Hedgehog Gall

This odd looking protrusion was found attached to the leaf of a burr oak tree. I had no idea what it was so I placed a photo of it on facebook and soon had an answer of something called a Hedgehog Gall. The name is very apt considering its appearance. It looks somewhat spikey, but is actually very soft and velvety to touch. Hedgehog galls are formed by a tiny gall-forming insect called a Hedgehog Gall Wasp (Acrapis erinacei). The lifecycle of most gall wasps is interesting and sometimes complex, and hedgehog gall wasps are no different. Mating takes place in the spring and the female will lay eggs on the leaves of various species of white oaks, like our burr oak. When the eggs hatch they irritate the tree causing it to form a gall that essentially protects the young larvae. Each gall may contain up to 5 larval cells. The larvae feed on the nutritious insides of the gall that the tree so kindly created for them.....well kindness probably had nothing to with it, but nonetheless the tiny gall wasps are provided all the nutrients they need as they grow inside the gall. As the insect feeds and grows so does the gall itself. This particular gall is approximately 1/2 inch in diameter and yellow in color with orangish colored hairs covering the surface.

When the larvae are ready to leave the protection of the gall they will emerge as adult females (no males). These unbred females will lay eggs on the leaf buds of the host tree. These eggs overwinter and hatch sometime in April or May. The newly hatched larva will develop inside other galls and emerge many weeks later as both males and females which will mate and begin the cycle over again. See, I told their life cycle was somewhat interesting and complex.

Galls have always fascinated me and I never get tired of finding them and I am always curious what type of insect created them. Most insects that form galls are so tiny as to go unseen by humans. Unlike larger, more defensive wasps, gall wasps are harmless to humans. Most are also harmless to trees, even if the tree is covered in a significant amount of galls it doesn't seem to have any ill effects on the trees overall health. If the tree is young and the number of galls is disproportionate to the size of the tree then it may stunt the tree or possibly kill it, but that would be rare. The wasp depends upon the host plant to survive, so it is not in the best interest of the wasp to destroy the tree by overpopulating it with offspring and galls to the point of damaging or killing the host. This spring as the leaves begin to open look for the many types of galls that are present and see if you can discover who formed them.