Basis for Listing
There are two subspecies of the arogos skipper, Atrytone arogos iowa (Scudder, 1869) and A. a. arogos. The range of subspecies iowa is roughly coincident with the midcontinent grassland biome, from North Dakota and eastern Montana east to Illinois and south to Texas; subspecies arogos is limited to the coastal states from New Jersey to Louisiana (Opler et al. 2006). Although the range of iowa is extensive, it is patchily distributed and generally considered uncommon to rare (subspecies arogos is now considered critically imperiled) (NatureServe 2008).
In Minnesota, the Iowa skipper occurs in the prairie region of the state as far north as Clay County, only in native prairie remnants. Most of the records are from the southwest part of the state. These reports are widely scattered and most have appeared to consist of only small numbers of individuals. About 40 colonies have been documented since 1970, but it is not known how many of these still survive. The species' disappearance from former habitats in the Twin Cities area has been documented (R. Huber, pers. comm.). Survey work since 1987 by the Minnesota Biological Survey and the Natural Heritage and Nongame Research Program has confirmed that the Iowa skipper is rare in the state. Although not in immediate danger of extirpation from Minnesota, this skipper is highly vulnerable to further population decline. For this reason, the Iowa skipper was listed as a special concern species in Minnesota in 1996.
The Iowa skipper is a medium- to small- sized skipper with a compact, tidy appearance. The forewing length in males is about 1.3 cm (0.5 in.); females are slightly larger, with a forewing length of about 1.5 cm (0.6 in.). Beneath, the sexes are similarly light yellow without markings except for faintly lighter scaling along the hind wing veins. Above, the wings are yellowish orange, with broad, grayish-brown borders. The dark shading is more extensive in the female, diffusing into the orange central area, whereas the 2 tones are neatly differentiated in the male. The male lacks a stigma or brand on the forewing, and there are no light or translucent spots on the forewing of the female. Except for the European skipper (Thymelicus lineola) and the Delaware skipper (Anatrytone logan), all skippers likely to be confused with the Iowa skipper have brands in males and pale spots in females. The European skipper is slightly smaller and more delicate than the Iowa skipper. The dark borders on the upper side are very narrow in both sexes of the European skipper, and dark scaling continues along the veins a short distance into the orange. The Delaware skipper is larger on average than the Iowa skipper. In Delaware skipper males, the dark border is narrower, and in females the boundary between the tones is sharp. The veins are dark, especially in the female. In the Delaware skipper, the dark tone is colder, more blackish, than in the Iowa skipper.
Other common names for the Iowa skipper include beardgrass skipper and arogos skipper.
In Minnesota, the Iowa skipper is restricted to native prairie habitat, generally where conditions are mesic or dry-mesic. It has been reported only once from sand prairie. Grassland dominated by non-native grasses does not appear to be suitable for the Iowa skipper in Minnesota or elsewhere in the Midwest.
Biology / Life History
The Iowa skipper has a single annual generation in Minnesota. The adult flight can begin in late June and last until the end of July, but most records are from the first three weeks of July. The partly grown larva overwinters and completes development the following spring.
Big bluestem (Andropogon gerardii) is the most commonly reported larval host in the Midwest, and little blustem (Schizachyrium scoparium var. scoparium) is also reported. The Iowa skipper has been observed ovipositing on big bluestem in Minnesota, and little bluestem is readily accepted by captive larvae (R. Dana, Minnesota DNR, pers. comm.). Larvae live in tubular shelters made by pulling together the edges of a grass blade or by fastening together two or more blades, and they feed on the portion of the blade beyond the shelter. As larvae grow they construct a series of these shelters. Because of the size of big bluestem, shelters can be 30 cm (11.8 in.) or more off the ground in the grass canopy. The partly grown larva overwinters in such a leaf-blade shelter (NatureServe 2008; R. Dana, pers. comm.). Larvae reared on potted little bluestem clumps pupated in the last constructed feeding shelter; one pupal shelter found in the wild was formed of several big bluestem blades near the ground and was clearly newly constructed for pupation (no sign of feeding on any blades) (R. Dana, Minnesota DNR, pers. comm.).
