Polites mardon (W.H. Edwards, 1881)

Introduction

Polites mardon (W.H. Edwards, 1881) is a small orange and brown skipper (Figure 1) found in scattered disjunct populations in western Washington, Oregon, and a tiny spot in northern California. It was one of the last non-cryptic species to be named in the United States and was initially believed to be a Washington endemic. It is considered “Endangered” by the Washington Department of Fish and Wildlife.

Throughout the Willamette Valley-Puget Trough-Georgia Straights ecoregion prairies with a significant native plant community have declined in extent to about 3% of the area they occupied in 1850 (Crawford and Hall 1997). Within this region, the larval host plant for P. mardon is probably Roemer’s fescue (Festuca roemeri) which is an important component of the native prairie plant community. Many species of prairie-dependent vertebrate animals have declined dramatically and some are threatened with extinction or have already been extirpated from prairies (Interior 1999; Leonard and Hallock 1997; Rogers et al. 1997; Rogers 2000; Ryan 1997). Plants of the prairie have declined as well, with the golden paintbrush (Castilleja levisecta), formerly found from southern British Columbia through the Willamette Valley, federally listed as threatened under the Endangered Species Act. Several other plants are listed as sensitive under the Washington State Natural Heritage Program.

Work on insects, thus far mostly confined to butterflies, is beginning to show the same pattern of population decline and endangerment in prairie obligates. Thirteen butterfly species that use grasslands in the ecoregion are listed as endangered, threatened, candidate, or extirpated (or equivalents) by national or state/provincial governments. Two butterfly species are Candidates for listing under the federal Endangered Species Act meaning that there is likely to be enough evidence to warrant listing. Polites mardon was designated “endangered” by the state (Potter et al. 1999) and federally listed as a candidate in 1999 (Interior 1999). Recently a petition from several conservation groups was submitted to the U.S. Fish and Wildlife Service for emergency listing of P. mardon (Vaughan and Black 2002b). Butterflies are often considered to be both good indicator (Black et al. 2001; Pollard and Yates 1993; Samways 1994) and umbrella (Launer and Murphy 1994; New 1997) taxa. Thus the imperiled nature of these butterflies is particularly troubling and indicates, along with the dramatic reduction in habitat, that prairie ecosystems are in crisis.

The butterfly has been relatively stable taxonomically. A subspecies, P.m. klamathensis, primarily based on wing pattern, was recently designated (Mattoon et al. 1998). For purposes of this paper, the species will be treated as a whole. Photos of pinned specimens are from the Oregon State Arthropod Collection at Oregon State University.

Description

This is a small butterfly (18-24mm wingspread) with a thick thorax and abdomen for its size, similar to other Hesperiinae or folded winged skippers. The dorsal surface of the wings is primarily brown with a fairly even, slightly orange infused, brown edge about 20% of the width of the wing along the outer margin and irregular lighter areas that correspond in part to spots on the ventral surface (Figures 2 and 3). The ventral surface is a lighter orange-brown-tan overall with distinctive patterns of lighter yellow-tan to white spots between the veins which are consistently the same ground color as the rest of the wing. The ventral surface also has an approximately 20% unmarked outer margin corresponding to the dorsal surface.

The flight of P. mardon, like other skippers, is characteristically low, fast, and with frequent short stops worthy of the name "skipper." When perched for basking P. mardon holds its wings at an appropriate angle to maximize or minimize sun exposure depending on temperature and other microclimate factors. Often they hold their hind wings at an approximately 45 degree angle and forewings at a nearly 90 degree angle as do other Hesperiinae.

Males are significantly smaller than females (18-20mm wingspread) and they appear even smaller than females because the abdomen is narrower. The stigma, a pheromone producing organ, appears as a dark patch in the center of the dorsal side of the forewing (Figure 2). The male wing gives the impression of being shorter in relation to overall size than the female wing. Males tend to fly in a more "aggressive" manner, often taking off in pursuit of other P. mardon , regardless of sex, other butterflies, or even insects of other orders. This is probably investigative behavior to find mating opportunities. When the pursued insect is not a female P. mardon the male quickly gives up the chase. The chase can be quite extended and sometimes involves several males if the object is a female P. mardon (personal observation).

