Gall (Fig. 5) –Conspicuous, solitary, integral outgrowths of the stem; only rarely two galls occurring on the same stem. Polythalamous; subelliptical but occasion- ally more clavate to fusiform. Surface variable; smooth to rugose, with or without con- spicuous pubescence. Color green when fresh to brown when dry. Usually 3–6 cm in both length and diameter. Usually developing such that upward growth of the stem is halted, causing galled stems to appear much shorter than those without galls.
Secondary plant modifications –Among all gall wasp species observed in our study, Antis- trophus silphii is the most likely to be detrimental to plant health. Fay and Hartnett (1991) found that stems galled by A. silphii exhibited reduced growth and produced fewer flowers; our obser- vations clearly reiterate these suggestions as A. silphii galls clearly halt the upward growth of the stem and were frequently observed to entirely prevent development of an inflorescence (Fig. 5). We found that colonization rates of A. silphii can vary widely between fields and individ- ual plants, but usually at least 50% of stems are colonized per plant, while upwards of 70% of stems were galled in heavily infested fields (Murrell, personal observation). While we have not yet evaluated the prevalence of these modifications through quantitative data, it is clear that a very large number of silflower plants exhibited greatly reduced growth patterns resulting from the presence of A. silphii galls. Further examination should evaluate the relationship between A. silphii galls and seed yield, especially as silflower continues to be developed as an oilseed crop.
Remarks –Antistrophus silphii was previously known in association with Silphium integrifolium, S. perfoliatum L., and potentially S. speciosum Nutt. (Nastasi and Deans, 2021; Nastasi et al., 2024b). The origin of crop silflower plants, as well as additional observations detailed in our dis- cussion, confirm S. speciosum as a true host of A. silphii. This gall wasp has been investigated in the context of cryptic species or incipient speciation (e.g., Deng, 2019), but existing studies have yet to identify clear phylogenetic signals that would support these hypotheses. We examined the lectotype female deposited at INHS (Fig. 6A) and found it to be morphologically identical to specimens we reared from silflower, with the exception of differences in color due to aging of the lectotype. Several paralectotypes also agreed with this assessment.

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.

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
Host plants: Silphium integrifolium Michx., S. perfoliatum L.; possibly S. speciosum Nutt. (see Nastasi et al. 2024c [Catalog of Silphium insects]) (Asteraceae).
Distribution: Nearctic, Eastern and Midwestern USA.

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.