Last year, on October 1st, I posted a BYGL Alert about small, fuzzy, reddish-brown to deep-red galls appearing on oaks in northern Ohio. The sheer number of the fuzzy oak leaf galls last year coupled with their alien appearance drew the attention of landowners as well as the news media once the galls started detaching from the oak leaves to rain down on sidewalks, decks, and parked cars.
The galls are produced under the direction of the gall-wasp, Callirhytis furva (family Cynipidae). I ran across a few of these oak leaf galls earlier this week on a shingle oak (Quercus imbricaria) in southwest Ohio. It’s the same tree where I found this gall makers handiwork last season. However, the population was not nearly as high as was observed and reported last year in the BYGL.
Neither the wasp nor its unusual hairy-looking gall has common names that are approved by the Entomological Society of America (ESA). You'll find a range of common names applied to this gall online from "plush galls" to "hairy oak galls." However, the galls bear a striking resemblance to a miniature version of Tribbles; the fictional alien species (truly) that overran the Enterprise in the 1967 Star Trek episode titled "The Trouble with Tribbles." As a Trekkie, I've taken advantage of the ESA common name being up for grabs.
Tribble galls may occur on the upper or lower leaf surface of oaks in the red oak group. They may arise singly to truly resemble their (proposed) namesake or in clusters to demonstrate what the fluffy aliens did to the Enterprise. They detach from the leaves of their oak hosts to apparently allow the gall-making larvae direct access to the soil where they pupate before winter.
Adult female wasps emerge in the spring to mate and sip plant nectar which gives them the energy to fly to developing leaf buds where they lay their eggs. Their ovipositors (ovi = egg; positor = deposit) are used solely for depositing eggs; they are not modified into stingers for defense. So, these wasps can't sting.
Don’t Fear the Tribbles or Any Other Oak Leaf Gall
The sole purpose of a plant gall is to house, protect, and nourish immature gall-makers. The vast majority of cynipid wasp galls, including tribble galls, cause no appreciable harm to the plant hosts. In fact, plant galls are considered an important strand in the fabric of many terrestrial ecosystems.
There are many odd-looking wasp galls that can develop on oak leaves. Some “mature” to release their gall-maker early in the season while others like the tribble galls mature late in the season. Some remain firmly attached to the leaves while others drop from the leaves; sometimes dramatically.
In general, galls in Ohio that occur on oaks in the red oak group are not found on oaks in the white oak group, and vice versa. Some gall-makers are confined be even narrower host ranges. This is important to consider with identifying the galls.
The location on the leaf is also important for identifying galls on oaks. Some galls only appear on the lower leaf surface, others on the upper leaf surface. Some like the tribble galls can appear on both the upper and lower leaf surfaces often at the same point. Some galls arise from the leaf veins while others appear between the veins, or the galls may randomly appear across the leaf.
As demonstrated last season with the trouble with tribbles, we occasionally see outbreaks for reasons that are not well understood. Oddly, these outbreaks may occur over large geographical regions even though the gall-wasps are very tiny insects.
Just as mystifying, the occurrence of a particular gall usually drops to almost undetectable levels the year after an outbreak, and they remain rare for many years. Last year, we predicted that it was unlikely the tribble galls would reappear in large numbers this season in areas where they were common. Of course, this prediction remains untested; outbreaks may have not yet been reported.