Females, larger than males (20-24mm wingspread), appear even larger because of their thicker abdomens. They lack the stigma on the dorsal forewing. Forewing shape is generally less pointed than in males with the apex angle less acute and the tornus angle more acute than in males. This is especially true in northern populations. Ovipositing females have a characteristic flight that is slow, very close to the ground, and contains many short half stops on vegetation. The stops are probably used to taste plants.

Immature stages

Larvae hatch from cream colored eggs as tiny, tan individuals with a dark, sclerotized head capsule. As larvae develop they quickly become a deep brown with a slight maroon tint. They have a distinctive narrowing of the "neck" behind the head common to Hesperiinae. Native bunchgrasses, especially Festuca, are used as hosts (A. Potter, personal communication). They produce solitary, silken larval nests that they can return to between bouts of feeding (Newcomer 1966).

Pupae are medium brown to dark brown in South Cascades populations in captivity. It is assumed that larvae overwinter in the leaf litter or in their modified larval nests similar to other skippers (MacNeill 1964). In captivity, larvae can eclose as soon as 9 days after pupation but this is unlikely in wild populations which are believed to be univoltine.

Diagnosis

Polites mardon can be separated from other skippers in the range by the use of field guides. Pyle (2002), Opler and Wright (1999), Dornfeld (1980), and Scott (1986) are particularly helpful. Polites m. klamathensis can be separated from the nominate P.m. mardon by its range (Mattoon et al. 1998). Polites mardon is the smallest butterfly in the region, which helps in identification.

In the lowland Puget Trough the butterfly is the only skipper flying at most locations in late May through mid June. In some areas, P. sonora flies at the same time. Polites sonora is somewhat larger, generally appears more infused with yellow at a distance, and prefers somewhat wetter microsites than P. mardon. When they fly at the same site, as at the Scatter Creek Wildlife Area, Thurston County, Washington, P. sonora is usually found in the edges of the grassland with intermixed shrubs and tall grasses while P. mardon stays mostly in open cespitose grassland.

Deposition of types

The type locality for P. mardon was incorrectly attributed to Mt. Hood in older literature (Mattoon et al. 1998). The type locality was corrected and reported as “small prairies near Puget Sound” by Miller and Brown (1981). I was unable to locate extant types and, as was common at the time the species was named, a holotype may not have been designated. Apparently, nominotypical (similar to named type) specimens reside at the Burke Museum of Natural History and Culture. The type locality for P.m. klamathensis is "Oregon: Jackson County: Soda Mountain road between first and second power line crossing, 3.0 to 3.8 road miles south of highway 66 at Greensprings summit, 4,500’ to 4,800’ elevation" (Mattoon et al. 1998). The holotype, allotype, and twelve paratypes of P.m. klamathensis are deposited at the Natural History Museum of Los Angeles County. Pairs of paratypes are also deposited at the Allyn Museum of Entomology, American Museum of Natural History, California Academy of Sciences, the National Museum of Natural History, and in the private collections of Matoon, Emmel, and Emmel (1998).

Material studied

All sorted specimens in the Oregon State Arthropod Collection and my personal collection were examined. The Oregon State Arthropod Collection is probably the most complete collection of Northwestern U.S. Lepidoptera and it contains P. mardon from nearly all U.S. counties from which the butterfly has been collected. Additional specimens reside at the American Museum of Natural History, Natural History Museum of Los Angeles County, Burke Museum of Natural History and Culture, several private collections, and probably elsewhere. Data collected from specimens at all the named locations above, were used for distribution records. Most specimens at the museums above have been entered into databases- either Hinchliff’s atlases (1996), Guppy and Shepard (2001), or the Washington Department of Fish and Wildlife’s database. Specimens at the M.T. James Museum at Washington State University have the potential for the most distributional surprises and they are not yet in a database. Additional efforts in curation there are slowly being accomplished by John Shepard working part time as a volunteer curator.

Etymology

According to Pyle (2002), Scudder had the speed of these skippers in mind when he named the genus after Polites, the speedy brother of Paris. However, the specific epithet mardon is shrouded in mystery as it is not mentioned in the original description. Polites mardon klamathensis is named after the type locality, near Ashland in the Klamath Basin of southern Oregon (Mattoon et al. 1998). The common name mardon skipper refers to the scientific name.

