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Joseph F. Boggs, Ohio State University Extension, Hamilton County/Southwest District; Curtis E. Young, David J. Shetlar, James A. Chatfield, Erik A. Draper, Pamela J. Bennett, Gary Y. Gao, |
Gall Makers
While a number of eriophyid mite species were found in abundance in Ohio landscapes, the most dramatic example this past season was the ash inflorescence gall produced by the mite, Eriophyes fraxinivorus. While these galls look like small witches' brooms growing just beneath the leaves, they cause no injury to the tree since only the flower parts are affected.
The USDA Agriculture Handbook Number 573, An Illustrated Guide to Plant Abnormalities Caused by Eriophyid Mites in North America, gives an excellent description of this gall. The narrative reads: "The infested inflorescence shows swollen and fused pedicels of individual flowers, often involving the peduncle or stalk. There is considerable distortion, and when the infestation is severe, every inflorescence is galled. Fully developed galls are conspicuously brownish, lumpy, and unsightly, usually remaining attached to the twigs from one season to the next."
As with previous seasons, samples of oak galls were common in OSU Extension offices throughout the state. A few of the more prevalent types found in Ohio landscapes this past season included the woolly leaf gall, the wool sower gall, the jumping oak gall, and the oak spangle gall. All are produced by tiny wasps in the family Cynipidae, and none cause significant harm to the host tree.
As their descriptive name implies, the woolly leaf gall looks like someone stuck a dense wad of light brown wool to a leaf vein. The galls are produced by the cynipid wasp, Andricus fullawayi, and are found on the underside of leaves where they are usually attached to the midvein, although they occasionally arise from lateral veins. They range in size from pea-sized to the diameter of a quarter. When oak leaves turn color in the fall, the galls often detach and fall to the ground. Large numbers littering the ground beneath an oak tree may cause concern, but no harm has been done to the tree.
The wool sower gall looks similar to the woolly leaf gall. However, it arises from the twigs. The galls are produced by the cynipid wasp, Callirhytis seminator. Immature galls can be cut open to reveal the interesting seedlike structures that contain the immature wasps. The off-white, woolly galls may expand to 1-2" in diameter. As with the wooly leaf gall, once the wasps mature and emerge as adults, the gall drops from the tree.
Jumping oak galls are produced by the cynipid wasp, Neuroterus saltatorius. This is by far the most entertaining of the oak galls. These light-tan, globular galls are about the size of a sesame seed and hang from the underside of the leaf. Each gall contains a single larva. When the galls mature, they drop from the leaves carrying the wasp larva to the ground. Once on the ground, they display an interesting behavior that gives them their common name. Larval activity causes the galls to jump around, as high as several inches! There has been no clear explanation for this unusual behavior, but it is speculated that the jumping activity carries the galls into soil crevices where the larvae are protected for the winter.
The flattened oak spangle galls resemble the spangles sown onto costumes. They form on the underside of the leaves and are attached at their centers by a tiny, post-like structure. The overall effect makes the leaves look like they are sprouting octopus suction cups! Immature galls are white to whitish-green. Mature galls are approximately 3/16" in diameter and as they mature, the galls turn pinkish-white and eventually purplish-brown. Mature galls detach from the tree carrying cynipid wasp pupae to the ground where they overwinter. Large numbers of galls may detach during rain storms and collect in puddles beneath infested trees. For example, dramatic displays of piles of the purplish-brown, saucer-shaped galls were found washed into gutters after a heavy rain this past season in southwestern Ohio.