Hesperia dacotae (Skinner, 1911)
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Basis for Listing
The Dakota Skipper (Hesperia dacotae) is a northern prairie endemic whose historical range extended from southern Saskatchewan and Manitoba through the Dakotas, Minnesota, and Iowa, and east to the Chicago area. Most of its native prairie habitat throughout this range has been lost through conversion to agriculture. The skipper has disappeared south and east of Minnesota and has become increasingly rare and local in its remaining range (Cochrane and Delphey 2002).
In pre-agricultural Minnesota, the Dakota Skipper probably occurred in about 40 counties where prairie predominated (Prairie Parkland Province) to at least the eastern limit of Des Moines Lobe calcareous glacial tills in Waseca and Freeborn counties. There are historical records from only 18 of these counties, most along the west edge of the state. As recently as the early 2000s this butterfly still occurred in 11 of these 18 counties with site complexes in four of them that supported good populations. Although observations suggested a possible decline in one of these sites beginning at this time, surveys in 2007 and 2008 still encountered this skipper in all four of these sites with robust numbers in two that were intensively surveyed. These surveys also found it present in several previously known sites that historically had smaller numbers of adults and in three sites searched for the first time. However, extensive surveys beginning in 2012 and continuing every year since have found only one Dakota Skipper population remaining in Minnesota in one of the four major site complexes. Intensive surveys at this site in 2014, 2015, and 2016 suggest that the total number of adults in each annual generation here has been in the low hundreds at most, compared with thousands of adults per year in the mid-1980s. Similar declines have been documented in the non-Minnesota portion of this skipper’s range.
The Dakota Skipper is completely dependent upon the survival of its native prairie habitat. There is no evidence to date that reconstructed prairie provides suitable habitat. Although much of the prairie in the four large complexes that formerly supported strong numbers of this skipper is protected from destruction by government or private conservation ownership, portions remain vulnerable to plowing, overgrazing, gravel mining, and housing development. This is also true for many of the smaller sites. However, the recent precipitous decline of the Dakota Skipper makes it clear that protecting prairie is not sufficient to guarantee the survival of this butterfly. At present, there is no obvious explanation for its decline. The use of prescribed burning to mimic the fire regime that was a crucial component of the tallgrass prairie ecosystem has been a concern—burning too much of a site at one time, or too often, may result in unsustainable mortality. However, the skipper has vanished from several sites with no known history of prescribed burning, and much of the one site where it persists has been managed with fire for many years. Another possibility is exposure to insecticides. All of the prairie remnants are tiny islands in a sea of cropland and are thus vulnerable to contamination by insecticides applied to the surrounding crops. Because these patches of habitat are small and widely separated, once the species is eliminated from a patch—by whatever cause— it is highly unlikely that a Dakota Skipper population will be reestablished by immigrants from other patches.
The Dakota Skipper was listed as a threatened species in Minnesota in 1984. In response to the recent deterioration in its situation, the Minnesota Department of Natural Resources elevated its status in 2013 to state endangered. In 2014, its range-wide decline prompted the U.S. Fish and Wildlife Service to list it as federally threatened (USFWS 2014).
The Dakota Skipper is a typical member of the "branded" skippers (subfamily Hesperiinae). It is a smallish butterfly with a robust body, narrow angular forewings, and shorter more rounded hind wings. Forewing length (base to apex) is 1.3-1.4 cm (0.51-0.55 in.) in males, slightly greater in females. The antennae are relatively short with clubbed ends that have a sharp recurved tip. Dakota Skippers are 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. Males are typically a smooth brownish orange above with diffuse, darker marginal coloration. There is a very narrow, almost linear, black "brand" of specialized scent scales used in courtship located centrally along the long axis of the forewing. Females are more variable but tend to be darker or duller with bands of lighter spots on both hind and forewings. The pattern has a soft smeared look. The underside of the hind wing (the forewing is mostly hidden at rest) is generally a dull yellowish color with an obscurely lighter spot band in males. Females tend to be grayish brown with a more evident lighter spot band. Both sexes become darker and duller as scales wear off with age.
Similar skippers that fly at the same time as the Dakota Skipper and in the same habitat include the Ottoe Skipper (Hesperia ottoe), the Long Dash (Polites mystic), the Tawny-edged Skipper (Polites themistocles), the Sachem (Atalopedes campestris), the Delaware Skipper (Anatrytone logan), and the Iowa Skipper (Atrytone arogos iowa). The Ottoe Skipper is quite similar but is larger and brighter straw-yellow beneath. The Long Dash is usually a richer orange above with a more contrastingly dark border, and the brand on the male forewing is broader and more conspicuous; beneath, the hind wing is a richer yellow-brown, sometimes with reddish tints. There is a broad band of lighter spots, often quite pronounced in females. The orange on the upperside of the Tawny-edged Skipper is restricted to the forward edge of the wing with the remainder very dark brown. The Sachem is larger than the Dakota Skipper, and the male brand is embedded in a large round patch of black scales. Beneath, both the Delaware and Arogos skippers are unmarked clear yellow. Males of both lack the brand on the forewing, and females have no whitish spots.
In Minnesota, the Dakota Skipper seems to prefer native dry-mesic to dry prairie where mid-height grasses such as little bluestem (Schizachyrium scoparium var. scoparium), prairie dropseed (Sporobolus heterolepis), and side-oats grama (Bouteloua curtipendula var. curtipendula) are a major component of the vegetation. In North Dakota, this skipper also occurs in more mesic prairie (Royer and Maronne 1992). The most productive sites in Minnesota feature some topographic variation (Swengel and Swengel 1999). Adults will forage into nearby lowland prairie (mesic and wet prairie) for nectar.
