Insect and mite galls
Orders: Acari, Hemiptera, Hymenoptera, Diptera
Families: Adelgidae, Agromyzidae, Aphalaridae, Aphididae, Eriophyidae, Cecidomyiidae, Cinipidae
Description and life history
Galls form when plant tissues grow abnormally in response to wounds, pathogens, or feeding and egg-laying by certain insects and mites. In the case of insect and mite feeding, galls may form in response to mechanical injury or due to compounds introduced in the leaf tissue that mimic plant growth hormones. There are different types of galls that most commonly form on leaves, stems, and flowers where insects and mites are most likely to feed. Galls often remain on plants long after the insects or mites have left. While their appearance may cause concern, galls rarely affect plant health, and their numbers tend to fluctuate from year to year as insect and mite populations fluctuate through the seasons. Many different insects and mites can produce galls on woody plants.
- Certain insects and mites introduce a compound to plant tissue during feeding. These compounds mimic plant growth hormones and stimulate abnormal growth of plant tissue, which results in the formation of a gall.
- Galls usually begin forming in late spring or early summer when insects become active and lay eggs.
- While galls are unattractive, they rarely threaten the health of the plant. Therefore, management is not usually necessary. In extreme cases, some of these pests can be controlled with dormant oils or foliar insecticide sprays.
Leaves with finger galls produced by eriophyid mites. Image credit: Bruce Watt, University of Maine, Bugwood.org
Eriophyid mites in a gall. Image credit: Tracy Wootten, University of Delaware, Bugwood.org
Adult of the hackberry nipplegall maker. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Oak rough bulletgall wasp. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Gall containing larvae and pupae of D. gleditchiae. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Poplar twiggall fly on quaking aspen (Populus tremuloides). Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Galls with a felt-like texture formed by eriophyid mites. Image credit: Steven Katovich, Bugwood.org
Leaves with nipple galls formed by psyllid activity. Image credit: Lacy L. Hyche, Auburn University, Bugwood.org
Spruce gall formed by cooley spruce gall adelgid. The gall has been cut open to expose the developing insects. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Marble-sized gall formed by activity of the poplar petiolegall aphid. Image credit: Ansel Oommen, Bugwood.org
Infestation of poplar petiolegall aphids on cabbage roots. Note the abundant white wax produced by these aphids. Image credit: Alton N. Sparks, Jr., University of Georgia, Bugwood.org
Fully formed galls on bur oak twig caused by the oak rough bulletgall wasp. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Honeylocust tree with podgalls formed by D. gleditchiae, a species of gall-forming midge. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Quaking aspen (Populus tremuloides) with galls on twigs caused by poplar twiggall fly. Image credit: William M. Ciesla, Forest Health Management International, Bugwood.org
Description and life history continued…
Eriophyid mites (Eriophyidae)
Eriophyid mites are very small arthropods that usually measure under 1 mm in length and are often found on leaves, flowers, fruits, or buds. These mites tend to form small, finger-like galls, pocket galls, or masses of plant hairs that have a felt-like texture. Eriophyid mites are typically white or yellow, and unlike most arachnids, eriophyid mites have carrot-shaped bodies with two pairs of legs instead of four. These mites develop through four stages that include an egg, two nymphal stages, and the adult stage. Adult females overwinter under the scales of buds or other protected sites, and there are often several overlapping generations produced each growing season. Most species of eriophyid mite feed on a single plant species, which includes various conifers and hardwoods such as birch (Betula spp.), mountain ash (Sorbus aucuparia), and willow (Salix spp.).
One common group of psyllids create galls on hackberry (Pachypsylla spp.). Species in this genus have similar morphology and produce nipple galls or blister galls on leaves. Adult psyllids are small, measuring 2-5 mm long, and hold their wings rooflike over the body. These pests resemble miniature cicadas that jump when disturbed. In Colorado, two notable species include the hackberry nipplegall maker (Pachypsylla celtidismamma), which produces nipple galls, and hackberry blistergall psyllid (Pachypsylla celtidisvesiculum), which produces blister galls. Both species likely produce one generation per year, with nymphs developing inside of galls through the summer and emerging as adults in late summer. There can be considerable variation in the number of galls that form on nearby leaves due to the synchronization of egg-laying and hatching with early leaf development. Different psyllid species tend to feed on one plant species or a group of closely related plant species. Collectively, these two Pachypsylla spp. can attack various Celtis spp., including hackberry, sugarberry, American or common hackberry, and net-leaf hackberry.
