Common name: plant galls
Scientific name: Plant galls are produced by a wide range of insect, mite, and pathogen species.
Hosts: There is a wide range of plants that are affected by gall-making insects and mites. Many gall-makers are host specific, and the gall produced by each species is very distinctive in size and shape.
Galls are abnormal growths that occur on leaves, twigs, roots, or flowers. Feeding or egg laying by the gall-making insect or mite causes stimulation of plant cells, and the plant’s tissues develop into unusual shapes. Some galls are the result of infections by bacteria, fungi, or nematodes, and are difficult to tell apart from insect-caused galls.The identity of the host plant and the shape of the gall may help to identify the organism causing the gall.
In the majority of cases, plant gall damage is an aesthetic problem and is not considered serious. Affected trees ordinarily show little injury, although foliage of young trees is sometimes extensively deformed. A few stem galls can do damage by girdling stems. Galls caused by pathogens may cause more serious damage.
Insect-caused galls provide a home for the insect, where it can feed, lay eggs, and develop. Galls may develop in a variety of shapes, each being characteristic of the causal organism. In addition to the unusual structure of galls, they draw attention due to their range of colors. Factors such as weather, plant susceptibility, and pest populations affect the occurrence of galls on plants from year to year. Oaks and hickories are common hosts for gall-making insects and mites. Numerous species of wasps, aphids, mites, and midges can cause galls on these trees.
Maple bladder gall is often seen as bright red bead-like growths on upper leaf surfaces of silver, red, and Freeman (hybrid) maples. The causal agent is an eriophyid mite (Vasates quadripedes). The mite overwinters at the base of the leaf buds. When the buds open, the mites feed on young leaf tissue and stimulate gall formation. The mites feed inside these galls, lay eggs and then die. The galls are green at first. Later they turn red and by the end of summer they may be almost black. By fall, the new generation of mites leaves the foliage to overwinter.
Various species of eriophyid mites are also responsible for spindle galls that can occur on a number of different tree species. They are common on lindens (Tilia species), maples (Acer species), and wild cherry trees (Prunus species). The galls get their name from their elongated, spindle-like shape. The feeding of the mites stimulates the galls to form. The adult females overwinter in crevices in the stems.
Bladder galls on fragrant sumac (Rhus aromatica) are also due to a species of eriophyid mite (Aculops rhois). Like most other galls, this one does little to no damage to the shrub, but in some years the population is so high that it does cause concern. The host plant will not look attractive, but it will still be healthy and continue to grow. This mite also affects poison ivy in a similar fashion, causing the poison ivy leaf gall.
Hackberry nipple gall occurs on almost every hackberry (Celtis occidentalis) tree. The tiny insect that causes the gall is called a psyllid (Pachypsylla species). It resembles a cicada, but is much smaller, about ⅛ inch. It overwinters as an adult in the crevices of the tree’s bark. The adults become active in spring and will lay eggs on the lower surfaces of newly emerged leaves. The nymphs that hatch from the eggs will feed on the leaves and stimulate the formation of the nipple-shaped gall. Galls can vary a little in shape, depending on which species of psyllid caused them. At maturity, the nymphs leave the gall through an opening and then fall to the ground before becoming adults.
Oak trees play host to a number of gall-makers. Various species of non-stinging wasps in the genus Neuroterus will produce galls on the lower side of the leaves.Two common galls in this group are oak spangles and jumping oak gall. Oak spangles galls (Neuroterus quercusbaccarum and other species) are small disc or saucer shaped structures on the lower side of the oak leaf in summer. On the upper side of the leaf, a small spot appears above where the gall is growing from the lower side of the leaves. When numerous, these spots are often mistaken for a fungal leaf spot disease. The galls fall off in autumn with the gall-maker inside.
Jumping oak galls (Neuroterus saltatorius) are similar to oak spangles. They form on the undersides of the leaves, but are more round in shape, looking somewhat like seeds. When these galls fall off in late summer, they will jump like jumping beans.
Even some aphids can be gall-makers. The witch-hazel cone gall aphid (Hormaphis hamamelidis) causes galls on witch-hazel (Hamamelis) leaves. The galls get their name from their very distinct cone shape. The aphid overwinters as eggs on the host plant. In spring, they begin to feed and stimulate the growth of the gall. The aphids grow and reproduce inside the gall. The aphids leave the gall later in the season and fly to an alternate host, birch trees. No gall is formed on the birch, and the insects look more like scale insects than aphids. They return to the witch-hazel host in autumn to overwinter.
Galls are not restricted to woody plants. They can also be found on perennials. Goldenrod (Solidago) is a good example of this. There are two different galls commonly found on goldenrod, the goldenrod bunch gall and the goldenrod ball gall. These two galls are caused by two different species of flies. The goldenrod ball gall (Eurosta solidaginis) causes round galls, slightly smaller than ping-pong balls, on the upper end of the stems of some species of goldenrod (Solidago). The insect overwinters in the larval stage inside the stem of the host plant. The larvae pupate into adult flies in spring. The adults mate and lay eggs on the stems, near a terminal bud. The newly emerged larva tunnels into the stem and downward. Chemicals in the larva’s saliva stimulate the gall to form. The larva spends the summer inside the stem, feeding and growing.
The goldenrod bunch gall is caused by a midge (Rhopalomyia solidaginis), which is a type of fly. The life cycle is similar to that of the goldenrod ball gall fly, but in this case the larval feeding stops the upward growth of the stem, but does not stop leaf production. This results in a tight bunch of leaves.
While most galls caused by insects and mites are relatively harmless, those caused by pathogens can be far more serious. These include crown gall and black knot.
If only a few galls are present, the affected part of the plant may be removed.
There are a number of predatory and parasitic species that will attack gall-makers. When the population of gall-makers rises, the population of predators and parasites will rise as well. They do not provide complete control, but help to keep the galls in check.
Fortunately, galls, while unusual and sometimes even alarming in appearance, cause little to no long-term injury. For this reason, chemical sprays are rarely necessary or recommended to treat gall infestations. By the time the galls become noticeable, the insect or mite causing the injury is protected from chemical sprays.
The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.
For more information, contact The Morton Arboretum Plant Clinic at 630-719-2424 or plantclinic@mortonarb.org.