Dromochorus pruininus, Casey, 1897

Duran, Daniel P., Herrmann, David P., Roman, Stephen J., Gwiazdowski, Rodger A., Drummond, Jennifer A., Hood, Glen R. & Egan, Scott P., 2019, Cryptic diversity in the North American Dromochorus tiger beetles (Coleoptera: Carabidae: Cicindelinae): a congruence-based method for species discovery, Zoological Journal of the Linnean Society 186, pp. 250-285 : 265-269

publication ID	https://doi.org/ 10.1093/zoolinnean/zly035
publication LSID	urn:lsid-:zoo-bank-.org-:pub:FB357841
persistent identifier	https://treatment.plazi.org/id/B83387B9-FF8B-F12F-FF59-7912FDB5EA73
treatment provided by	Plazi
scientific name	Dromochorus pruininus
status

Treatment

DROMOCHORUS PRUININUS View in CoL CASEY, 1897

( FIGS 7A View Figure 7 , 9A View Figure 9 )

Common name Frosted tiger beetle.

Type locality ‘Kansas’. Syntypes (3) in USNM, Washington DC ( Fig. 9A View Figure 9 ; map Fig. 3 View Figure 3 ).

Synonymy

Cicindela pruinina Horn, 1915 .

Dromochorus pruinius Johnson, 1991 (unjustified emendation, misspelling).

Taxonomic history

Dromochorus pruininus was described by Casey (1897) as a species, but its taxonomic status has been contentious in recent decades. Freitag (1999) regarded D. pruininus as conspecific with D. belfragei . Pearson

et al. (2006) acknowledged the uncertainty of its placement and assessed that D. pruininus could be either a ‘form’ or subspecies of D. belfragei , or possibly a full species. Other workers have regarded it as a distinct species (e.g. Johnson, 1991; Wiesner, 1992; Bousquet, 2012). Due to the relative paucity of specimens in most museums, previous assessments of species status were necessarily made with limited morphological and geographical data. Here we recognize D. pruininus as a distinct species from D. belfragei and all others, based on reciprocal monophyly (mtDNA) ( Fig. 2 View Figure 2 ), multiple diagnostic morphological characteristics and ecological characteristics.

Distribution

Dromochorus pruininus is the northernmost species in the genus, occurring from Missouri, Kansas and Oklahoma to Central Texas. It appears to be allopatric with D. belfragei , however, as the two ranges come in close proximity in several areas ( Fig. 3 View Figure 3 ). Despite the near geographic overlap of the two taxa, there was no evidence of interbreeding based on in the mtDNA genealogy ( Fig. 2B View Figure 2 ).

Diagnosis

This species is distinctive, and can be distinguished from all other Dromochorus by the presence of a frosted, darkblue dorsum, in conjunction with maxillary palpi that are yellow-ochre with a contrasting dark apical segment ( Fig. 7 View Figure 7 ). Dromochorus pruininus also lacks any elytral pitting, subsutural foveae or dark infuscations on the elytra.

Description

L a r g e -s i z e d D r o m o ch o r u s. B o d y l e n g t h 1 2.3– 14.6 mm, mean ♀ 14.0 mm, mean ♂ 13.1 mm. Head slightly wider than pronotum. Head black with frosted blue and/or violet sheen. Fine rugosity often present on the frons and vertex. All head portions glabrous, except for two supraorbital setae next to each eye. Frons concave in median area, especially in male, bulging towards slightly convex near the anterior margin, clearly delimited from clypeus, gradually blending into vertex. Genae bright polished metallic blue or green, blending to violet posteriorly, with shallow longitudinal striae gradually ending at border of vertex. Clypeus shimmering blue, occasionally with violet reflections along the margins. Male labrum tridentate with 6–8 setae, central area pale ochre-testaceous, with a thin, dark-brown to black border anteriorly and posteriorly, dark brown to black laterally; female labrum tridentate with 6–8 setae, entirely dark brown to black with polished metallic cupreous to green reflections. Maxillary and labial palpi yellow-ochre to pale amber; apical segment dark brown to black, often with metallic purple and green reflections. Antennae normal length, reaching back to humerus and basal third of elytron, slightly longer in male than female; scape dark testaceous to black with metallic reflections of violet, cupreous and green, with 2–3 apical setae; pedicel dark testaceous with metallic reflections of violet, cupreous and green, lacking any setae; flagellum dark testaceous, antennomeres 3–4 with metallic violet and green reflections, densely clothed in short white setae, antennomeres 5–11 dull-textured without metallic reflections and possessing erect setae in apical rings only, covered with fine pubescence throughout.

Thorax: Pronotum 2.4–3.3 mm in length, mean ♀ 2.9 mm, mean ♂ 2.8 mm; width 2.8–3.4 mm, mean ♀ 3.2 mm, mean ♂ 3.1 mm. Pronotum black, with dark-blue, frosted surface, slightly wider than long, widest near anterior margin, width to length ratio 1.0 to 1.3, setae sparse and irregular, mostly present along lateral third of dorsal surface; disc finely rugose, with thin but distinct median line, with less well-defined shallow sulci present anteriorly and posteriorly; notopleural sutures clearly defined, not visible from dorsal view; proepisternum black with weak to strong iridescent blue reflections, glabrous. Elytra elongate, convex, 7.5–9.3 mm length, mean ♀ 8.5 mm, mean ♂ 8.0 mm, shape similar in both sexes, but slightly wider in female, especially toward apical third; sutural spine absent, microserrations not present on elytral apices; elytral texture dull throughout, elytral coloration black with frosted blue to violet sheen; maculations absent; infuscations absent; subsutural foveae absent.