Biology / Life History
The Dakota Skipper has a single annual generation. Adults emerge from pupae during a 2-3 week period beginning near the summer solstice (males a few days earlier than females, on average). Adults can live up to three weeks in captivity, but adult survival in the wild is probably a few days to a week. Eggs hatch in about 10 days. The partly grown larvae enter winter diapause in the fall and complete their growth the following spring. Adults of the new generation emerge after about a 2-3 week pupal stage.
Although females place eggs (usually singly) on both grasses and forbs, larvae feed on grasses (possibly on sedges to a limited extent). Larvae construct shelters in which they rest, emerging only briefly to clip blades of grass that they then pull back inside the shelter to consume. Initially, these are vertical tubes mostly or entirely below the soil surface, usually hidden among the crowded culm bases of grass tufts. As they grow, they construct a series of increasingly larger but otherwise identical shelters. Partly grown larvae hibernate in a shelter of this type. When this is outgrown in the spring, subsequent shelters are constructed of silk and debris along the soil surface, including the final shelter in which pupation occurs.
Larvae feed on several grass species in their habitat. Structural properties of the grasses may be more significant than chemical ones: the smaller bunch grasses such as little bluestem and prairie dropseed appear to be more suitable for shelter construction than larger species such as big bluestem (Andropogon gerardii) or Indian grass (Sorghastrum nutans) or than rhizomatous non-native grasses such as Kentucky bluegrass (Poa pratensis) or smooth brome (Bromus inermis). Adults are avid seekers of nectar. Narrow-leaved purple coneflower (Echinacea angustifolia) is particularly attractive, but they visit several other species as well, especially where the coneflower is absent (Dana 1991; Swengel and Swengel 1999).
Females mate shortly after emergence and rarely mate a second time. Males seek receptive females primarily by perching on vegetation and pursuing any passing insect that might be a female Dakota Skipper. Females respond to pursuit by descending into the vegetation where coupling follows almost immediately, if the female is receptive; unreceptive females respond with emphatic wing jerks and attempts to crawl away, eventually losing the male. Loose aggregations of perching males form on the upper slopes and tops of prominences in the prairie. It may be that unmated females are drawn to these same features. The dispersal behavior of mated females is poorly known, though they move freely within suitable habitat (Dana 1991). Dakota Skippers are rarely encountered away from native prairie suggesting that non-prairie habitat is a barrier to dispersal.
Conservation / Management
Habitat destruction (Minnesota’s Remaining Native Prairie) has been the primary threat to the Dakota Skipper. All prairie habitat that is not protected by permanent dedication for conservation is at risk of destruction for agricultural production, aggregate mining, or development. Wind power development is a recently added threat. This skipper is reportedly sensitive to grazing (McCabe 1981). If not properly managed, long-term grazing can convert prairie to a sod dominated by exotic species that is unsuitable as skipper habitat. Episodic, heavy grazing that does not degrade the prairie may still eliminate the Dakota Skipper by removing all necter flowers during the flight period. Use of herbicides to control weeds or shrubs can permanently eliminate critical nectar sources, and insecticide drift from nearby agricultural fields can kill this skipper in all stages of development. In some locations in Minnesota the habitat is seriously threatened by the encroachment of shrubs and trees. Small isolated colonies of the Dakota Skipper are at high risk of extirpation resulting from both natural events (such as severe drought or hailstorms) and human caused events (such as accidental 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 also a threat (Britten and Glasford 2002).
Conservation management of prairie remnants inhabited by this skipper is an important concern. Immature stages of Dakota Skippers are susceptible to being killed by prairie fire, and small populations will be especially at risk of extirpation. Prescribed burns when larvae are in buried shelters cause less mortality than burns when larvae or pupae are in surface shelters (Dana 1991). Mortality rate also varies directly with fire intensity (Dana 1991) so factors that influence the latter such as fuel load, fuel moisture content, and ambient temperature are important to consider in planning burns. Regardless of the timing, prescribed burns will always cause some mortality; subdividing a site and burning the units in a rotation that leaves enough larval habitat unburned to assure population survival and recolonization of burned areas between burns is recommended. This can be difficult for small sites. Haying can provide a suitable option in these cases; Swengel (1998) provides evidence that late-summer haying is more favorable for the Dakota Skipper than rotational burning.
Conservation Efforts in Minnesota
Much of the prairie habitat that supported each of the four largest known Dakota Skipper populations is protected through ownership and management by public agencies or private conservation organizations. This is also true for several of the sites that had smaller populations. The Minnesota DNR has sponsored or supported several survey efforts to find new locations of Dakota Skippers and updates information on known locations. Guidelines for protecting skipper populations within a fire-management program are employed by the major owners of Dakota Skipper habitat in Minnesota, and efforts have been made to educate other land managers. The U.S. Fish and Wildlife Service has developed conservation guidelines for prescribed fire, grazing, haying, habitat restoration, and weed control activities. Further research on the effects of cattle grazing on this species is needed. The Minnesota Zoo has recently begun a project to breed the Dakota Skipper in captivity to shore up the one remaining Minnesota population if needed and to reestablish populations in former sites. It has also initiated studies of possible insecticide contamination of remnant prairie sites.
Robert P, Dana, Ph.D. (MNDNR), 2018
(Note: all content ©MNDNR)