known to cause economic damage and, in some cases, if left unchecked will cause death of the tree. Pruning the galls off the tree in May, when the wasps are inside, is the only known effective control.
Dozens of additional wasp-induced galls occur on oak, rose, and brambles. Many of the insects are not well studied, and the galls do not have accepted common names. The wasps that emerge from the galls will not sting humans.
Revised by Eric Day Prepared by D. J. Hilburn
Department of Entomology Virginia Polytechnic Institute and State University Blacksburg, Virginia 24061-0319
