Hesperia ottoe W. H. Edwards, 1866
Basis for Listing
The Ottoe Skipper (Hesperia ottoe) historically ranged from northern Texas north through the eastern Great Plains to southwestern Manitoba and east through most of the tallgrass prairie region to northern Indiana and southern Michigan. Prior to the destruction of native prairie and barrens habitats, it must have been a fairly common butterfly. Lindsey (1942) reported that it was "moderately plentiful...on virgin prairie" in Iowa. Although it still occurs throughout most of this range, it is spottily distributed and generally uncommon. Of the 16 states and one Canadian province in which it occurs, about 80% rank it as Critically Imperiled or Imperiled (94% if it were similarly ranked, as it probably should be, in the two states that haven’t assigned ranks). No state ranks it better than Vulnerable (NatureServe 2016). It is unclear whether this species is secure in any part of its range.
There is no early documentation of the Ottoe Skipper’s distribution and abundance in Minnesota, but it is reasonable to surmise that it was fairly common before the prairie disappeared (Minnesota’s Remaining Native Prairie). Since the first Minnesota report of the species in 1965, it has been encountered at 13 locations distributed among ten counties, with a possible location in each of two additional counties (Dakota and Rock) for which there are no confirming specimens. All locations lie south of a line from Dakota County (Eastern Broadleaf Forest Province) west to Big Stone County (Prairie Parkland Province), which is probably the northern limit of the species’ natural range in Minnesota. Only two sites are documented to have supported strong populations, one in Lincoln County and one in Wabasha County, both are large prairie remnants. One site in Pipestone County, near the Lincoln County site and similar to it, supported a persistent but small population. The other records are mostly of one to a few skippers observed on a single visit to a site. Most of these also involve small habitat remnants.
This skipper is completely dependent upon native prairie and barrens habitat. There is no evidence that reconstructed prairie provides suitable habitat for this species. Therefore, additional loss of native prairie habitat threatens its survival in Minnesota. Unprotected native prairie remnants are at risk of being destroyed to grow crops, to mine sand and gravel, or to build housing or other facilities. Less dramatically, they can be converted to non-native pasture through overgrazing. In addition to protection from destructive activities, prairie requires active management to prevent encroachment by trees and eventual succession to woodland and to suppress nonnative invasive species. The principal tool for this purpose is prescribed fire, and this itself poses some threat to the persistence of this skipper in remnant habitat patches, especially small ones (Dana 1991; Swengel and Swengel 1999). In most cases, extirpation would not be reversed by recolonization as remnants are too far apart in an inhospitable agricultural landscape.
For these reasons the Ottoe Skipper was listed as state threatened in 1984, and there was no evidence indicating a change in the skipper’s status when the state list was reviewed in 1996. However, a dramatic decline has since become evident. The last confirmed observation of an Ottoe Skipper in western Minnesota was in 1995 in the Lincoln County site that formerly supported a strong population. Repeated surveys since the early 2000s have not encountered it at any site in this part of the state. The strong southeast population (Wabasha County) persisted longer, but no Ottoe Skippers have been documented there since 2012. In response to these new data, the status of the Ottoe Skipper was elevated to state endangered in 2013.
The Ottoe Skipper is a typical member of the “grass skippers” (subfamily Hesperiinae). It has a robust body, narrow angular forewings, and shorter more rounded hind wings. Although all grass skippers in Minnesota are small, the Ottoe Skipper is one of the larger species in this group. Forewing length (base to apex) is 1.6-1.7 cm (0.63-0.67 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 in wing markings most notably on the upper surface of the forewings. Males are typically a smooth golden orange above with diffuse, darker marginal coloration. There is a narrow, almost linear, black “brand” containing specialized scent scales used in courtship centrally placed along the long axis of the forewing. Females are variable but tend to be darker or duller with bands of lighter spots on both hind and forewings. The largest spot, near the center of the forewing, is translucent. The pattern has a soft smeared look. The underside of the hind wing (the forewing is mostly hidden at rest) is generally buff-colored to yellow. Fresh males show no evidence of spots, but in females the absence of yellow overscaling on the spots makes them appear dark against the surrounding yellow. As the overscaling wears off with age both sexes become darker and duller and a lighter spotband becomes evident on the hind wing underside of females.
Similar skippers that fly at the same time and in the same habitat as the Ottoe Skipper include the Dakota Skipper (Hesperia dacotae), the Long Dash (Polites mystic), the Sachem (Atalopedes campestris), the Delaware Skipper (Anatrytone logan), and the Arogos Skipper (Atrytone arogos). The Dakota Skipper is quite similar but is smaller and grayer or browner beneath. The Long Dash is usually a richer orange above with a more strongly contrasting 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, with a contrasting broad band of lighter spots that are often quite pronounced in females. The Sachem is as large as the Ottoe Skipper, but the male brand is a large very black rectangular patch. A pale spotband is usually evident on the hind wing underside in both sexes forming a sharp chevron. Beneath, both the Delaware and Arogos skippers are unmarked clear yellow. On the upperside, males lack the black brand on the forewing, and females have no whitish spots. The Arogos Skipper is noticeably smaller than the Ottoe Skipper. In western Minnesota, the Pawnee Skipper (Hesperia leonardus pawnee) begins to fly when some Ottoe Skippers (primarily females) are still around. Reliably distinguishing males requires examining technical characters (color of the scent scales within the forewing brand or genitalic structures); females usually have contrasting lighter spots on the hind wing beneath, and none of the pale spots on the forewing above are translucent.
