Galls on oak leaves: I've been told over the years that these particular galls are caused by certain species of wasp. The gall tissue is created by a mature insect laying eggs in the budding leaves. A biochemical reaction causes the swelling to occur, and the larva develops in the hardening chamber within. In a way, the oaks foster parent the wasps. These images were taken September 30, 2012 in Lafayette, LA. You can tell by the pinholes in the galls that the young wasps have exited their cozy compartments, and entered the larger world without.