Horned oak gallmaker adult wasps have started to emerge in southern Illinois. Adults were observed on pin oak in the Effingham area on April 23, 2008. The wasps are too small to sting humans, being about 1/8 inch long, and are reddish brown, with clear, reflective wings that overlap and lie flat across the top of the abdomen. Very few adults were observed, which probably indicates that emergence had just begun. The pin oak buds had broken and elongated, but no leaves had yet emerged.
Application of bifenthrin (Onyx, Talstar) is recommended at this time to kill the egg-laying wasps if gall infestations are severe. Trunk injections of bidrin (Injecticide-B) or abamectin (Abacide) at this time should also be effective in killing the larvae in the leaves. Foliar sprays of the translaminar systemic insecticides acephate (Orthene), abamectin (Avid), imidacloprid (Merit), or dimethoate (Cygon) should be effective in controlling the larvae in the leaves once the leaves have expanded to at least 30 to 60 percent of full size. Dimethoate is an option only for nurseries.
The problem with any of these insecticide applications is that they greatly reduce the number of parasitic wasps that naturally keep this gall under control. Parasitic wasps and other competing insects in the galls typically provide 70 to 80 percent control of the gallmakers in leaf galls, and 20 to 100 percent of the gallmakers in stem galls. What this means is that in areas where the horned oak galls are very numerous, insecticide applications should be appropriate. However, in areas where the galls are not numerous, treating with insecticides may cause the galls to become more numerous. Where insecticides have been applied, scout trees in nearby natural and other areas where insecticides to control these galls have not been applied. When the number of galls in these untreated areas drops, reduce or stop treating trees. This allows the rising numbers of natural enemies to control the galls on your trees as well.
Heavy infestations of horned oak gall are found from the Mattoon area on south through Illinois, particularly in the eastern half of the state. Scattered infestations are seen north to the Champaign and Peoria areas of central Illinois. Galls on trees farther north may have been brought in on the tree. Without a nearby source of additional gallmakers in these northern Illinois trees, pruning off and destroying the galls probably eliminates the infestation without the use of insecticides. For additional information, refer to 2008 issue no. 1 of this newsletter.