Pawnee Montane Skipper

Hesperia leonardus montana

Description

The adult Pawnee montane skipper, Hesperia leonardus montana, is a brownish yellow butterfly with a wingspan of slightly more than 1 in (2.5 cm). Distinct, yellowish spots occur near the outer margins of the upper surface of the wings; there are one to four tan or off-white spots on the lower (ventral) surface. Ventral spots are larger on the hindwings and are generally whiter in females.

Behavior

Pawnee montane skipper females deposit single eggs directly on leaves of blue grama grass (Bouteloua gracilis ), the only known larval host plant. Larvae overwinter; pupation lasts 13-23 days. Adult males emerge in late July, followed by females up to ten days later. Adults feed principally on the nectar of the prairie gayfeather. The musk thistle is also an important nectar source. Adults live until the first strong frost.

Habitat

The Pawnee montane skipper inhabits a rugged mountainous region of plateaus cut by deep canyons and narrow river valleys. The skipper is found in dry, open, ponderosa pine forests on outcrops of Pikes Peak granite where soils are thin, unstable, and susceptible to water erosion. Slopes are moderately steep with a south, west, or east aspect. The understory is very sparse, generally with less than 30% ground cover.

Blue grama grass occurs in clumps across the hot, open slopes inhabited by skippers but actually covers only about 5% of the surface area. Prairie gayfeather occurs in patches throughout the ponderosa pine woodlands.

Distribution

The Pawnee montane skipper is endemic to habitat associated with the Pike's Peak granite formation in the South Platte river drainage of Colorado, and has probably always had a very limited range.

This species is found in four Colorado counties directly southwest of Denver: Teller, Park, Jefferson, and Douglas. Within these counties, the skipper is restricted to the South Platte River drainage in a band roughly 25 mi (40 km) long and 5 mi (8 km) wide. A three-year study of pawnee populations conducted by the Denver Water Department as part of the study to construct dams, concluded in 1986, estimated the population at 85,000-116,000 individuals.

Threats

Sections of the current range are managed by the Forest Service, the Bureau of Land Management, Denver Water Department, county governments, and private landowners. Currently, the skipper's limited numbers threaten its survival more than do environmental factors. If, however, plans to construct the Two Forks Dam on the South Platte River are implemented, a portion of the butterfly's habitat would be eliminated. Residential development of the area would probably accelerate if the reservoir were built.

Insecticides aimed at controlling the mountain pine beetle and the spruce bud worm would also eradicate the butterfly population if allowed to be applied near the habitat.

The prairie gayfeather, which provides nectar for adult skippers, appears to grow in areas subject to occasional fire or logging, and the skipper does not recolonize these areas for several years after such disturbances.

Conservation and Recovery

Government lands within the butterfly's range are already managed to conserve the species. Control of logging leases and burning may allow the skipper to expand to additional sites.

Contact

U.S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
P.O. Box 25486
Denver Federal Center
Denver, Colorado 80225
http://www.r6.fws.gov/

References

ERT Company. 1986. "1986 Pawnee Montane Skipper Field Studies." Prepared for the Denver Water Department, Denver.

Scott, J. A., and R. E. Stanford. 1982. "Geographic Variation and Ecology of Hesperia leonardus (Hesperiidae)." Journal of Research on the Lepidoptera 20 (1): 18-35.

U.S. Fish and Wildlife Service. 25 September 1987. "Determination to List the Pawnee montane skipper, Hesperia leonardus montana, as a Threatened Species." Federal Register 52 (186): 36176-36180.