Ecology

The life cycle of P. mardon is initiated by eggs laid singly from late May through early August, depending on the latitude and elevation (Pyle 2002). Eggs hatch in seven to twelve days to become first instar larvae. Caterpillars feed throughout midsummer and pupate in late summer or early fall. In captivity larvae originating from eggs can develop fairly synchronously in some cases but in a highly asynchronous manner in others. With eggs collected from females from Puget Trough lowland locations and larvae fed lawn grasses, larvae generally developed together (Newcomer 1966, Nunnallee personal communication). In a recent rearing attempt, with eggs collected from females captured near Mt. Adams and larvae fed Festuca roemeri, the larvae developed asynchronously. At one point in November, 3rd, 4th, 5th instar larvae, pupae, and adult were all present in a group of captive individuals that hatched nearly together and were treated identically (unpublished data). It is uncertain if hostplant, temperature, larval genotype, or some other factor causes these differences in larval development. Pupae overwinter and eclose from late May (south Puget Trough) through early August (near 5000ft. elevation near Mt. Adams) in the wild. In captivity, pupae can eclose as soon as 8 days after pupation (unpublished data). Many aspects of the life history of P. mardon, such as the extent to which females choose their oviposition site, the diet of larvae in the wild, and the location of overwintering pupae are unknown .

Within the Cascade, Siskiyou, and Puget prairie grassland environments occupied by P. mardon, a variety of nectar source plants are important for adults. On Puget prairies they strongly prefer early blue violet (Viola adunca) and common vetch (Vicia sativa) (Hays et al., Washington Dept. of Fish and Wildlife, unpublished data). This is the only area for which nectar preferences have been investigated. Adults showed strong, significant differences between years, preferring Viola one year and Vicia the next. Thus, multiple usable nectar sources are probably needed for the conservation of this species. Adults have been observed nectaring on Agoseris spp. and Fragaria spp. in the south Cascades (personal observation) and Pyle (2002) indicates a wide variety of nectar sources in the south Cascades. Which of these nectar sources are preferred and which are only used in times of stress when other sources are not available is unknown.

The butterfly shows a strong preference for areas of high native plant quality that have low cover of Scot’s broom (Cytisus scoparius) and invasive exotic grasses (Hays et al., Washington Dept. of Fish and Wildlife, unpublished data). The short, open stature of native fescue bunchgrass stands allows P. mardon to access nectar and oviposition plants.

Distribution and conservation

Polites mardon was historically known from gravelly outwash prairies in Thurston and Pierce counties. Populations have also been found in southern Washington near Mt. Adams as well as a few scattered locations in southwestern Oregon and extreme northwestern California. Even prior to any habitat destruction, it probably had a limited distribution and a highly fragmented population structure. Lowland Puget Trough, Mt. Adams area, and southern Oregon populations may have been disjunct at the time of European settlement. The species has declined in many areas of western Washington and its small remaining populations are threatened by many land management activities as well as stochastic processes.

This species was likely more widespread and abundant prior to large-scale loss of most of their open, fescue dominated, grassland habitat (Potter et al. 1999). The grassland and savanna landscapes upon which P. mardon populations depend continue to be threatened today by forest encroachment, invasion by native and non-native plants, residential and commercial development, recreational activities, grazing, agricultural practices, and possibly application of herbicides. The butterflies are directly threatened by insecticides, control practices for invasive plants (this is essential to protect habitat), military training, fire (this is needed to restore habitat), and recreational activities (Vaughan and Black 2002a).

Currently in the Puget lowlands, P. mardon is found in open grasslands with abundant Roemer’s fescue (Festuca roemeri) interspersed with early blue violet (Viola adunca) (Hays et al., Washington Dept. of Fish and Wildlife, unpublished data). In the southern Cascades, P. mardon is found in open, fescue grasslands within Ponderosa pine savanna/woodland, at elevations ranging from 1900' to 5100'. South Cascade sites vary in size from small, half acre or less meadows to large grassland complexes, and site conditions range from dry, open ridgetops to areas associated with wetlands or riparian habitats (Potter et al. 1999).

