5 June 2007
Virginia Cooperative Extension, Charlottesville/Albemarle County Office
460 Stagecoach Road, Charlottesville, Virginia 22902
phone: 434.872.4580 fax: 434.872.4578
Has anyone ever told you that you have a lot of gall? Some plants hear this often, only with them it has nothing to do with impudence. Galls are abnormal growths of plant tissue induced by insects and other organisms. Gall-making parasites release growth-regulating chemicals as they feed, causing adjacent plant tissues to form a gall. The parasite then develops within the relative security of the gall. Several different groups of insects and one family of mites have developed the ability to induce plant galls. In addition, there are a few galls produced by nematodes, bacteria, fungi, and viruses.
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. Most galls are formed by two kinds of insects or mites: gall wasps, gall midges, and gall mites. Other less common gall producing insects are aphids, psyllids, and gall flies. Since most galls seem to do no permanent damage to their host plants, limited research has been done on the biology or control.
Minor Gall Producers
Many aphids are well known plant pests, but only a few cause galls. Slippery Elm Pouch Galls are elongated pouches on the upper surface of elm leave. Elm Cockscomb Galls on American and slippery elm are named for their resemblance to the comb of a rooster.
Adelgids are commonly called aphids, but they are only close relatives. Several different species produce galls on spruce. Cooley Spruce Galls are elongated cone-like galls on the new growing trips of Colorado blue spruce. Eastern Spruce Galls are similar to the above gall, but occur at the base of new shoots.
Phylloxerans are insects also closely related to aphids and they produce a variety of galls on hickory. Hickory Leaf Stem Galls appear as irregular, globular growths on the leaves, petioles, and twigs. Other phylloxeran galls on hickory usually occur on the leaves and may be disk-, button-, or bead-shaped. Psyllids, commonly known as jumping plant lice, resemble miniature cicadas. All species feed on plant juices, but only a few produce galls. Psyllids that feed on hackberry cause the Hackberry Button Gall, Hackberry Flask Gall, Hackberry Nipple Gall, Hackberry Star Gall, and the Hackberry Melon Gall.
Major Gall Producers
Mites who cause galls are members of the family Eriophyidae. Many Eriophyid mites live in buds or are free-living on the surface of leaves and do not produce galls. Some are serious pests; the Privet Rust Mite, Hemlock Rust Mite, and Juniper Bud Mite are examples.
Gall midges are a large group of tiny, delicate flies about l/8 inch long. Most species lay their eggs in plant tissue, and feeding by their larvae produces a wide variety of galls. Dogwood Club Galls are elongate swellings at the tips of small twigs of flowering dogwood. 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.
Wherever oaks occur, they are attacked by a group of tiny wasps, mostly less than l/8 inch in length. Gall wasps attack all parts of oak trees, including roots, flowers, and acorns, but especially the leaves and twigs. Roses and brambles (blackberries and raspberries) also are attacked by gall wasps.
Examples include Oak Apple Galls, golf ball-sized growths with thin shells and spongy cores. 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.
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.
Damage and Control
Controlling gall insects is difficult, and at present there are no insecticides registered for this use by homeowners. Any treatment applied after galls are already present is useless, because galls will not go away even if the parasite is killed. Fortunately, the vast majority of galls are not particularly injurious and are of no economic significance. Most plants can support a large number of galls and continue to grow normally. Pruning out heavily galled portions of a plant is sometimes feasible and may help reduce populations of the gall insects. When this is not possible, it is best to accept galls as curiosities of nature. Enjoy watching their development if you are interested; simply ignore them if you are not.
For more information about this and other landscape topics contact your local Virginia Cooperative Extension office. The local Virginia Cooperative Extension office numbers are Albemarle 872-4580, Fluvanna 591-1950, Greene 985-5236, Louisa 540-967-3422, and Nelson 263-4035.