Photo Gallery of Insects and Mites on Ornamental Plants
Some of the more spectacular galls of oaks occur in spring and early summer. Gall wasps have alternating generations in which the offspring resemble the grandparents, not the parents. Not only that, but the galls of the two generations are usually quite different. One generation produces stem galls in which development takes two or three years and the other generation produces leaf galls in which development takes less than a growing season. Adults sometimes emerge from the stem galls in mid-winter and lay their eggs in the buds. Adults from leaf galls emerge in late spring and lay their eggs in stems (or roots or immature acorns).
The wool sower gall is a distinct and unusual plant growth induced by the secretions of the grubs of a tiny gall wasp. If a fresh wool sower gall is held in a plastic bag out of the sun so it will not get too hot, within a week or three weeks tiny gall wasps will emerge. These wasps are harmless to people. The wool sower gall is specific to white oak and only occurs in the spring. When the gall is pulled apart, inside are small seed-like structures inside of which the gall wasp grubs develop. This gall is also called the oak seed gall. Fortunately, wool sower galls are hardly ever abundant enough to cause real harm to white oaks. The eggs hatch just as the new growth emerges in spring and the leaf galls form quickly. Gall wasps invariably have alternation of generations in which one generation develops in one type of gall (leaf gall) and their children develop in another type (stem gall). Wasps of each alternate generation are slightly different is size, and the galls of each generation are enormously different from the parents. The wool sower gall is probably the leaf gall of this species because of its transient nature; the stem gall is unknown.
Roly-poly galls are caused by larvae of a gall wasp in the genus Dryocosmus. Roly-poly galls are also called succulent oak galls. The leaf galls of the roly-poly gall wasp are amazing in that the wasp grub develops inside a seed-like structure that rolls around freely inside the hollow sphere of the gall, hence the name roly-poly galls. The biology of roly-poly galls has not been worked out in detail.
Small, leafy or flower-like galls on live oak are caused by the gall wasp, Andricus quercusfoliatus. Females lay their eggs in the buds which causes the buds of live oaks to form the galls inside of which their larvae mature. Dryocosmus floridensis causes a leafy gall on red oak.
Various sorts of knot galls, horned galls, potato galls, and gouty galls on the leaves and twigs of oaks are caused by gall wasps in the genus Callirhytis. These tiny wasps apparently lay their eggs on the limbs of oaks and their larvae cause galls which eventually may cause the stem to die from the gall outward. The gall wasp, Callirhytis quercusoperator, causes woolly galls on the catkins of oaks in which their larvae develop inside seed-like structures. When the new wasps emerge, they lay their eggs in immature acorns. Callirhytis claviger causes an unattractive stem gall on willow oak. Callirhytis crypta forms stem galls on blackjack oak. The stems are slightly swollen and if the gall wasps have emerged, their emergence holes are also noticeable.
Galls wasps in the genus Neuroterus have some of the most unusual galls. Neuroterus irregularis causes an irregular gall on the leaves of post oak. Neuroterus saltarius forms tiny galls on the leaves of post oak that are dehiscent, that is, they drop off of the leaf. A sunken scar marks the spot of the gall. If enough galls form on a leaf, the leaf may die back. This year David Goforth, County Agent, Cabarrus County, reported that so many Neuroterus galls had dropped out of one tree that the ground appeared to be covered with sawdust!
Philonix nigra causes small, spherical, fuzzy galls on the leaves of northern red oak.
Large oak-apple gall wasps form galls on the leaves or leaf petioles of various red, black and scarlet oaks. These galls are up to two inches in diameter, are green tinged with red when fresh, and gradually turn brown.
If galls are actually damaging trees, the best time to try to control the wasps is in mid-winter when the wasps are laying their eggs or in spring just as the buds are breaking. By the time stem galls are noticed, it is too late to effectively control the gall wasps. With pests such as the gouty oak gall wasp, it is not possible to control the stem gall generation, perhaps because the tissue surrounding the larvae is so disrupted by the influence of the larvae that the tissue does not translocate the pesticide to the larvae in large enough quantities to kill them. Because gall wasps have a leaf gall generation as well as a stem gall generation, spraying the new leaves should disrupt their life cycle and after a year or three years, the number of new galls should decline. It would be a good idea to take a soil sample from under the infested trees and submit it to the North Carolina Department of Agriculture and Consumer Services Soils Laboratory. If the pH or nutrients are out of balance, the soil should be amended, but not over-fertilized. During periods of prolonged drought, heavily-infested trees could be irrigated. Round Upping the grass under heavily-infested trees and mulching the soil should help conserve soil moisture and keep the roots cooler. Any means within reason that can enhance the vitality of the trees should be tried. It is known that trees under stress have more simple sugars (rather than starches) and more free amino acids (rather than more complex proteins) in the sap. Thus stressed trees are more nutritious to the gall wasps than healthy trees. Providing an infested tree with the optimum growing condition should make it better able to compensate for the damage caused by the galls and less susceptible to the gall wasps. Gall wasps also have parasites and often gall wasp populations decline naturally. Ornamentals and Turf Insect Information Note Number 5 explains the biology of gall-forming insects.
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Last modified on May 17, 1999 by Stephen J. Toth, Jr.