For more detailed information, read and download the free Extension publication, Gall-making Insects and Mites – E-397.

Cynipid wasp emerging from an oak gall

In early spring, when oak tree leaves first appear, species of gall wasps (Hymenoptera: Cynipidae) from last year’s galls emerge, males and females mate and females insert eggs in expanding leaf tissue. If successful, the presence of developing cynipid wasp larvae stimulate development of galls – often specific in shape and color to a particular species. Once galls begin to form, insecticides are ineffective in stopping growth or affecting developing larvae.