Montana Field Guide

The following is taken from Wallis (1961), Kritsky and Horner (1998), and Pearson et al. (2015), including description of C. t borealis. Body length is 12-15 mm, variable in size but one of the larger forms. Color above is brown, green, dull coppery-green to dark brown, black, or dark green. Maculations are complete and usually relatively thin, the oblique shoulder (humeral) maculation running toward the midline with the rear tip not expanded, sometimes reduced to a short or broken line (vibex); sometimes a partial marginal band on the outer edge of the elytra. Ventral surfaces metallic green and copper to purple on abdomen (borealis), or metallic green to purple (vibex). Hairy forehead with erect setae, short upper lip, labrum short with 3 teeth.

Tiger beetle life cycles fit two general categories based on adult activity periods. “Spring-fall” beetles emerge as adults in late summer and fall, then overwinter in burrows before emerging again in spring when mature and ready to mate and lay eggs. The life cycle may take 1-4 years. “Summer” beetles emerge as adults in early summer, then mate and lay eggs before dying. The life cycle may take 1-2 years, possibly longer depending on latitude and elevation (Kippenhan 1994, Knisley and Schultz 1997, Leonard and Bell 1999). Adult Cicindela tranquebarica vibex (= borealis) a spring-fall beetle found April-October (Larochelle and Larivière 2001). In Montana probably varies with elevation, reported (as C. t. borealis) from late March to mid-September (Nate Kohler personal communication).

The following come largely from Wallis (1961), Kritsky and Horner (1998), Acorn (2001), and Pearson et al. (2015) for Cicindela tranquebarica vibex (= borealis). For our region, the long obliquely-descending arm of the shoulder (humeral) maculation on the dark elytra differentiates this from all other tiger beetle species. C. t. vibex (= borealis) differs from C. t. kirbyi, the other subspecies of Oblique-lined Tiger Beetle in Montana, by being dark brown, dark green, or black above with thin maculations (including a thin shoulder maculation that may be short or broken), instead of dark brown with a relatively thick and unbroken shoulder maculation. Intergrades of the two subspecies may occur west of the Continental Divide. Validity of several subspecies designations are in need of reevaluation (Sumlin 1976, Shook 1984, Kippenhan 1994, Kritsky and Horner 1998, Acorn 2001, Knisley and Haines 2007, and Pearson et al. 2015) although C. t. vibex is accepted by all authorities but not with the same distribution. Validity of C. t. borealis is questioned by Pearson et al. (2015) but which they include as a subspecies separate from C. t. vibex, and with a range touching Montana.

Non-migratory but capable of dispersal. When wings are fully developed (macropterous), it is a strong agile flier and fast runner (Larochelle and Larivière 2001).

Adult and larval tiger beetle habitat essentially identical, the larvae live in soil burrows (Knisley and Schultz 1997). Across the range, Cicindela tranquebarica vibex (= borealis) occurs in a variety of habitats including beaches, mud flats along lakes, saline and alkali flats, ponds, streams, sagebrush, dry lake beds, irrigated pastures, ditches, roads near lakes, forest openings, river sandbars (Wallis 1961, Acorn 2001, Larochelle and Larivière 2001, Pearson et al. 2015, and Sauder 2017). In Montana, documented habitats include ditches, sand pits, riverine dunes, sand flats near ponds, old river crossings and fishing access sites, sparse creek-side vegetation (Nate Kohler personal communication).

Larval and adult tiger beetles are predaceous. In general, both feed considerably on ants (Wallis 1961, and Knisley and Schultz 1997). Cicindela tranquebarica vibex diet is not described but probably includes a variety of small insects and spiders for both the larvae and adults (Larochelle and Larivière 2001).

Larval tiger beetles live in burrows and molt through three instars to pupation, which also occurs in the larval burrow. Adults make shallow burrows in soil for overnight protection, deeper burrows for overwintering. Adults are sensitive to heat and light and most active during sunny conditions. Excessive heat during midday on sunny days drives adults to seek shelter among vegetation or in burrows (Wallis 1961, and Knisley and Schultz 1997). Cicindela tranquebarica vibex has a broad range of ecological tolerance (eurytopic). Adults are diurnal, and gregarious (Larochelle and Larivière 2001). Noted hiding during the night under dry cow chips. Predators of adults are not described but probably include birds and robber (asilid) flies. Larval parasites probably include bombyliid flies. Escapes by flying slowly short distances (3-5 m), turns to face pursuer once landing then seeks cover quickly (Larochelle and Larivière 2001). Associated tiger beetle species include Cicindela columbica, C. hirticollis, C. oregona, C. repanda (Sauder 2017).

Life cycle of Cicindela tranquebarica vibex is probably 2 years, with both third instar larvae and tenerals (fresh adults) overwintering (Acorn 2001, Larochelle and Larivière 2001, and Pearson et al. 2015). Little information on many reproductive characteristics, but probably similar to other subspecies (see the full species account). Mating probably May to August (Larochelle and Larivière 2001). Females typically oviposit in sites where the water table is near the ground surface, often in moist, sandy soil that is sparsely vegetated and contains some humus. Larval burrows dug in saltgrass areas. Tenerals probably appear in August then overwinter, reemerge the following spring to mate. Duration of larval life probably 12-15 months, of adult life about 11 months (Criddle 1907, 1910, and Larochelle and Larivière 2001). No information for Montana.

Not considered rare or in need of special conservation management overall (Knisley et al. 2014). Benefits from many man-made habitats, including agricultural fields and pastures, rangelands, sand and gravel pits, clay roads, powerline corridors. Native habitats favored by this species experience vegetation encroachment and stabilization as succession proceeds, especially from shrubs and trees which cast too much shade on larval burrows, and benefit from disturbance that retains a variety of succession conditions. Some colonies (particularly the larval burrows) could be impacted by trampling through livestock overgrazing, but grazing at appropriate times and stocking levels could also be beneficial by keeping vegetation cover more open (Knisley 2011). Prescribed fire is also a useful tool for maintaining a variety of successional conditions in appropriate situations (Winton 2010).