Gallmaking Insects and Mites

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.

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 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 (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, 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 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.

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.

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.

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.

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 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.