Gall (Fig. 9) – Conspicuous, solitary, integral outgrowths of the leaf midrib; apparently restricted to the lower cauline leaves. Polythalamous; blister-like, with surface mostly smooth and without conspicuous pubescence. Occasionally present as multiple isolated galls along the same midrib. Color green when fresh to yellow or brown when dry. 0.25–3 cm in length and usually around 0.25–1 cm wide; occasionally with clusters reaching across half the leaf midrib or more and thus appearing up to 8 cm in length.

Secondary plant modifications – We did not notice any obvious changes to the host plant resulting from the presence of galls induced by this species; leaves bearing these galls appeared identical to other cauline leaves on the same plants excluding the presence of the gall itself. Given that basal and lower cauline leaves in crop silflower and related taxa shrivel and break away during the growing season (Brock and Weakley, 2020), galls of this species likely do not persist on the host plant for long after induction. Further, this gall was quite rarely observed; only around ten galls were located over the several years of sampling. A lack of obvious secondary plant modifications coupled with narrow phenological opportunity to impact the host plant and low abundance suggests that these galls do not have detrimental impacts on plant health or crop yield.

Remarks – Antistrophus sp. 2 differs morphologically from all known Antistrophus species and species complexes (Table 1). This, along with the highly unique gall of this species among Silphium gallers, suggests that Antistrophus sp. 2 is an undescribed species that is in need of further study (Nastasi et al., 2025).