Photos by James Kalisch, UNL.

For most of us, when the word insect is mentioned, the common response is often “kill them.” Yet, for some insects, a different response may be more appropriate. A few insects nearly always cause damage, some cause damage, but only on occasion, and most rarely cause any damage at all. Galls and gall-forming insects are usually harmless.

Integrated pest management

Two of the more important foundations of integrated pest management (IPM) are correct pest identification and establishment of damage threshold levels. These factors are especially important when making management decisions regarding galls and gall-maker insects.

Symptoms of galls and gall makers

Galls appear as bumps on leaves or stems. They come in all sorts of colors, from red to green to brown, and in all kinds of shapes, from oval to round to pyramidal. The vast majority are small, in the .125 to .25-inch range. Overall, the key characteristic of a gall is that it produces a substantial growth change in the host plant. When attempting to identify gall-forming insects and their galls, a good reference book, such as Dr. Whitney Cranshaw’s “Garden Insects of North America,” can be helpful.

Common gall makers and galls

Aphid galls—Petiole gall aphids infest the stems, petioles and leaves of poplar and cottonwood trees.

These insects cause roundish, usually medium green galls about the size of a penny or nickel. If cut in half, small whitish aphids will be found inside. The poplar vagabond aphid produces a small mass of distorted growth near the base of developing leaves. They are usually lime green in color. Vagabond galls are readily visible on smaller trees, but can be more difficult to spot on hidden or larger specimens, as they tend to congregate on the upper branches. The witch hazel leaf gall aphid makes small, cone-shaped growths on the upper sides of leaves. Birch may also be infested with witch hazel galls.

Adelgid galls—Adelgids are curious insects, almost always associated with conifers. Cooley spruce gall adelgids produce thumb-sized growths on stems that are rather intricate in detail. The galls themselves appear as a small pinecone with projecting dead needles. They take the place of the normal stem. Adelgid feeding causes cavities to form inside the gall, where the insects develop. Prior to forming the gall, the small whitish nymphs look much like scales on needles and stems. The related Eastern spruce gall adelgid produces a smaller and less visible pineapple-shaped gall. Pine leaf adelgids also make galls in the terminal growth of spruce and pine trees, but their galls are leafier in appearance than Cooley spruce galls.

Phylloxeran galls—Mostly a pest of grapes, phylloxera galls can also infest certain tree species. The hickory leaf stem gall phylloxera produces roundish galls on shoots and petioles. The pecan leaf phylloxera produce nondescript, beige, oval bump-like galls on the surface of pecan leaves.

Psyllid galls—The poster child of the psyllids is the hackberry nipple gall maker, which produces medium-sized green galls on the undersides of hackberry leaves. While development of psyllids from egg to nymph is rapid, some trees that form leaves late due to shading or genetic factors may escape infestation. Feeding by psyllid nymphs causes an overgrowth of leaf tissue, which initially appears as a raised swelling. As the season progresses, a gall is produced to cover the insect. The nymphs grow and develop over the summer. Adults emerge from galls in late summer or early fall and seek sheltered sites to overwinter. Eugenia psyllids produce small pits that surround a scale-like nymph, usually on new eugenia (Brazilian cherry) leaves. Red bay and laurel psyllids induce a thickened curling of the leaves, with raised, blister-like galls.

Gall-making flies—Leaf thickening and folding is also a common symptom of gall- making flies. Such is the case for the maple gouty vein gall midge and the pear leaf-curling midge. The honeylocust pod gall midge causes thickened, pod-like galls, whereas the ash midrib gall midge produces a thickening along the midrib of ash leaves in the spring. Other gall midges in-festing woody plants include the willow cone gall midge, juniper tip midge, pinyon spindle gall midge and the willow-beaked gall midge.

Eriophyid mite galls—Abnormal plant growth is normal for trees infested with gall-producing insects, but perhaps the most unusual are the structures produced by eriophyid mites. The maple bladder gall maker produces small brightly colored growths. Typically red, yellow or light purple, these small galls appear as raised warts on the leaf surface. Ash flower galls appear as large (.5 to 1-inch diameter) distinctive aberrations of the ash flowers. Initially green in color, these galls turn from brown to black as the summer progresses and remain on the tree long after the leaves have dropped. This is also the case for the hackberry witches’ broom gall. These eriophyid mites often work in concert with the powdery mildew fungus to produce odd distorted growths from flower stems.

What to do about galls

Instead of reaching for the bottle of insecticide, consider the following options when dealing with galls.

Point them out to your clients; you’ll earn a lot of goodwill by doing so. In this day and age of “everything is a huge problem and you gotta spray to keep the tree alive” mentality, informing your customer that most galls are benign will score a lot of points for your company.

Teach your crew about galls. As you scout properties with your technician, make sure they can recognize the most common gall species. Teach them the difference between galls and truly damaging insects, such as aphids and scale insects.

Point gall infestations out to noncustomers. Take some to your church or Kiwanis club meetings. Again, there’s lots of potential for goodwill here.

Send some galls with your elementary school-aged children (or with kids down the block) for show and tell. Many elementary school teachers are not well trained in the sciences and would welcome the opportunity to expose their class to this interesting facet of nature.

Keep records on gall infestations. Monitor the presence and numbers of galls from year to year. Take digital photographs and distribute them in your promotional newsletter.

John C. Fech is a horticulturist, certified arborist and frequent contributor located in Omaha, Neb. Frederick P. Baxendale is a professor and extension entomologist with the University of Nebraska-Lincoln.