Gall (Fig. 3) –Conspicuous when in larger numbers occur within a single flowerhead; concealed within flowerhead in smaller numbers. Solitary to confluent, integral outgrowths of the disc florets, forming clusters at base of flower and fusing florets to the receptacle when numerous. Individual galls monothalamous; subelliptical; occasionally curved to clavate when heavily distorting the floret. Surface mostly smooth except for unmodified floret retained above gall tissue. Color yellow to green when fresh and tan to brown when dry. Individual galls 0.5–1 cm in length including retained floret tissue and about 0.25 cm in width; dimensions more variable when confluent but clusters usually less than 3 cm in diameter.

Secondary plant modifications –Inflorescences galled by A. bicolor are often superficially identical to those without galls. Heavily galled inflorescences may be identified by the presence of abundant modified florets (Fig. 3). The male florets in the center of the inflorescence appear to be more commonly galled than the female ray florets (Murrell, personal observation), but direct or indirect effects of infestation level on seed set have not yet been measured. Beyond alterations to the flowers, plants with abundant A. bicolor galls are not readily identifiable.

Remarks –Henderson et al. (in review) found that flower gall wasps on S. integrifolium represented A. bicolor Gillette rather than A. laciniatus as originally reported by Buffington et al. (2017). Antistrophus laciniatus is restricted to other host plant species and is morphologically distinct from A. bicolor. We compared newly reared adult wasps from flower galls with additional specimens of A. bicolor, including the holotype (Fig. 4A) and non-type specimens reared during previous studies (Buffington et al., 2017, Henderson et al., in review). All specimens were morphologically identical. While Buffington et al. reported A. bicolor from S. integrifolium, their material was also collected from the same crop silflower fields as we did in this study, albeit at different stages in the domestication process. Further consideration of taxonomic placement of crop silflower and other entities related to S. integrifolium is given in our discussion. A comprehensive discussion of flower-galling Antistrophus species, including identification keys and DNA barcode data, will be given in a forthcoming revisionary study of Aulacideini (Nastasi et al., in prep.).

Antistrophus bicolor Gillette, 1891

Feeds on: May induce galls on Silphium integrifolium Michx. but claim is unsubstantiated

Distribution: United States: Illinois

Antistrophus bicolor

Described from one specimen from Normal IL. Type dated July 6 1884. Accession catalogue says: From Silphium integrifolium."

Antistrophus bicolor n. sp.

Gall unknown.

[Antistrophus] bicolor Gillette, 1891
Host plant: Presumably a species of Silphium L. (Asteraceae) (see Nastasi and Deans 2021; Nastasi et al. 2024c). Distribution: Nearctic, Illinois (USA).

Gillette did not offer much information about this species. He described it from single specimen from Normal, Illinois. Bothe the host plant and gall of the wasp were unknown to him. Seventy years later, Weld reports the collection date of Gillettes's type specimen as being July, 6, 1884 and the accompanying accession catalogue as stating "from Silphium integrifolium".

Aylax bicolor (Gillette)

Antistrophus bicolor Gillette
Aulax bicolor Kieffer

Habitat. Illinois (normal).

The type of this species was sent to me for examination by Mr. C. A. Hart. In general appearance and color it is like Aulacidea bicolor, but the parapsidal grooves are different, the radial area is open at the costa, and the cubitus almost reaches the first cross-vein. The gall is not known.