P&PDL Picture of the Week for
July 5, 2010

Of All The Gall: Horned Oak Gall in Pin Oaks

Timothy J Gibb, Extension Entomologist, Department of Entomology, Purdue University

One of the dictionary definitions for ‘gall’ is as follows. Gall:  brazen boldness coupled with impudent assurance and insolence

Surely reading this would be enough of an insult that even a marginally, self-respecting insect such the gall wasp should feel enough shame to desist in the gall-making practice.  But apparently not so for the tiny gall wasp (Cynipidae: Callirhytis cornigera), that is responsible for the horned oak gall growths in pin oaks.

Most galls that form in oak trees, and there are quite a number, do not cause significant harm to the tree.  They are many and vary widely in size, shape and color. The galls are formed by the plant as a result of chemicals injected by these insects. The plant response is a gall, actually plant tissue, around the developing insects thereby providing them both food and protection.

Even though they may become a bit of an eyesore, galls are most often small and because they are plant tissue, they do not cause enough damage to be of any real concern.  Most times, they do not even justify any chemical or cultural treatment. 

Click image to enlarge

Gall

Horned oak gall

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service