Gall (Fig. 7)–Inconspicuous; integral chambers concealed within the stem. Apparent within internal tissue as small, ellipsoid chambers, each 2–4 mm long.
Secondary plant modifications–Almost every silflower stem we dissected (> 90%) contained dense numbers of the concealed galls of this species. Considering the imperceptible nature of the galls (often referred to as “cryptic galls” in the literature, e.g., Nastasi et al., 2024c), we noticed no obvious changes to the plants despite dense internal infestations. Tooker and Hanks (2006) found that dense infestations of Antistrophus rufus on Silphium laciniatum could delay flowering, reduce seed germination rates, and reduce mass of mature seeds. However, further study is needed to examine whether these impacts are induced by the silflower stem gall wasp.
Remarks–Morphologically, this species is a member of the Antistrophus rufus complex follow- ing diagnostic characters given by recent taxonomic studies (Tooker et al., 2004; Nastasi et al., 2024c; see also Table 1 below). Following the key to A. rufus complex species given by Nastasi et al. (2024c), our specimens do not route past couplet 3 based on the subquadrate mesoscutel- lar foveae, which occupy only the anterior third of the mesoscutellum, in combination with the POL being longer than the OOL. In comparing our specimens from crop silflower to the diag- noses and descriptions given therein, our specimens appear morphologically closest to Antis- trophus jeanae Tooker and Hanks, 2004. However, our specimens differ from A. jeanae by the dimensions of the ocellar triangle, especially by the POL, which is longer than the OOL in our specimens but shorter than the OOL in A. jeanae. We initially expected our specimens to be conspecific with or similar to Antistrophus laurenae Nastasi, 2024 due to crop silflower being closely related to Silphium integrifolium, the known host plant of A. laurenae (Nastasi et al., 2024c). This was not supported by morphological examination, as our specimens had the POL longer than the OOL (shorter than the OOL in A. laurenae), and had a much longer F2 at about 3.8 × as long as wide (3.3 × as long as wide in A. laurenae). Given that our specimens were mor- phologically distinct from all described species of the A. rufus complex, they probably repre- sent an undescribed species; further study including collection of genetic data would elucidate whether this is the case.