Antistrophus sp.

Hosts: slender rosinweed (S. gracile)

Gall location: In the disk flowers, often concealed by the outer parts of the flower.

Description: Oval-shaped, kernel-like, single-chambered galls, typically clustered at the base of the disk flowers. Galls are often hidden, but carefully teasing away the outer flower layers will reveal the galls at the center. In some host plants, the flower is disfigured by the presence of galls (see photo below).

Range: This species group has been found throughout the Midwestern, Eastern, and South Central United States. Like many other herb galls, this group probably occurs throughout the entire ranges of each species’ respective host.

Historically, it was believed that all flower galls found on rosinweeds were induced by a single species, Antistrophus laciniatus; this species was described by Clarence Gillette in 1891. However, molecular evidence, along with careful morphological observations, suggests that there are several distinct flower gall wasp species on rosinweeds, each of which uses only a subset of the host plants from which flower galls are known. The identities of several of these putative species are still being examined; for instance, it is still unclear whether flower gall wasps on prairie rosinweed (Silphium integrifolium) and cup plant (S. perfoliatum) are induced by one species of gall wasp, or two separate species, each restricted to one host plant.

An additional species, Antistrophus bicolor, may be a rosinweed flower gall wasp, as it closely resembles other species in this complex. However, this species is known only from a single female collected in the late 1800s, and its host plant is not known. Hopefully, further collecting and rearing of disc flower galls on rosinweed will allow a verifiable association of this species and its true host plant.

- Louis Nastasi, Charles Davis: (2022) Field Guide to the Herb and Bramble Gall Wasps of North America^©