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.

Antistrophus silphii Gillette, 1891

Nomenclature: Antistrophus leavenworthi Bassett, 1900

Feeds on: Induces galls on Silphium integrifolium Michx. and S. perfoliatum L.

Distribution: United States: Illinois, Wisconsin

Antistrophus silphii

Silphium perfoliatum
Club-shaped subterminal stem swelling on stem of Silphium perfoliatum.

[Photo caption]

353. On Silphium perfoliatum. The adults emerged May 2-June 12 (Nebr).

Antistrophus silphii

Galls. — Abrupt sub-globular swellings from 1 to If inches in diameter at the tips of the stems of Silphium integrifolium and perfoliatum (Plate IX., Fig. 1). The inner portion of the gall is made up almost entirely of a rather dense pithy material that cuts with some difficulty. Interspersed through the gall are numerous oval larval cells, and also open spaces or cavities that do not contain insects. (Plate IX.,Fig. 2.) The larval cells are not woody, as is usually the case in cynipidous galls, but their walls are of pith like the sur-rounding gall substance.

These galls are very common in the vicinity of Champaign, on stems of Silphiiim integrifolium, and Mr. Hart had collected similar galls at Normal, III, from S. perfoliatum, from which flies were reared that were in every way identical with those from galls of the other species.

Gillettte described this species as forming abrupt, large (2.5-4.5 cm) subglobular swellings at the tips of the stems of rosinweed and cup-plant. Each gall supported may wasp larvae.

Reported discovery of A silphii galls in Wisconsin.

This rather uncommon gall appears at the tip of a growing stem of Cup Plant. It is green and often irregular in shape. The gall growth strongly re- sembles an unopened flower bud. (KB)

Aylax leavenworthi (Bassett)

Antistrophus leavenworthi BASSETT
Aulax leavenworthi KIEFFER
Antistrophus silphii GILLETTE
Aulax silphii

Gall. (Plate XIV, Figs. 4, 5.) On the tips of the stems of rosin-weeds (Silphium integrifolium and Silphium perfoliatum). Polythalamous. Abrupt, subglobular, tumor-like swellings, usually stunted with aborted growths of leaves and stems. Green when fresh, dark brown and woody when dry. Internally (fig. 5) it is composed of white pith in which are many larval cells. Length, 30-75 mm.

Habitat. Illinois (Champaign and Normal); Virginia (Petersburg); Mts. of Georgia (Macon Co.).

The types of A. leavenworthi and A. silphii are before me and I cannot find any differences between the two. The gall on Plate XIV was made from one of Prof. Gillette's types of A. silphii. The type of A. leavenworthi is in the collection of the American Entomological Society and the gall is in poor condition.