General Information
CAUTION: This information is for North Carolina and may not apply to other areas.
Some insects and mites cause unusual growths on plants called galls. Galls may form on any part of the plant from the flowers, leaves and stems to the roots. The abnormal growths developing in plant tissue are due to the powerful enzymes given off by the immature gall-forming insect as it grows. The plant tissue is remarkably altered even to the point of replication of chromosomes without cell division.
Most galls are formed by three kinds of insects or mites: gall wasps, gall midges, and gall mites. Other less common gall producing insects are aphids, psyllids, and gall flies. Since most galls seem to do no permanent damage to their host plants, limited research has been done on the biology or control. This note is about galls formed on oak by gall wasps. Gall wasps are small, brown wasps with the abdomen flattened side to side.
Many gall wasps develop for 2 or 3 years in woody galls on the twigs of oaks. Adults then emerge from the twig galls during the winter. They lay eggs in the buds and die. When these eggs hatch, and new growth resumes on the oak, salivary secretions of the gall wasp grub act as powerful plant growth regulators that force the tree to form the gall. Gall wasp galls typically have an outer wall, a spongy fiber layer and a hard, seedlike structure inside of which the gall wasp grub develops. Although gall wasp grubs have chewing mouthparts, they do not seem to chew plant tissue. Evidently the gall secretes nutrients that the grubs lap up.
The wool sower gall is caused by secretions of grubs of a small gall wasp, Callirhytis seminator. The wool sower gall is specific to white oak and only occurs in the spring. The galls contain seed-like structures. The gall wasps develop inside these structures. (This gall is also called the oak seed gall.) Fortunately, wool sower galls are usually never abundant so that the health of infested trees is rarely threatened. These wasps probably lay their eggs in midwinter and the eggs hatch just as the new growth emerges in spring. By the time the galls are noticed, it is too late to effectively control the gall wasps. Wool sower gall wasps probably have an alternate generation of wasps which develops in galls in the buds, twigs or on the leaves.
Roly-poly galls are caused by gall wasps in the genus Andricus and are called roly-poly galls because the wasp grub develops in a seedlike shell and nutritive layer structure loose inside the hollow gall. The roly-poly gall is probably an alternate generation for a twig gall not now recognized. The roly-poly gall is a very specialized gall as it has no spongy layer of plant fibers. Evidently the nutritive layer absorbs nutrients directly from the outer wall as it rolls around in the gall. Andricus coronus causes the crowned oak gall, another roly-poly gall. Roly-poly galls are not likely to cause significant plant injury.
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. One Extension agent reported that so many Neuroterus galls had dropped out of one tree that the ground appeared to be covered with sawdust!
Figure 1. The wool sower gall on white oak.
Figure 2. Immature gall wasp grubs induce their host plants to form protective galls.
Figure 3. The gouty oak gall wasp is typical of many of the gall wasps on oaks.
Figure 4. The abdomen of most gall wasps is flattened side to side.
Figure 5. These are "seeds" in a wool sower gall inside of which tiny gall wasps grubs develop.
Figure 6. Roly-poly gall on willow oak. The grub develops in a spherical "seed" that rolls around inside the hollow gall.
Figure 7. Roly-poly galls are not unusual on willow oaks in spring.
Figure 8. Andricus coronus, roly-poly gall.
Figure 9. Neuroterus irregularis galls sometimes greatly distort blackjack oak leaves.
Figure 10. More typical galls caused by Neuroterus irregularis.
Figure 11. Neuroterus saltarius galls on oak (arrows) and the scars left when the galls drop.
Figure 12. Bassettia pallida twig galls on live oak.
Figure 13. Callirhytis clavula caused this stem gall on white oak.
Figure 14. Amphibolips confluenta causes oak apple galls, a source of tannic acid.
Figure 15. Leafy oak galls are caused by Andricus quercusfoliata only on live oak.
Figure 16. Cynipid Amphibolips quercusjuglans, acorn plum gall.
Control
Usually by the time a gall is noticed, it is too late to carry out adequate control measures. In order to properly time spray applications for gall wasp control, collect some of the galls and store them in plastic bags outside, in the shade. When the small, dark wasps emerge, it is time to spray. If after a few weeks no wasps have emerged, then collect another sample and store them in the shade and wait. Gouty oak gall wasps (Figure 17) emerge from the stem galls in winter. Many of the wasps from leaves and succulent stem gall emerge in late spring or early summer. When the gall wasps emerge, spray the tree thoroughly. Sevin (carbaryl) pesticide is labeled for gall wasp control on shade trees and park trees. Do not allow public use of treated areas during applications or until sprays have dried. These formulations are suitable for home use, however, chemical control is rarely effective and often not worth the expense. These galls are generally not life-threatening to the tree. Damage is mainly cosmetic to a mature tree. Damage to young trees by stem galls in a nursery setting may cause weakened branches in a tree as it matures. Attempts at protective sprays may be warranted in such a setting.
Whether the gall wasps are causing tree decline or the declining health of a tree makes it susceptible to gall wasps is a matter of conjecture. Because most heavily infested trees are obviously declining, it is a good idea to try to improve growing conditions for infested trees. A soil sample from under an infested tree should be submitted it to the NCDA&CS Soils Lab. If the pH or nutrients are out of whack, the soil should be amended. (Don't just apply a bunch of 10-10-10. Excessive nitrogen is likely to make these and other pests worse!) During periods of prolonged drought consider irrigating the infested tree. "Round Upping" the grass under the tree and mulching the soil will help conserve soil moisture and keep the roots somewhat cooler. In other words, anything within reason that can be done to enhance the vitality of infested 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 likely more nutritious to the gall wasps than healthy trees. By getting an infested tree into better growing condition, it should be less susceptible to the gall wasps and the gall wasps will not reproduce as prolifically. As a consequence, the gall wasp population may die away naturally. In addition, gall wasps are plagued by parasitic wasps, which may also help the gall wasp population die away naturally.
Figure 17. Gouty gall wasps on oak.
References
- Common name: gall wasps, scientific name: Callirhytis quercusclaviger (Ashmead) and Callirhytis cornigera (Osten Sacken) (Insecta: Hymenoptera: Cynipidae). Dixon, W. N. 2015 (revised). Featured Creatures. Entomology & Nematology, FDACS/DPI, EDIS.
- Galls on Oak. Hoover, G. A., Sr. 2013 (revised). Dept. Entomology, PennState College of Agr. Sci.
- Biology and management of the horned oak gall on pin oak . Eliason, E. A. and D. A. Potter. 2001.J. Arbor. 27: 92-101.
- Horned Oak Gall. Billingsley, E. 2006. University of Illinois Extension.
- Spatial distribution and parasitism of leaf galls induced by Callirhytis cornigera (Hymenoptera: Cynipidae) on pin oak in Kentucky. Eliason, E. A. and D. A. Potter. 2001. Environ. Entomol. 30: 280-287.
- Understanding Oak Galls. Bratsch, T. No Date. University of Illinois Extension.
- NC State Extension Plant Pathology Publications and Factsheets
- NC State Horticultural Science Publications
- North Carolina Agricultural Chemicals Manual
For assistance with a specific problem, contact your local Cooperative Extension center.
Publication date: Nov. 4, 2002
Revised: Sept. 18, 2019
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