Gall-making aphids, adelgids, or woolly aphids (Aphididae, Adelgidae)
These gall-making insects primarily attack coniferous trees, with the most common being the Cooley spruce gall adelgid (Adelges cooleyi) in Colorado, which forms cone-like galls on spruce trees. In addition to the Cooley spruce gall adelgid, some Pemphigus spp. can form stem and petiole galls on certain Poplus spp. such as aspen and cottonwood. The Pemphigus spp. are roughly 2 mm long and have pale green bodies with a dark thorax. Wingless forms are more frequently observed than wing forms, and the nymphs are like adults but smaller. These aphid species have a similar 1-year life cycle that involves two sets of hosts. For example, the poplar petiolegall aphid (Pemphigus populitransversus) lays eggs in the fall to coincide with changes in foliage. Eggs hatch the following spring, and the newly emerged nymphs begin to feed on developing leaf petioles, which results in the formation of a gall that envelops the aphid. The aphid reproduces asexually within the gall until developing wings in late June or July, at which point the gall splits open and the winged aphid migrates to cruciferous plants where it colonizes roots and produces multiple generations of asexually reproducing females through the rest of the summer. The nymphs tend to produce large amounts of white wax when colonizing the roots of cruciferous plants. At the end of summer, winged adults are again produced and migrate back to their winter hosts. Small, mouthless males and females are produced, and the males die shortly after mating. Each mated female lays a single egg on the winter host and then dies. Winter hosts include spruce, Douglas-fir, and Populus spp., while cruciferous plants often serve as hosts for summer generations. High densities of certain species, like the lettuce root aphid (Pemphigus bursarius), can affect the productivity of cruciferous garden crops.
Gall wasps (Cinipidae)
Gall wasps comprise the largest group of gall-making insects that produce a range of galls, including woody, round galls on stems or leaves to woolly or mossy galls. One species, the oak rough bulletgall wasp (Disholcaspis quercusmamma), is native to the midwestern United States and is likely found in most locations that contain bur oak (Quercus macrocarpa). Adults have stout, dark brown bodies, and a rounded abdomen. In Colorado, the wasps emerge from galls in late October or early November, leaving a small circular exit hole in the gall. Females lay eggs in dormant buds, which results in the formation of a minute, light tan gall the following spring after budbreak. Each gall contains a single wasp that emerges in mid-late spring. After emerging, females lay eggs on green stems, which results in the characteristic large, rounded galls that are initially pale brown and soft but gradually darken and harden. Pupation occurs in the center of the gall, and adults emerge in fall. Galls formed on oaks (Quercus spp.) and roses (Rosa spp.) are likely created by gall wasps.
Gall midges (Cecidomyiidae)
Gall midges are common in Colorado and can form a wide range of galls that vary in appearance, from woody, rounded galls on stems or leaves to woolly or mossy galls. These flies are small and have long, slender legs and antennae. The exact appearance and life cycle of the insect depends on the species. For example, adults of Dasineura gleditchiae, commonly referred to as the honeylocust pod gall midge, are about 2.5-3.2 mm (1/10-1/8 inch) long with two black stripes on a gray thorax. The larvae of D. gleditchiae are white or yellow maggots that measure about 2.5 mm (1/10 inch) long, while the pupae are about the same size but have oblong, orange, or white bodies.
Dasineura gleditchiae overwinters as adults near honeylocust trees. The following spring, adults emerge prior to budbreak and lay eggs on individual leaflets. Larvae develop within curled leaves where they feed and cause galls to form. Different species of gall midge feed on one or a few related hosts, which collectively include honeylocust (Gleditsia triacanthos), coyote brush (Baccharis pilularis), dogwood (Cornus spp.), Douglas-fir, ficus (Ficus spp.), oak (Quercus spp.), pines (Pinus spp.), willow (Salix spp.), sagebrush (Artemisia spp.), and creosote bush (Larrea tridentata).
Gall flies (Agromyzidae)
In Colorado, the poplar twiggall fly (Hexomyza schineri) commonly forms galls that appear as smooth, spherical swellings. Adults are shiny, dark flies with stout bodies that measure about 4.2 mm (1/6 inch) long. The larvae are green or yellow maggots that reside in the gall through the summer, with each gall containing two or three larvae.
Mature larvae overwinter within galls and most drop to the ground in late winter or early spring to begin pupating. When new growth begins forming, adult flies emerge from the pupae and tend to remain on the sunny sides of leaves during the day. After mating, females insert eggs in developing stems, and the larvae begin feeding after hatching which produces the distinctive swelling. Galls are often formed in the areas below buds, and the egg-laying sites become increasingly swollen as the stem grows. The poplar twiggall fly can attack aspen, cottonwoods, and poplars.
In addition to the formation of galls, these insects and mites can cause unsightly plant injuries that may reduce plant health. Feeding of eriophyid mites causes plants to have hairy or fuzzy pads on the surface of leaves and other plant structures. Infestations can also cause excessive production of buds, twig elongation, and other forms of broom-like growth in the host plant. Leaves can also roll and blister in response to eriophyid mites.
Cruciferous vegetables with high densities of the lettuce root aphid display wilting of foliage and fail to develop heads, resulting in yield reductions. Sustained high densities of these aphids can kill the plant over time as individual rootlets turn brown and die.
Infestations of D. gleditchiae can cause foliage to turn brown and drop prematurely, leaving entire portions of branches without leaves. Heavy infestations of the rough bulletgall wasp can cause galls to cover much of the infested twig, which reduces the host plant’s growth rate. After leaves drop, the galls can be easily noticed and appear as warts.
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