The Ottoe Skipper occurs in native dry-mesic to dry prairie where mid-height grasses (little bluestem [Schizachyrium scoparium var. scoparium], prairie dropseed [Sporobolus heterolepis], side-oats grama [Bouteloua curtipendula]) are a major component of the vegetation. This includes prairies on deep sands, on steep bedrock-controlled slopes, and on slopes and hills in unsorted glacial till. Adults will forage into nearby moister prairie (mesic and wet prairie) for nectar.
Biology / Life History
The Ottoe Skipper has a single annual generation. Adults emerge from pupae during a 3-4 week period beginning at about the summer solstice (males, on average, a few days earlier than females). Adult survival in the wild is probably a few days to a week. Eggs hatch in about ten 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 (Dana 1991).
The grass skippers are so called because the larval food plants are all grasses or sedges. Ottoe Skipper larvae feed on grasses, but females lay eggs (singly) on both grasses and forbs. Newly hatched larvae are left with the challenge of finding a suitable host plant. In southwestern Minnesota, females have frequently been observed laying eggs in the central disk of purple coneflower (Echinacea purpurea) heads (Dana 1981), and a similar behavior has been reported in Illinois (McGuire 1982). Larvae live in shelters that they construct of plant materials, emerging only briefly to clip blades of grass that they then pull back inside to consume. Initially these shelters are vertical tubes formed by fastening two or more grass blades together with silk. After constructing and outgrowing two or three of these above-ground shelters, larvae shift to subterranean shelters formed among the bases of grass culms similar to those of Dakota Skipper larvae. Hibernation of partly grown larvae occurs in a shelter of this type. When this is outgrown in the spring, subsequent shelters are constructed of silk and debris on the soil surface, including the final shelter in which pupation occurs (Dana 1991).
Larvae feed on several grass species that are common in their habitat. Structural properties of the grasses may be more significant than chemical ones: smaller bunch grasses such as little bluestem appear to be more suitable for shelter construction than larger species such as big bluestem (Andropogon gerardii) or Indian grass (Sorghastrum nutans) or rhizomatous grasses such as Kentucky bluegrass (Poa pratensis) and smooth brome (Bromus inermis). Adults are avid seekers of nectar. They will visit almost any flowers available, though narrow-leaved purple coneflower (Echinacea angustifolia) is particularly attractive where it occurs (Dana 1991; Swengel and Swengel 1999). Males also visit damp soil for moisture and nutrients.
Females mate shortly after emergence and probably rarely mate with more than one male. Males seek receptive females primarily by perching on vegetation and pursuing any passing insect that might be a female Ottoe Skipper. Females respond to pursuit by descending into the vegetation; if the female is receptive, coupling follows almost immediately. Unreceptive females jerk their wings sharply and attempt to clamber away, eventually escaping. 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). Ottoe 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 Ottoe 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 (Lindsey 1942; McCabe 1981; Royer 1988). If not properly managed, long-term grazing can easily degrade prairie and destroy it as skipper habitat, and episodes of heavy grazing may eliminate this skipper even if the prairie is not degraded. Light grazing, however, may produce a habitat structure that is favorable to this skipper. The Ottoe Skipper was common in the Lincoln County site for several years after grazing was discontinued but eventually declined and ultimately disappeared as the prairie vegetation became taller and denser. Similar declines have been observed in Wisconsin sites after historical grazing management ended (Swengel et al. 2010). Use of herbicides to control weeds or shrubs can eliminate critical nectar sources, and insecticide drift from nearby agricultural fields can kill this skipper. In some locations in Minnesota the habitat is seriously threatened by the encroachment of shrubs and trees. Small isolated colonies of the Ottoe Skipper are at high risk of extirpation resulting from both natural events (such as severe drought or hailstorms) and human caused ones (such as accidental insecticide application) as well as from the vagaries of normal population processes (for example, by chance the few adults in one generation of a small population are all male). Loss of genetic diversity in small isolated populations is another possible threat.
Use of fire in the management of prairie remnants inhabited by this skipper is an important concern. Immature stages of the species can be killed by prairie fire, and small populations will be especially at risk of extirpation. Burns when larvae or pupae are in surface shelters cause greater mortality than burns when larvae are in buried 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 conducting 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 may provide a suitable option in these cases; Swengel (1998) provides evidence that late-summer haying is more favorable for several prairie skippers than rotational burning.
Conservation Efforts in Minnesota
Much of the prairie habitat that supported both of the two largest known Ottoe Skipper populations is protected through ownership and management by public agencies or private conservation organizations. This is also true for several of the smaller populations. The Minnesota DNR has conducted or supported a number of survey efforts to find new locations and update information for known locations of this species. Guidelines for protecting skipper populations within a fire-management program are employed by the major owners of Ottoe Skipper habitat in Minnesota, and efforts have been made to generally increase awareness among land managers of the potential negative impacts of prescribed fire on insects and of strategies to minimize them.
Robert P. Dana, Ph.D. (MNDNR), 2018
(Note: all content ©MNDNR)