Biology
Antistrophus rufus induces inconspicuous, externally imperceptible galls in stems of Silphium laciniatum (Fig. 8H) (Tooker et al. 2004; Nastasi and Deans 2021). Records associating A. rufus with other host plants most likely represent distinct species including the other species addressed in the present work.

Distribution
Antistrophus rufus was described from material collected in Illinois (USA) and has since been reported from Kansas (Tooker et al. 2004; Nastasi and Deans 2021). Additional material that we examined revealed additional records from Iowa and Wisconsin, and reiterated occurrence of this species in Kansas (Suppl. material 1: table 1). Known and potential distribution are summarized in Fig. 9.

Remarks
Kieffer (1902) synonymized Antistrophus with Aylax (=Aulax) Hartig, 1840. Some authors (e.g., Beutenmüller 1910) follow Kieffer’s synonymy, but others appear to have rejected or otherwise omitted this change from many works treating the group (e.g., Weld 1951; Nieves-Aldrey 1994). This generic synonymy and the resulting species synonyms were accidentally omitted from Nastasi and Deans’ (2021) catalogue, and we are unable to locate a nomenclatural act reinstating Antistrophus as a valid genus. The genus Aylax has now long been regarded to include only two species of gall wasps inducing galls on Papaver (Papaveraceae) (Nieves-Aldrey 1994; Ronquist et al. 2015). Additionally, the two genera are now placed in different tribes: Aylax is now placed in the tribe Aylacini, while Antistrophus is placed in the Aulacideini. We regard here Antistrophus as a valid genus independent of Aylax, although Antistrophus represents a heterogeneous assemblage as currently defined based on preliminary data from an ongoing revisionary study (Nastasi et al., unpublished data).

Antistrophus rufus Gillette, 1891

Feeds on: Induces galls on Silphium laciniatum L.

Distribution: United States: Illinois

Antistrophus rufus

Silphium laciniatum
Cells hidden in stem [shares its description and photo with A minor]

[Photo caption]

347. On Silphium laciniatum. Adults emerged May 15- June 24.

Antistrophus rufus Gillette

Larvae developing in galls in cambium and pith of flowering stems of S. laciniatum.

Biology. — Larvae develop within galls in flowering stems of S. laciniatum (for biology, see Tooker et al. 2002, Tooker and Hanks 2004a). They overwinter in dead stems, pupate in the early spring, and adults emerge in late May through late June. Although adults of both sexes have fully developed and functional wings, they do not fly readily but rather walk, occasionally taking hopping flights to nearby plants. Males emerge before females and search for mates on dead stems of the previous season, guarding areas of stem where females eventually emerge, and driving off competitors by charging or butting with the head. Females mate immediately upon emerging and then move to nearby developing stems of S. laciniatum where they oviposit in axils. As many as 20 females oviposit in a stem at the same time and require less than one minute to insert an egg. Egg load upon emergence averages —165 eggs per female. Densities of galls commonly reach hundreds per stem. Parasitic wasps are the most significant source of mortality for A. rufus larvae with rates of parasitism often exceeding 90% (Tooker and Hanks 2004a). Species of parasitoids reared from A. rufus galls include eurytomids (Eurytoma lutea Bugbee and an unidentified Eurytoma species), an ormyrid (Ormyrus labotus Walker), eupelmids (Eupelmus vesicularis [Retzius] and two unidentified Brasema species), and a pteromalid (unidentified Homoporus species; Tooker and Hanks 2004a). Populations of gall wasps are decimated when prairies are burned, but quickly recolonize (Tooker and Hanks 2004b).

Antistrophus rufus n. sp.

While looking through the Laboratory collection for Cynipidae I was much interested in finding a vial containing a section of a stem of Silphiam laciniatum and a number of two species of Cynipidae bred from it. There was not the slightest indication of a gall upon the stem, and it was found that the flies had emerged from little cells in the pith exactly like the cells in the pithy substance of the galls of A. silphii^ above described. In company with Mr. C. A. Hart I visited fields where this species of Silphiiim was growing, and we found that the majority of the stems were more or less infested with cynipidous larvae, hundreds of which could, in some cases, be found in a single stem; but in no case was there any indication of the formation of a gall. An illustration of a stem containing these cells is given at Fig. 4, (PL IX). After finding the stems of Silphium laciniatum so much infested, we pushed our investigations further and found similar larval cells abundant in Silphium perfoliatum^S. terebinthinaceum^ and S. integrifolium. Whether the flies when bred from these stems will all prove to be one or the other of the two species here described cannot yet be told

[Tooker and Hanks list Antistrophus rufus and minor emerging in numbers from both Silphium laciniatum and terebinthinaceum in the spring.]

