Walking Beau Diddley (my black lab) and blowing leaves today gave me an opportunity to glimpse into the world of leaf galls. Lots of organisms, both animal and disease, cause these bizarre structures to grow on the leaves and stems of various plants. In the case of the two shown here, they are cause by tiny wasps called cynipid gall wasps. There are thousands of different species of gall wasps, each creating their own characteristic type of gall and restricted to one or a few closely related host plants, but in each case the adult female wasp lays one or more eggs in the leaf—their “stinger” being used like a hypodermic needle to inject the eggs inside the leaf tissues (but completely unable to sting humans). When the eggs hatch, the larvae (called grubs) do not begin feeding directly on the existing leaf tissue, but instead secrete plant growth-like hormones that cause the plant to grow a specialized structure—called a gall—inside which the grub lives and feeds. It’s sort of like “Invasion of the Body Snatchers” on a micro-scale. When the grub has completed its development, it transforms into a pupa (kind of a wasp version of a butterfly chrysalis), and eventually the adult wasp emerges and chews its way out of the gall. It’s a marvelously elegant life cycle that goes unnoticed by most people.
©️ Ted C. MacRae 2021