In 1999, nine of 18 historic Washington sites were known to be occupied (Potter et al. 1999). Based on several years of repeated survey effort, populations at five historic sites appear to be extinct. Four of these are in south Puget Sound and one is in the southern Cascades. The current status of four other sites is uncertain. Surveys conducted in subsequent years, primarily by the Washington Department of Fish and Wildlife, U.S. Fish and Wildlife Service, and the U.S. Forest Service have resulted in locating additional southern Washington Cascades sites. Grasslands of the Puget prairies and Washington’s southern Cascades are believed to support just a few hundred individuals. All known populations are estimated to contain fewer than 1000 individuals each (Vaughan and Black 2002a).

Polites mardon’s small population sizes, limited distribution, and habitat fragmentation indicate the species is vulnerable to extinction. The Puget Trough populations are particularly threatened . In the south Cascades, areas that used to be extensive meadows have become small scattered meadows interspersed with trees, a consequence of fire suppression. Thus far, management and recovery efforts have concentrated on control of invasive plants in existing populations, population monitoring, searches for new populations, research on nectaring preferences, and initial development of captive rearing techniques (Potter et al. 2002, Hays et al., Washington Dept. of Fish and Wildlife, unpublished data; Potter et al. 1999).

Discussion

Polites mardon has been well accepted as a species ever since it was first named. It has a highly unusual disjunct population structure and research into the genetics of these disjunct populations should clarify the relationship between the populations. This might indicate whether or not variable captive rearing results among populations (unpublished data) could be the result of speciation or other genetic differences between these disjunct populations. Additional research into larval feeding, oviposition site, host and nectar species preferences, predators, and the effects of management activities would all contribute to the conservation of the species.

References

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Crawford, R. C., and H. Hall. 1997. Changes in the south Puget prairie landscape. In Ecology and conservation of the south Puget Sound prairie landscape, edited by P.V. Dunn and K. Ewing, 11-16. Seattle, WA: The Nature Conservancy.

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Interior, U. S. D. o. 1999. Endangered and threatened wildlife and plants; review of plant and animal taxa that are candidates or proposed for listing as endangered or threatened; annual notice of findings on recycled petitions; annual description of progress on listing actions; proposed rules. Federal Register 64 (205):57534-57547.

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Mattoon, S. O., J. F. Emmel, and T. C. Emmel. 1998. The distribution of Polites mardon (Lepidoptera: Hesperidae) in North America, and description of a new subspecies from southern Oregon. In Systematics of western North American Butterflies, edited by T.C. Emmel, 767-774. Gainsville, FL: Mariposa Press.

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Potter, A., J. Fleckenstein, and J. Feen. 2002. Mardon skipper range and distribution in Washington in relation to state and federal highways with a habitat description and survey method guidelines. Olympia, WA: Washington Department of Fish and Wildlife.

Potter, A., J. Fleckenstein, S. Richardson, and D. Hays. 1999. Washington state status report for the mardon skipper. Olympia, WA: Washington Department of Fish and Wildlife.

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Rogers, R., D. Norman, and D. Rolph. 1997. The status of neotropical migrant birds in the prairie landscape. In Ecology and Conservation of the south Puget sound prairie landscape, edited by P.V. Dunn and K. Ewing. Seattle, WA: The Nature Conservancy.

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Vaughan, M., and S. H. Black. 2002a. Petition to List the Mardon Skipper Butterfly (Polites mardon) as an endangered species under the U.S. Endangered Species Act: The Xerces Society, Gifford Pinchot Task Force, The Northwest Environmental Defense Center, Center for Biological Diversity, Oregon Natural Resources Council, Friends of the San Juan's, Northwest Ecosystem Alliance.

———. 2002b. Petition to List the Taylor's (Whulge) checkerspot (Euphydryas editha taylori) as an endangered species under the U.S. Endangered Species Act. Portland, OR: The Xerces Society, Gifford Pinchot Task Force, The Northwest Environmental Defense Center, Center for Biological Diversity, Oregon Natural Resources Council, Friends of the San Juan's, Northwest Ecosystem Alliance.

(This document was completed as part of an Individual Learning Contract for the MES Program at The Evergreen State College, with faculty advisor John T. Longino.)