Hesperia assiniboia, Hesperia comma assiniboia
Basis for Listing
The Assiniboia skipper occurs in aspen parkland and prairie from central Alberta to Manitoba and southward into Montana, North Dakota, and northern South Dakota. Minnesota is on the eastern margin of the species' range. Although not uncommon in the core of its range, the Assiniboia skipper is reportedly declining as its native prairie habitat disappears (Layberry et al. 1998).
Presumably, the Assiniboia skipper was not common in Minnesota even before prairie destruction. Its southern range limit in the state suggests that it was confined to northwestern Minnesota, where the dry, sandy habitat it requires was a minor component of the prairie landscape. Nonetheless, this skipper was probably well distributed along the sandy ridges left as shoreline features by Glacial Lake Agassiz. The destruction of the native prairie on these ridges for agricultural purposes or for aggregate material is nearly complete, and today the Assiniboia skipper is extremely rare in the state. The primary threats to its remaining habitat are overgrazing and aggregate mining.
The Assiniboia skipper is completely dependent upon the survival of its native prairie habitat. There are only two protected sites where this skipper has been observed in any numbers, and one of these sites is only partly protected. Active aggregate mining continues in the unprotected portion. The isolation and small size of the remnant prairie habitat makes all other occurrences highly susceptible to extirpation. For these reasons, the Assiniboia skipper was listed as an endangered species in Minnesota in 1984.
The Assiniboia skipper is a typical member of the "branded" skippers, or subfamily Hesperiinae. It is a small butterfly with a robust body, narrow, angular forewings, and shorter, more rounded hind wings. Forewing length (base to apex) is 1.3-1.5 cm (0.51-0.59 in.) in males, slightly greater in females. The antennae have clubbed ends with a sharp, backward-pointing tip. Assiniboia skippers are strong, fast fliers, with a very rapid wing beat that appears as a blur to the human eye.
Males and females differ in wing markings, most notably on the upper surface of the forewings. In males these are typically tawny orange, with a broad grayish-brown outer margin that includes a cluster of small whitish spots near the apex. There is a narrow, black "brand" containing specialized scent scales used in courtship centrally placed along the long axis of their forewing. A chevron of lighter orange spots is present on the upper hind wings. Females are more variable, tending to be darker or more grayish, with bands of lighter angular spots on both hind and forewings. The underside of the hind wing (the forewing is mostly hidden at rest) in males is pale ochreous, sometimes more grayish; in females, it's a distinctive grayish-green. White or pale yellow spots are usually present, though sometimes greatly reduced in males. These have the appearance of separate clusters rather than a band. Both sexes become darker and duller with age as scales wear off.
The only similar skippers that fly in Minnesota at the same time as the Assiniboia skipper are the Pawnee skipper (Hesperia leonardus pawnee) and the sachem (Atalopedes campestris). The Assiniboia skipper, when fresh, is a colder color (grayish or grayish green) below than the Pawnee skipper, and the spotting is more prominent. Reliably distinguishing worn males may require examination of technical characters, such as color of the scent scales within the forewing brand or genitalic structures. Sachem males have a large, rectangular black patch surrounding the brand above, and females have a large translucent central spot on the forewing (all spots on the Assiniboia female are opaque). The hind wing spotband is a sharp, continuous chevron of more or less equal sized spots compared to the separated clusters in the Assiniboia skipper.
The Assiniboia skipper is found in native dry prairie where midheight and short grasses such as needle-and-thread (Stipa comata), prairie junegrass (Koeleria macrantha), and blue grama (Bouteloua gracilis) are a major component of the vegetation. Crests of glacial lake beach ridges or upper slopes of shoreline scarps, as well as old sand dune areas are the principal sites of suitable habitat. Adults will range more widely for nectar.
Biology / Life History
The Assiniboia skipper has a single annual generation. All observations of this species in Minnesota have been in August, although the flight period probably extends somewhat into September, as it does in North Dakota and Manitoba (McCabe and Post 1977; Klassen et al. 1989). Very little has been reported on the life history details of this skipper. The eggs or hatchling larvae overwinter (Royer 1988), and larval development occurs the following spring and summer.
Larvae of this species, as with other members of the genus Hesperia, live within hidden shelters constructed of silk, plant material, and debris. McCabe and Post (1977) report that larval shelters are often concealed under dried cow pies in pastures in North Dakota. As is the case in other Hesperia, larvae feed on grasses (possibly on some sedges also), but whether particular species are favored or required is not known. Adults are avid seekers of nectar; dotted blazing star (Liatris punctata var. punctata) is especially utilized. Males also visit bare, damp soil for moisture and nutrients.