Antistrophus species overwintered in dead Silphium stems and pupated in the early spring. A. rufus adults began emerging from S. laciniatum in mid-May and from S. terebinthinaceum in mid-June 2000. Although adults of both sexes have fully developed and functional wings, they did not fly readily but rather walked, with occasional hopping flights to nearby plants. On both plant species, males emerged first and were present only on dead stems of the previous year. There they guarded areas of stem where females later emerged, driving off other males by charging or butting with the head (N> 10 observations). Females mated immediately upon emerging and then moved to nearby developing Silphium stems (N> 10). As many as 20 females oviposited in one stem at the same time. Oviposition usually lasted less than 1 min (N> 10). Females oviposited in axils of S. laciniatum before bolting, but in S. terebinthinaceum they oviposited in the internodes after bolting, accounting for the greater proportion of emergence holes near stipules of S. laciniatum.

Adult A. rufus emerged in cages from S. laciniatum about 1 month earlier than from S. terebinthinaceum.

Densities of galls were significantly greater in S. laciniatum than in S. terebinthinaceum, perhaps due to its thicker stems. Galls were concentrated in the basal portions of S. laciniatumstems, but at high densities there was a greater proportion of galls in stem tops. Most galls in S. laciniatum were in proximity to stipules, which is consistent with oviposition behavior of A. rufus females. Thick clasping stipules may offer ovipositing females protection from predators and refuge for larvae from parasitoids, but offer no protection from fire (Tooker and Hanks 2004a). Galls in S. terebinthinaceum were more evenly distributed along stems, suggesting that the thinner stipules of this species provide no refuge from natural enemies

Antistrophus rufus
Rosinweed cryptic stem gall wasp

Hosts: compass plant (S. laciniatum)

Gall location and description: Galls of species in this group are cryptic, meaning they are not outwardly visible and can only be located upon dissection of a galled stem. When dissected, the stem will have numerous brown, oval-shaped chambers along the inside of the pith, each with a single insect.

Range: This species group has been found throughout the Midwestern, Eastern, and South Central United States. Like many other herb galls, this complex probably occurs throughout the entire ranges of each species’ respective host plant(s).

As with the rosinweed flower gall wasps, it was once believed that cryptic galls on rosinweeds were all induced by either of two species, Antistrophus minor and A. rufus. Recent data shows that there are multiple species of cryptic stem gall wasps on these plants, each preferring a single host plant.

Unlike other rosinweeds, compass plant (S. laciniatum) has been found to host two distinct species of cryptic stem gall wasps. These two species are found in the same habitats, and often induce galls on the same individual host plant. The reason for this bizarre occurrence is currently unknown.

Many unrelated groups of gall wasps seem to have indepdently developed the ability to induce cryptic, inconspicuous galls. The mechanism by which this has been made possible is not known, but it is likely that having hidden galls decreases the odds of attack by predators or parasites. Due to the known diversity of cryptic gall wasps and hidden nature of cryptic galls, it is quite likely that many cryptic galls have thus far evaded detection and await discovery!

Gillette reared both of these species (editor: A minor as well) from flower stalk stems of compassplant, but unlike the previous two species, there was no evidence of gall formation. Instead he found that the wasps pupated in little cells within the pith just under the outer wall of the stem. A minor was smaller and less numerous than A rufus. Dr. Gillette along with Mr. Hart visited "fields" of compassplant and "found that the majority of the stems were more or less infested with cynipidous larvae, hundreds of which could. in some cases, be found in a single stem.". They went on to investigate other Silphium species in the area and "found similar larval cells abundant in S. perfoliatum, S terebinthinaceum, and S. integrifolium". However, they did not rear specimens from these stems to see what species of Antistrophus wasps were present.

Found wasps in stems of compassplant in Wisconsin and Iowa.