Throughout the summer you can find a variety of galls – the wool sower gall, the oak apple gall, the scrub oak gall and many others. Many galls, like the ones listed here, are found on oaks, which are common host plants for many gall-producing insects. 

But a few weeks ago, I noticed a gall on staghorn sumac (Rhus typhina) for the first time. If I hadn’t been inspecting the undersides of the leaves closely, in search of a butterfly that had just fluttered in that direction, I would have completely missed the surprisingly large pendulous galls present under the leaves. Cloaked in the same green as the leaves, they were almost invisible from the trail. 

These turned out to be galls formed by the sumac gall aphid (Melaphis rhois). Also known as the staghorn sumac aphid, it is the only species in the genus Melaphis. M. rhois is a type of woolly aphid, which are sap-sucking insects that produce a fuzzy-looking waxy covering that resembles cotton or wool. The sumac gall aphid is one of the few aphids that induce the formation of galls – most galls are the result of wasp, midge or sawfly activity.

The sumac plant is triggered to produce the gall when a female aphid lays a single egg on the underside of the leaf. To cordon off the intruder, the leaf then forms a large sac around the egg – the ones I found were approximately an inch and a half in diameter. Although the galls are some of the biggest I’ve seen, the aphids do not significantly impact the health of the host plant. However, if there are numerous galls, some leaves may change color or fall off early. Interesting, these galls turn red in the fall, just like the leaves, providing continued camouflage as the season progresses. 

The sumac gall aphid has a complex life cycle, called alternation of generations, where summer generations are produced within galls on the sumac plant, while winter generations are produced within colonies of moss beneath or near the sumac. The initial gall-producing egg laid on the sumac leaf will result in a single female aphid. This female then reproduces asexually within the gall, through a process called parthenogenesis, creating enough clones of herself to fill the gall. If the gall “matures” without be predated, these cloned females will be released at the end of the summer or early fall and drop onto mosses below where they will pass the winter, establishing asexually reproducing colonies. Ultimately, these moss-dwelling aphid colonies produce the males and sexual females responsible for recolonizing sumac each spring, restarting the cycle. Just another example of how complex and interwoven everything in the environment is.