WASPS AND TREES: TO PARAPHRASE AN AGE-OLD QUESTION, WHICH CAME FIRST? THE CYNIPID WASP OR THE TREE GALL?

by Jessica Mora, Intern, Plateau Land & Wildlife Management

Recently there has been a movement for a greater understanding of the relationship between plants and insects, such as that between live oaks and a particular type of gall-forming cynipid wasp, Belonocnema treatae. Gall-formers are insects that lay their eggs on a host plant. The insect diverts the plant's energy, forming a hard substance that grows around the insect. This is called a gall. The wasp absorbs the tree's nutrients while growing within this protective shell.

This particular wasp goes through a sexual and asexual reproductive cycle. In the spring, newborn wasps hatch fully matured from root-galls, which are galls that form on the root of a tree. They then reproduce and lay their eggs onto the live oak leaf veins. These develop into tree galls, which are galls that grow on the leaves of the tree. After the insect emerges from the tree gall it leaves a scar on the leaf. In the fall, the wasp comes out of its tree gall and undergoes an asexual reproductive cycle. It then lays eggs on the trees roots, forming more root-galls. So, which came first, the cynipid wasp or the tree gall? We may never know.

These wasps act as parasites on live oaks, but some live oaks have developed a resistance to this specific gall-former invasion through a defense mechanism called hypersensitive response. The tree detects the foreign substance (the egg) being deposited on its leaf, and sends out a signal, which triggers a chemical reaction that causes localized cell death where the egg was deposited. This process leaves many holes in the leaves of the tree.

Research conducted at Texas State University indicates that about half of the live oak population in Texas has developed hypersensitive response. It also demonstrates adaptation within the process of plant/insect co-evolution.

Wasps lay eggs on live oak leaf veins, which develop into tree galls. Photograph courtesy Jessica Mora.