What Are Galls?

Have you ever noticed unusual lumps, spheres, or masses on trees or other plants? These strange formations are called galls. Galls come in many sizes and shapes. Some galls look and feel like pompoms, while others are hard as rocks. Galls may occur on every part of plants, from the leaves to the roots.

What Are Galls?

Galls are abnormal growths of plant tissue trigger in response to an injury to or an irritation of the plant, usually (but not always) caused by some living organism. Nematodes, bacteria, fungi, and viruses can all cause the formation of galls on trees, shrubs, and other plants. Most galls, however, result from insect or mite activity.

Gallmaking insects or mites initiate gall formation by feeding on a plant, or by laying eggs on plant tissues. The insects or mites interact with the plant during a period of rapid growth, such as when leaves are opening. Scientists believe that gallmakers secrete chemicals that regulate or stimulate plant growth. These secretions cause rapid cell multiplication in the affected area of the meristematic tissue. Galls can only form on growing tissue. Most gallmaking activity occurs in the spring or early summer.

Galls serve several important purposes for the gallmaker. The developing insect or mite resides within the gall, where it is sheltered from the weather and from predators. The young insect or mite also feeds on the gall. Eventually, the mature insect or mite emerges from the gall.

After the gallmaking insect or mite leaves, the gall remains behind on the host plant. Other insects, such as beetles or caterpillars, may move into the gall for shelter or to feed.

Which Insects Make Galls?

Insects that make galls include certain kinds of wasps, beetles, aphids, and flies. Other arthropods, like mites, can cause gall formations, too. Each gallmaker produces its own unique gall, and you can often tell which kind of insect made the gall by its shape, texture, size, and host plant.

• Psyllids - Some jumping plant lice, or psyllids, produce galls. If you find galls on hackberry leaves, there's a good chance it was caused by a psyllid. They feed in spring, triggering the formation of two well-known leaf galls: hackberry nipple galls, and hackberry blister galls.
• Gallmaking aphids - Aphids belonging to the subfamily Eriosomatinae cause gall formations on the stems and petioles of certain trees, most notably cottonwood and poplar. Aphid galls vary in shape, from a cockscomb-shaped growth on elm leaves to a cone-shaped gall that forms on witch hazel.
• Gallmaking adelgids - Gallmaking adelgids target conifers, for the most part. One common species, Adelges abietis, causes pineapple-shaped galls on Norway and white spruce twigs, as well as on Douglas fir. Another, the Cooley spruce gall adelgid, makes galls that look like cones on Colorado blue spruce and white spruce.
• Phylloxerans - Phylloxerans (family Phylloxeridae), though tiny, do their share of gallmaking, too. The most notorious of the group is the grape phylloxera, which produces galls on both the roots and leaves of grape plants. In 1860, this North American insect was accidentally introduced into France, where it nearly destroyed the wine industry. French vineyards had to graft their grape vines onto phylloxera-resistant rootstock from the U.S. to save their industry.
• Gall wasps - Gall wasps, or cynipid wasps, comprise the largest group of gallmaking insects, with over 1,000 species known globally. Cynipid wasps produce most of the galls on oak trees and plants within the rose family. Some gall wasps oviposit in galls created by other species, rather than induce the growth of their own. Cynipid wasps sometimes develop within galls that have fallen from the host plant. Jumping oak galls are so named because they roll and bounce around the forest floor as the larva inside moves.
• Gall midges - Gall midges or gall gnats make up the second largest group of gallmaking insects. These true flies belong to the family Cecidomyiidae, and are quite tiny, measuring 1-5 mm in length. The maggots, which develop within the gall, come in strangely bright colors like orange and pink. Midge galls form on various parts of plants, from the leaves to the roots. Common galls formed by gall midges include the pinecone willow gall and the maple leaf spot.
• Gall flies - Some genera of fruit flies produce stem galls. Eurosta gall flies develop and overwinter within goldenrod galls. Some Urophora gall flies were introduced into North America from their native Europe, as biocontrols for invasive plants like knapweed and bull thistle.
• Gallmaking sawflies - Sawflies produce some unusual galls, most commonly on willows and poplars. The leaf galls induced by Phyllocolpa sawflies look like someone crimped or folded the leaves. The sawfly larva feeds within the crinkled leaf. Pontania sawflies produce strange, globular galls that protrude through both sides of a willow leaf. Some Euura sawflies cause petiole swelling in willows.
• Gallmaking moths - A few moths make galls, too. Some micromoths in the genus Gnorimoschema induce stem galls in goldenrod, where the larvae pupate. The midrib gall moth produces an odd leaf formation in buckthorn. The center of the leaf is rolled tight, with the sides joining to form a pouch in which the larva resides.
• Beetles and weevils - A handful of metallic wood-boring beetles (Buprestridae) are known to produce galls in their host plants. Agrilus ruficollis induces galls in blackberries. Ruficollis translates to "redneck," a specific name that refers to this insect's red pronotum. Another species, Agrilus champlaini, creates galls in Ironwood. Long-horned beetles of the genus Saperda also produce galls, in stems and twigs of alder, hawthorn, and poplar. A few weevils also cause swellings in their host plants' tissues. Podapion gallicola, for example, causes galls in pine twigs.
• Gall mites - Gall mites of the family Eriophyidae produce unusual galls on leaves and flowers. The mites begin feeding on their host plants just as buds are opening in the spring. Eriophyid galls may form as finger-like projections or warty bumps on leaves. Some gall mites produce a velvety discoloration of the leaves.

Will Galls Damage My Plants?

Insect enthusiasts and naturalists probably find insect galls interesting or even beautiful. Gardeners and landscapers, though, may be less enthused to discover insect galls on trees and shrubs and may be concerned about insect gall damage.

Fortunately, with few exceptions, insects galls do not damage trees and shrubs. While they may look unsightly, particularly on specimen trees, most healthy, well-established trees and shrubs will be unaffected by galls in the long run. Heavy gall formations may slow growth.

Because the negative impact of galls on plants is largely aesthetic, control measures for galls or gallmaking insects are rarely warranted. Leaf galls will fall off, either with the leaves themselves or from the leaves once the insect or mite has emerged. Galls on twigs and branches can be pruned out. A gall that has already formed cannot be treated or sprayed to eliminate it. The gall is part of the plant itself.

Gallmaking insects, it should be noted, will attract their own biological controls in the form of parasitoids and predators. If your landscape is riddled with galls this year, give it time. Nature will restore the balance in your ecosystem.