This time of year, with the leaves still absent, it’s hard not to notice the tan spheres attached to the ends of some oak branches.  These ball-like objects are oak apple galls, and can range in size from 1 to 2 inches in diameter (approximately golf ball sized).  A “gall” is a general term for a plant deformity caused by an insect or a fungus.  These oak apple galls are caused by gall wasps in the family Cynipidae. Female wasps will lay a single egg in a developing leaf bud or on the central vein of an existing leaf.  Chemicals and hormones secreted by the egg cause the oak to produce the gall, likely in an attempt by the plant to isolate the source of irritation.  The larva can then feed on the gall tissue produced in reaction to these secretions, as well as find shelter and protection inside this orb that the oak has produced around it.  When the larva matures into an adult later in the summer, and exits the gall, it leaves a sheltered vacant space that other insects will occasionally utilize as well.

By winter, these galls are brown crunchy balls, with a small holes in them (where adult wasp that developed from the larva exited the gall), but in the summer when they develop, they are green and a little rubbery. Inside, the larvae are located in a dense structure in the center of the gall, and spongy fibers radiate out of this central point and attach onto the gall’s external shell to hold the larva in place. The remains of this center spongy structure can be seen in the photo below, which shows the inside of the gall.

Although there are a number of gall forming wasps, one of the most common in eastern North America is Amphibolips confluenta.  To take a look at this tiny (~5mm) gall-forming wasp, check out this Bug Guide page here.