Males seek receptive females primarily by perching on low vegetation or bare spots and pursuing any passing insect that might be a female Assiniboia skipper. Females respond to pursuit by descending into the vegetation where coupling probably follows almost immediately if the female is receptive. Females mate shortly after emerging as adults, but whether they remate is not known. Loose aggregations of perching males form on upper slopes and prominences in the prairie. It may be that unmated females are drawn to these same features. Nothing is known about the dispersal behavior of mated females.
Conservation / Management
Small colony sizes due to past habitat loss, and further habitat destruction are the primary threats facing the Aassiniboia skipper in Minnesota. All habitat that is not protected by permanent dedication for conservation is at risk of destruction for agricultural production, aggregate mining, or development. The effects of livestock grazing on this skipper is unclear. McCabe and Post (1977) assert that the Assiniboia skipper is most common in North Dakota in moderately grazed prairie, but that in Minnesota it needs ungrazed "virgin" prairie. However, one colony documented in Clay County, Minnesota occurred in a moderately heavily grazed prairie pasture (R. Dana, Minnesota DNR, pers. comm.). On some sites with droughty, sandy-loam soils, short, sparse prairie suitable for this skipper may be the product of a long history of moderate grazing or annual haying, the removal of which would result in a gradual change to taller, denser prairie than the Assiniboia skipper would tolerate.
Even though the Assiniboia skipper has a stronger dispersal tendency than other prairie-dependent skippers in Minnesota, the small size of most habitat patches and the great distances separating them suggest that the population is comprised of isolated colonies. Small colonies are vulnerable to extripation as a result of natural events (such as severe drought or hailstorms) or human caused ones (such as insecticide application), as well as from the vagaries of normal population processes (for example, by chance all adults in one generation are males). Loss of genetic diversity is another possible threat.
The Assiniboia skipper may be highly sensitive to fall or spring prairie fires as eggs and small larvae would be the exposed stages. Fires in habitat favored by this skipper will generally be on the cool side, as fuel loads are relatively low, but until larval habits are known well enough to permit more precise estimates of risk, the use of prescribed fire as a habitat management tool should be judicious.
Several programs and resources are available to land managers and landowners to help protect and manage remaining prairie parcels including the Native Prairie Bank Program, the Native Prairie Tax Exemption Program, and a prairie restoration handbook.
Conservation Efforts in Minnesota
Some of the prairie habitat supporting the largest known Assiniboia skipper colony is protected through ownership and management by public agencies and private conservation organizations. Considerable effort has been devoted to reducing the amount of habitat destroyed by aggregate mining in the non-protected portion. The largest of the smaller colonies is also substantially protected. The Minnesota DNR has sponsored or supported several survey efforts to find new Assiniboia skipper locations and update information for known locations. Guidelines for protecting skipper populations within a fire-management program are employed by the major owners of Assiniboia skipper habitat in Minnesota, and efforts have been made to educate other land managers in the state.
References and Additional Information
Bird, C. D., G. J. Hilchie, N. G. Kondla, E. M. Pike, and F. A. H. Sperling. 1995. Alberta butterflies. The Provincial Museum of Alberta, Edmonton, Alberta. 349 pp.
Klassen, P., A. R. Westwood, W. B. Preston, and W. B. McKillop. 1989. The butterflies of Manitoba. Manitoba Museum of Man and Nature, Winnipeg, Manitoba. 290 pp.
Layberry, R. A., P. W. Hall, and J. D. LaFontaine. 1998. The butterflies of Canada. University of Toronto Press, Toronto, Ontario. 280 pp. + color plates.
McCabe, T. L., and R. L. Post. 1977. Skippers (Hesperioidea) of North Dakota. North Dakota Insects Publication No. 11, Schafer-Post Series. Agricultural Experiment Station, University of North Dakota, Fargo, North Dakota. 70 pp.
Royer, R. A. 1988. Butterflies of North Dakota: an atlas and guide. Science Monograph Number 1, Minot State University, Minot, North Dakota. 192 pp.
Swengel, A. B. 1998. Effects of management on butterfly abundance in tallgrass prairie and pine barrens. Biological Conservation 83(1):77-89.