Mating activity seems to be concentrated in the afternoon, with males perching on low vegetation and chasing insects that fly past in attempt to intercept receptive females. During this activity, males fly close to the grass tops at high speed and are very difficult to follow with the eye. During the morning and early afternoon, both males and females are usually observed nectaring, where their near motionless devotion to the task is distinctive. Narrow-leaved purple coneflower (Echinacea angustifolia) is an attractive nectar flower, but the Iowa skipper will utilize other flowers (R. Dana, pers. comm.). Mating frequency has not been reported, nor has fecundity. Eggs are laid singly; females appear to oviposit only on host grasses, unlike many other grass-feeding butterflies. Life expectancy of adult Iowa skippers has not been investigated but is not likely to be more than several days, possibly a week. Evidence for long-distance dispersal exists for subspecies arogos (NatureServe 2008), but not for the prairie subspecies. Occurrences of the latter appear to consist of highly local colonies.
Conservation / Management
Small colony sizes and isolation due to past habitat loss are the primary threats facing the Iowa skipper in Minnesota. This is aggravated by continuing habitat destruction. All habitat that is not protected by permanent dedication for conservation is at risk of destruction for agricultural production, aggregate mining, or development. Small colonies are vulnerable to extirpation as a result of natural events (such as severe drought or hailstorms) or human caused ones (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). Dispersal capabilities and propensities have not been investigated, but low numbers typical of most Iowa skipper colonies and the isolation of colonies in a landscape of non-habitat suggest that immigration is unlikely to help sustain colonies or to reestablish them in suitable habitat after extirpation events. Loss of genetic diversity is thus another possible threat.
Even where protected, prairie in Minnesota is strongly susceptible to woodland invasion. Invasion by non-native herbaceous species such as smooth brome (Bromus inermis) and leafy spurge (Euphorbia virgata) is also a serious threat. Prescribed burning to replicate the natural fire regime that created and maintained the tallgrass prairie is the usual management tool for preventing succession and for suppressing non-native species. The Iowa skipper is probably highly sensitive to fall or spring prescribed burns (Swengel 1998). Larvae and pupae are likely to be exposed to lethal temperatures, especially in the fall when the grass blades with overwintering shelters are still erect. Accordingly, the use of prescribed fire as a habitat management tool should be judicious. A site should be subdivided and the units burned in a rotation that leaves enough larval habitat unburned to assure population survival and recolonization of burned areas between burns. This may be difficult for small sites. Swengel (1998) provides evidence that haying is more favorable for the Iowa skipper than rotational burning in Missouri, but apparently harmful in the upper Midwest. The elevated position of larval shelters in late summer when most prairie haying is done in Minnesota probably results in most of these being taken in the hay. The Iowa skipper also appears to be adversely affected by grazing (R. Dana, pers. comm.).
Several programs and resources are available to land managers and landowners to help protect and manage remaining prairie parcels including the Native Prairie Bank Program, the Native Prairie Tax Exemption Program, and a prairie restoration handbook.
Conservation Efforts in Minnesota
Many colonies of the Iowa skipper occur in prairies partly or completely protected as preserves by The Nature Conservancy; as Scientific and Natural Areas, State Parks, or Wildlife Management Areas by the Minnesota DNR; or as Waterfowl Production Areas or National Wildlife Refuges by the U.S. Fish and Wildlife Service. Detailed information about known occurrences within these protected areas is available to land managers. Most managers are knowledgeable about the potential negative impacts of prescribed burning on the arthropod fauna of prairies and follow guidelines to ameliorate threats. There has been an effort to educate all land managers regarding this. The Minnesota DNR has sponsored or supported a number of survey efforts to locate new populations of Iowa skippers and update information for previously known populations. Information about known occurrences is taken into account as part of the state environmental review process to help avoid negative impacts.