Basis for Listing
The Uncas skipper has an extensive range in western North America, from the Canadian prairie southward into Mexico (MacNeill 1964). Its occurrence in Minnesota is restricted to an isolated outlier population in Sherburne County, about 300 km (186 mi.) east of its closest established populations in eastern South Dakota. The Minnesota population is possibly a relict of the post-glacial thermal maximum (ca. 8000-4000 B.P.) when major vegetation zones shifted well east of their present positions. This skipper has also been recorded as a rare stray into southwest Minnesota, but there is no evidence of establishment there.
The central Minnesota Uncas skipper population is confined to a large complex of sand dunes that formed within an extensive sand plain during the previously mentioned warm period. These dune blankets were covered with a mosaic of dry sand prairie and open bur oak savanna or scrub, separated by wet prairies and oak brushland. The Uncas skipper was not discovered here until 1961, after a state forest had been created to include much of the dune areas and the planting of pines and other conifers on the dunes was well underway.
Today, most of the former habitat within the state forest is densely stocked pine plantation and oak forest that has grown up in the absence of fire, and dune areas outside the state forest have mostly become housing developments. The Uncas skipper disappeared from the locations in the state forest where it had formerly been encountered, and was feared to have been extirpated before being relocated in 1983 in a small area of sand prairie openings on south-facing dune slopes. Oak forest expansion and young conifer plantations are a threat to these few remaining patches of habitat. The one remaining large dune area in private ownership is a prime candidate for development. Given its extreme rarity in the state, the Uncas skipper was listed as an endangered species in Minnesota in 1984.
The Uncas 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 1.4-1.5 cm (0.55-0.59 in.) in males, slightly greater in females. The antennae are relatively short and have clubbed ends with a sharp, backward-pointing tip. They are strong, fast fliers, with a very rapid wing beat that appears as a blur to the human eye.
Males and females differ significantly in appearance on the upper side of the wings. Fresh males are typically a brownish orange with an olivaceous tendancy, with diffuse, darker marginal coloration. Several small light-colored spots are visible in the darker area near the forewing tips. There is a narrow, almost linear black "brand" containing specialized scent scales used in courtship centrally placed along the long axis of the forewing. The pattern above is less contrasty in males from the central Minnesota population compared to those from farther west. Females are variable, but tend to be darker, with large whitish spots on the forewings and pale brown spots on the hind wings. The underside of the hind wing (the forewing is mostly hidden at rest) is similar in the two sexes, and is distinctively marked with prominent rectangular white spots that are linked by white-scaled veins to form a web-like pattern. This is set off from the yellowish-brown ground color by dark brown shading around the white. Both sexes become darker and duller with age as scales wear off, but the distinctive patterning never disappears.
The underside pattern of the Uncas skipper will reliably separate it from all other skippers of similar size and shape that may occur in the same area. These include the long dash (Polites mystic), the tawny-edged skipper (Polites themistocles), and the sachem (Atalopedes campestris) in central Minnesota.
The Uncas skipper inhabits dry native prairie or barrens prairie on sand dune forms. More sparsely vegetated slopes and summits are especially critical. Adults will range more widely in search of nectar.
Biology / Life History
The Uncas skipper in central Minnesota appears to have only a single generation in a year, although in South Dakota and throughout most of its range it produces two or more generations per year (Scott 1986; Marrone 2002). Adults have been observed from mid-June to mid-July, but in any given year the flight period is rarely more than three weeks. No detailed study of the life history of this skipper in Minnesota has been made, but in rearings from eggs observed by R. Dana (Minnesota DNR, unpublished data), the timing of the various stages appeared similar to that of the Dakota skipper (Hesperia dacotae). Larvae reached the fourth or fifth stage before hibernating for the winter and finished larval development the following spring.
All observed ovipositions have been on hairy grama (Bouteloua hirsuta), a small tufted grass that is common on dune slopes. Females typically lay only a single egg before moving on to another tuft. Larvae live in a series of increasingly larger shelters that they construct of plant materials and silk. Details of larval behavior have not been observed. Hibernation of partly grown larvae occurs in shelters burrowed into the soil beneath the grass tufts (R. Dana, Minnesota DNR, pers. comm.).
So far as is known, hairy grama is the obligate host plant for larvae of the central Minnesota Uncas skipper population. The very similar relative, blue grama (Bouteloua gracilis), is most commonly reported to be the larval host throughout the main part of the skipper's range. Adults are avid seekers of nectar. They visit almost any flowers available, but hairy puccoon (Lithospermum caroliniense) and prairie golden aster (Heterotheca villosa) are favorites.
Based on what is reported for other species in the genus, female Uncas skippers probably mate shortly after emergence and rarely remate. Males seek receptive females primarily by perching on low vegetation or bare sand and pursuing any passing insect that might be a female Uncas skipper. Uncas skipper males elsewhere in the range frequently perch on soil or rock. Receptive females respond to pursuit by descending into the vegetation where coupling follows almost immediately. Loose aggregations of perching males form on upper slopes and tops of prominences. It may be that unmated females are drawn to these same features. The dispersal behavior of mated females is not known. In particular, it is not known how much the woodland that now separates the habitat openings is a barrier to movement among the patches in Minnesota.
Conservation / Management
Small population size due to past habitat loss, and further habitat destruction are the primary threats facing the Uncas skipper in Minnesota. The largest remaining continuous patch of habitat in the state is privately owned and vulnerable to development. The colony in the state forest is confined to a few small habitat patches separated from each other by dense woodland or forest, and it is separated from the larger colony on private land by over 10 km (6 mi.) of forest and increasing suburban development. Such a small, isolated colony is at high risk of extirpation as a result of natural events (such as severe drought or hailstorms) or human-caused events (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.
Survival of the Minnesota Uncas skipper population depends upon protection of the large private tract and on reclamation of habitat in the state forest from conifer plantation and oak woodland. The latter will take a number of years, although removing young conifers from the remaining habitat patches and opening corridors between these can be accomplished quickly. The use of prescribed fire as a habitat management tool must be very judicious; the core habitat areas on the dune slopes should probably not be burned at all. Abundance of the host grass, hairy grama, appears to be susceptible to rapid change, and maintenance of adequate amounts to sustain the fragile butterfly population may require special management intervention. Research is needed into the factors driving population dynamics of this grass in the dune habitat. Arrival of the spongy moth (Lymantria dispar) is inevitable in the near future, and it will be critical to keep insecticide applications for moth control well away from the areas where the Uncas skipper occurs.
Conservation Efforts in Minnesota
Most of the Uncas skipper habitat remaining in the state forest has been dedicated as a state Scientific and Natural Area and is being managed to protect and restore the native sand prairie and savanna. Conifers in the core habitat patches have been removed and management to open up additional habitat and to restore intervening woodlands to oak scrub are underway. There have also been discussions about removal of conifers from the rest of the remaining habitat patches, which are within a recreation area in the state forest.
References and Additional Information
Lindsey, A. W. 1942. A preliminary revision of Hesperia. Denison University Bulletin, Journal of the Scientific Laboratories 37:1-50 + 6 plates.
MacNeill, C. D. 1964. The skippers of the genus Hesperia in western North America, with special reference to California (Lepidoptera: Hesperiidae). University of California Publications in Entomology 35:1-230.
Marrone, G. M. 2002. Field guide to butterflies of South Dakota. South Dakota Department of Game, Fish, and Parks, Pierre, South Dakota. 478 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.
Scott, J. A. 1986. The butterflies of North America: a natural history and field guide. Stanford University Press, Stanford, California. 583 pp.