Antistrophus silphii species group
Rosinweed terminal stem gall wasps

Hosts: Whorled rosinweed (S. asteriscus var. trifoliatum), prairie rosinweed (S. integrifolium), and cup plant (S. perfoliatum).

Gall location: On the stem, almost always at the apex.

Description: Many-chambered, oval-shaped to spherical galls, 1-5 centimeters in length and usually of similar diameter. Often adorned with aborted leaves, and only rarely with the stem continuing beyond the end of the gall. Green and fleshy when fresh to brown and rough when dry. Both fresh and dry galls are rather hard and corky.

Range: This species group has been found throughout the Midwestern and Eastern United States. Each of the three hosts overlap over much of their range. They are apparently most common in the Midwest.

Terminal stem galls on prairie rosinweed and cup plant have long been considered the same species, A. silphii. However, it has long been suspected that galls on either host plant are induced by separate but closely-related species; the active periods of wasps on either host do not largely overlap, and gall size seems to vary between the two host. Terminal stem galls on whorled rosinweed were just discovered for the first time within the last year. Further research will unveil the true identities of gall wasps across the three host plants.

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