Oak trees a favorite host for gall wasps

Hundreds of species of small wasps called gall wasps live in the forests of North America. Hundreds more species of them are spread worldwide.

In Southeast Missouri oak trees are a favorite host plant for gall wasps. A gall (hollow ball-like growth) will develop on an oak leaf after a female gall wasp lays one or more eggs inside the walls of a growing leaf.

It is not known for certain what causes the gall to develop on the leaf after the female wasp lays her egg, but it is likely a chemical reaction. It is known, however, that the gall both protects the wasp egg as it develops, and provides food for the wasp larva to survive upon.

In the interest of education, I have pinched off a portion of one of the galls in this photo taken shortly after rain April 24 revealing a single light blue gall wasp egg inside.

An interesting fact about gall wasps is that some species exist by seeking out galls made by others. The second female wasp will penetrate the wall of the gall and lay an egg upon the one already there. As the second egg grows into a larva it will parasitize the first.

Through the Woods is a weekly nature photo column by Aaron Horrell. Find this column at semissourian.com to order a reprint of the photo. Find more work by him at the O'Tenem Gallery.