On October 1, 2020, I posted a BYGL Alert titled, “Are Oaks Raining Tribbles?” The Alert focused on small, fuzzy, reddish-brown to deep-red leaf galls detaching from oaks in Ohio, primarily in the northern part of the state.
The galls are produced under the direction of the gall-wasp, Callirhytis furva (family Cynipidae). Neither the cynipid wasp nor its unusual hairy-looking gall has a common name that’s been approved by the Entomological Society of America (ESA). You'll find a range of common names online from “furry oak galls” to "plush galls" to "hairy oak galls." None carry any official recognition, so one name is as good as another.
However, I believe the galls bear a striking resemblance to a miniature version of Tribbles; the fictional alien species that overran the Enterprise in the 1967 Star Trek episode titled "The Trouble with Tribbles." As a Trekkie, I've taken advantage of the lack of any common name oversight by applying the common name “oak tribble galls.”
Oak tribble galls occur on the upper or lower leaf surface of oaks in the red oak group. They may arise singly to truly resemble their tribble namesake or in clusters to demonstrate what the fluffy aliens did to the Enterprise.
The tribble galls detach from the leaves of their oak hosts in early fall to allow the gall-making larvae direct access to the soil where they pupate before winter. The sheer number of oak tribble galls in 2020 coupled with their alien appearance drew the attention of landowners as well as the news media from Maine to Ohio once the galls started detaching from the oak leaves to rain down on sidewalks, decks, and parked cars.
Adult female wasps emerge from the soil 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.
I ran across a few oak tribble galls late last week on a pin oak (Quercus palustris) in southwest Ohio. Amy Stone (OSU Extension, Lucas County) has reported receiving a question about these galls in the Toledo area. Although the populations do not appear to be nearly as high as was observed in 2020, it may be too early to tell because the galls have not yet started to detach from leaves. Gall detachment triggered the trouble with tribbles in 2020.
Don’t Fear the Tribbles or Other Oak Leaf Galls
The sole purpose of a plant gall is to house, protect, and nourish immature gall-makers. The vast majority of cynipid wasp galls, including oak 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 oak tribble galls mature late in the season. Some remain firmly attached to the leaves while others drop from the leaves; sometimes dramatically.
Other cynipid wasp oak leaf galls that are notable for detaching late in the growing season include Wooly Oak Galls produced under the direction of C. lanata. The galls appear on oaks belonging to the red oak group most commonly on pin oaks. Like the oak tribble galls, these galls appear as a fuzzy mass consisting of individual galls each housing a single wasp larva.
Clustered Midrib Oak Galls produced under the direction of Andricus dimorphus look like clusters of tiny red berries. The cynipid gall wasp targets the leaves of oaks belonging to the white oak group. As their common name implies, the galls sprout from the central leaf vein (midrib) on the underside of the leaves. Each gall contains a single wasp larva, and the galls are beginning to detach from the leaves of their oak hosts.
As demonstrated with the trouble with tribble galls in 2020, we occasionally see outbreaks of a particular type of oak gall for reasons that are not well understood. Oddly, these outbreaks may occur over large geographical regions including multiple states even though the gall wasps are very tiny insects. Just as mystifying, the occurrence of a particular gall usually drops to almost undetectable levels over large regions the year after an outbreak, and they remain rare for many years.