GALLS on OAKS
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.
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
and 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 which 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. Roly-poly galls are not likely to cause significant plant injury. (Andricus
coronus at right.)
Cynipid Dyrocosmus floridensis
stem gall on red oak (left).
Stem
or flower-like gall (Andricus quercusfoliatus)
on live oak(right).
Oak Apple Gall Development - Cynipid wasp Amphibolips confluenta
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 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.
See also, the Pest
News article on Oak Galls .
Doug Caldwell's Note on galls on live oak.
Florida Insect Note and also .
References of interest:
Eliason, E. A. and D. A. Potter. 2001. Spatial distribution and parasitism of leaf galls induced by Callirhytis cornigera (Hymenoptera: Cynipidae) on pin oak in Kentucky. Environ. Entomol. 30: 280-287.
Eliason, E. A. and D. A. Potter. 2001. Horned oak gall biology and management. J. Arbor. 27: 92-101.
Eliason, E. A. and D. A. Potter. 2000. Budburst phenology, plant vigor, and host genotype effects on the leaf-galling generation of Callirhytis cornigera (Hymenoptera: Cynipidae) on pin oak. Environ. Entomol. 29: 1199-1207.
Eliason, E. A. and D. A. Potter. 2000. Biology of Callirhytis cornigera (Hymenoptera: Cynipidae) and its associated gall community in Kentucky. Environ. Entomol. 29: 551-559.
Published by North Carolina Cooperative Extension Service
Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University at Raleigh, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.
ENT/ort-05 May 1994 (Revised) November, 2002
Web page last reviewed January, 2008 by the webperson.