Legs: Pro-, meso- and metacoxae dark testaceous to black with iridescent blue to violet and cupreous reflections, with numerous setae; pro- and mesotrochanters with a single erect seta, metatrochanter glabrous, trochanters dark brown-testaceous; femora metallic green to violet, densely clothed in decumbent white setae; tibiae brown, clothed with setae of two types: sparser brown-testaceous long setae and dense short decumbent white setae; two tibial spines present; tarsi brown-testaceous, first three dilated protarsomeres in male with dense greyish-white setal pad.

Abdomen: Venter black with metallic violet to greenish reflections throughout most surfaces. Decumbent white setae present on ventrite 1. Ventrites 2–6 have scattered short brown recumbent setae present throughout, but often abraded.

Ecology/natural history

Dromochorus pruininus appears to have a 2-year life cycle based on observations from our Dallas-Fort Worth (DFW) area study sites. Adults are active between mid-May and early July in the southern part of its range (e.g. central to north TX) and late June to early August in northern part of its range (e.g. north-west OK to KS). Based on detailed observations of populations in the DFW area, peak adult activity is approximately 3–4 weeks after initial emergence.

Dromochorus pruininus is mostly associated with riparian systems with strong clay content. However, it is rarely found on muddy streambanks, but instead is typically found on higher elevation or relief within the riparian system. The species also occurs in sodded and cultivated fields, hill-tops, road-side ditches and meadows ( Larochelle & Larivière, 2001; Pearson et al., 2006). Dromochorus pruininus appears to be most associated with clay soils that exhibit large cracks when dry, and beetles have been observed using these cracks to escape. Although it has been considered a habitat generalist ( Larochelle & Larivière, 2001), proximal optimal oviposition habitat is likely involved in determining where adults are most active, and this observation is reinforced by the majority of our observations of adults tending to be concentrated near larval burrow sites. The eurytopic (able to tolerate a wide range of habitats) point of view is probably due to a lack of understanding regarding the preferred microhabitat association of D. pruininus , coupled with the fact that this species cannot fly and must walk through these areas of suboptimal habitat to disperse. Although adult D. pruininus must move through thick vegetation, this species requires semi-open areas for mating, foraging and oviposition. Like all other Dromochorus , D. pruininus will run into dense grasses to hide when pursued.

Various accounts have been reported regarding daily activity for the adults. They are thought to be most active in early morning and afternoon/evening ( Pearson et al., 2015), with most activity in late afternoon/early evening ( MacRae & Brown, 2011) or active after 4pm ( Larochelle & Larivière, 2001). Between 2012 and 2016, the authors have observed the species active at every hour of daylight. Most activity occurs in the 2–3 h after sunrise, and in the 2–3 h before dark, but when there is significant cloud cover, beetles may be present at any time of the day. In addition, shaded microhabitats may permit beetles to be more active than in nearby open microhabitats, suggesting that adults are more photophobic than crepuscular. Despite a tendency to generally avoid strong sunlight, our observations indicate that this species appears to tolerate direct sun much more than D. belfragei . Dromochorus pruininus are not active at night, and no beetles were ever observed at light traps, even those that were placed at sites where adults were observed during the same day.

At one DFW area study site, larval burrows were observed in a variety of open areas and vegetated areas, especially along loose soil berms, small mammalian trails that cut through heavy vegetation along steep/eroding clay banks, cracked clay areas with heavy grasses along upper banks, freshly disturbed areas (e.g. tracks from off-road vehicles that cut through grasses), the middle and edges of open trails, flat areas near the water’s edge of rivers, the base of ant mounds, clay areas with>50% grassy cover and the base of mesquite trees in riparian zones. Females seem to prefer crusty or loose clay for oviposition, even in areas with extensive vegetation (above 150 cm in height in late summer). Spomer et al. (2008) demonstrated that in a laboratory setting this species showed a preference for oviposition along sloped surfaces; however, we have observed larval burrows in flat, level areas, as often as sloped, in the field. In the autumn, burrows have been found in heavily shaded and soggy areas along riparian wooded middle banks under clover.

P e a r s o n e t a l. (2 0 0 6) s u g g e s t t h a t D. pruininus / belfragei have been impacted by the introduction of the red imported fire ant, Solenopsis invicta , and report that Dromochorus has all but disappeared in certain areas. Our observations do not support this hypothesis. In our fieldwork, we found D. pruininus sympatric with S. invicta at three sites in the DFW area. S. invicta is abundant at two of the sites and at lower densities at the third. The distribution of S. invicta extended to sites within Dallas and Tarrant Counties as early 1958–67 ( Cokendolpher & Phillips, 1989), and we have monitored beetles at the sites between 2012 and 2014. We observed that D. pruininus are abundant near fire ant mounds, with>81 beetles seen per hour at peak activity (Herrmann, Duran & Egan, unpublished). Moreover, using S. invicta -specific mtDNA markers, we screened the gut contents of hundreds of beetles, and found that ~70% had recently fed on fire ants (Duran, Herrmann & Egan, unpublished). All data to date suggest that D. pruininus , and likely all other Dromochorus , are not displaced by fire ants and, moreover, they appear to frequently prey upon them.