By Jodi Zirbel
Horned oak gall (Callirhytis cornigera
Photo by A. Steven Munson, USDA Forest Service, Bugwood.orgWhat does it look like?
Although causes vary, all galls are characterized as abnormal plant growths, resulting from an adverse reaction between the plant and chemical leave-behinds from an insect, mite or fungus. Most often, galls are characterized as “tumor-like,” varying from spheres to tubes in a variety of colors and sizes. The surface of the galls also varies, ranging from smooth to hairy to spine-covered. These growths typically occur on leaves and twigs, but can also form on the tree trunk, flowers, buds, fruit and roots.
Host material and range
It is estimated that there are more than 2,000 gall-producing pests in the United States. The majority of these insects fall within a species of either gall gnats or gall wasps and many of these insects — about 60 percent — affect the oak family.
Two very common species that target oak trees are the horned oak gall wasp, Callirhytis cornigera, and the gouty oak gall wasp, C. quercuspunctata. Their range extends throughout the United States impacting specific oak varieties. In particular, the horned oak gall affects black, blackjack, pin and water oaks while gouty oak galls form on black, pin, red and scarlet oaks.
Although oak galls are seldom fatal, their presence is often aesthetically unpleasing and may cause otherwise healthy leaves to turn brown or drop prematurely. In some circumstances, the sheer weight of the galls can cause twigs to droop, as well as branch dieback.
The formation of galls is a relatively complex process that takes up to three years to develop. The process starts in spring when buds first begin to emerge. At this time, adult wasps use the expanding plant tissue to lay their eggs. At some point during the reproduction process, a secretion chemical negatively interacts with the host plant, causing an abnormal growth or gall. As the gall develops, it provides a safe, confined area for the larvae to grow, as well as a source of nutrients. In fact, once the gall forms, the insect no longer needs to feed on the host plant.
It is not uncommon for galls to grow more than two inches in diameter, which can take two or three years to occur. After adults emerge from the galls, they lay eggs on new twigs and the process continues.
Gouty oak gall (Callirhytis quercuspunctata)Photos by Jim Baker, North Carolina State University, Bugwood.orgPreventative treatments aren’t usually recommended; however, a variety of techniques can be employed once damage is detected.
Once a gall has formed, it is difficult to reverse its development. However, one of the most highly recommended courses of treatment for lightly affected trees is to remove and destroy infested plant material. If galls are found, prune affected twigs and collect fallen leaves which may contain the gall-producing wasps.
Because galls are rarely fatal, pesticide applications are usually not recommended to treat oak gall. If a spray insecticide application is used, look for products containing bifenthrin or chlorpyrifos, and time treatments for early spring to coincide with female wasps emerging from galls. The goal is to treat before they’ve laid their eggs on fresh buds. Another course of treatment is the use of a trunk injection application with concentrated solutions of bidrin, which target developing larvae and should take place in mid-spring.
What can you do?
Monitor trees early in the growing season to look for signs of infestation. If detected, quickly remove those isolated areas where the wasps reside.
Jodi Zirbel is with Epic Creative, Wis. Article provided by Mauget, a leader in micro-injection and micro-infusion tree care. Contact Mauget or visit www.Mauget.com to learn more about ways to treat oak galls and tips on how to prevent gall-producing insects.