Hesperia dacotae

This butterfly, dependent on native, alkaline prairie, has lost the vast majority of its historical habitat and continues to decline within the fragments of its former range.

Range now restricted to southern Manitoba and Saskatchewan, Canada, south through North Dakota and eastern South Dakota to western Minnesota, USA. Range extent calculated at 206,000 km2, but would only be 174,000 km2 if two southernmost sites are no longer extant (Royer 2018). Formerly had a larger range that also included northern Illinois, parts of Iowa, and much of Minnesota. Now occurs in isolated remnant colonies and a few metapopulations, but was once a landscape level species over a vast northern prairie range. The last collection in Illinois was in 1888. Held on into 1990s in northern Iowa but since has died out. Britten and Glasford (2002) interpret genetic evidence as suggesting most of their sample sites were originally connected.

USFWS (2018) estimated that in 2017, 75 metapopulations (here considered equivalent to occurrences) persisted across three states and two provinces.

Threats include: loss and fragmentation of habitat (to intensive agriculture, residential and other development), grazing, early- or mid-summer haying, fire (controlled burning and wildfire), flooding, pesticides, habitat succession, and invasive species (COSEWIC 2014, USFWS 2014, USFWS 2018).

Dakota Skipper populations require periodic habitat disturbance in order to thrive (USFWS 2018); however, because they now persist only in small fragments habitat, disturbance of any kind can be a threat.

Historically, fire affected only a small proportion of Dakota Skipper habitat annually. Today, however, both prescribed burning and wildfires are a serious threats at some sites, and almost certainly account for some past extirpations. Larvae may escape some spring fires as documented by Dana (1991), but most or all are killed in fires in late spring through fall. Thus the response of the Dakota Skipper to fires is typical of prairie specialist butterflies and skippers in general (e.g. Swengel, 1996, 1998, Swengel and Swengel,1999), although recovery is possibly slower. Overall, fire could be regarded as a threat if more than half the occurrence is likely to burn in a two year period. Full recovery of populations to pre-burn levels might take several years and densities are significantly lower at sites managed by fire than those managed otherwise (Swengel and Swengel, 1999). In summary, fire is a threat at any site where too little habitat is left unburned, or where patches are burned too frequently (USFWS 2018).

Flooding can threaten or even eliminate Dakota Skipper populations when it occurs too frequently or lasts too long (USFWS 2018); this is the major threat to Canadian populations, especially now that the skipper's habitat fragments are no longer interconnected and recolonization is impossible (COSEWIC 2014).

Conversion of native prairie to cropland or tame grass continues to claim sites (COSEWIC 2014). Dakota Skipper populations are now isolated in the fragments of a once vast grassland; those separated by more than 1 km may be effectively isolated from each other (USFWS 2018). Small, isolated populations may be lost as a result of stochastic events. The likelihood of extirpation is probably directly related to habitat fragment size; in surveys in Minnesota, Swengel and Swengel (1997, 1999) found no Dakota Skippers on prairie remnants less than 20 ha and significantly lower abundance on remnants 30-140 ha, relative to those on larger tracts (reviewed in USFWS 2018).

Grazing can have both positive and negative impacts, depending on its timing, frequency, and intensity (USFWS 2018). Stocking rates that are too heavy, or season-long grazing can result in the removal of forage for Dakota Skipper larvae and floral nectar resources for adults, and may result in long-term changes to vegetation structure and composition (USFWS 2018). Cochrane and Delphey (2002) suggest that grazing is more tolerated in tall grass habitats than in mixed grass prairies, and light grazing might even be beneficial in these habitats. Rotational grazing can be favorable for Dakota Skippers, especially where it is sufficiently light to maintain native plant species diversity (Skadsen 1997, USFWS 2018). Grazing may favor key components of Dakota Skipper habitat while minimizing mortality compared to prescribed fire (Dana 1991, Schlicht 1997, Skadsen 1997, all cited in USFWS 2018).

Although haying can be a good method to maintain prairie habitat and Dakota Skipper populations, it should be used judicially and in the late summer or fall, after the adult flight season has ended; rotating mowed areas over years may further enhance persistence (USFWS 2018). Mid-summer haying can destroy nectar plants and may kill individual skippers directly.

Paradoxically, lack of disturbance in prairie habitats allows for invasion of non-native plants, particularly Smooth Brome and Kentucky Bluegrass. This invasion reduces native plant diversity and consequently reduces important larval food plants and adult nectar resources for Dakota Skipper (reviewed in USFWS 2018).

Herbicides, which are commonly used to control weeds in both crop fields and grasslands, can result in skipper population declines through the loss of adult nectar sources, especially when nectar sources are limited (USFWS 2018).

Before the arrival of soybean aphid within the range of the Dakota Skipper in about 2000, aerial insecticide applications were rare in that region. To control the aphid, a variety of insecticides have been used, and these applications have coincided with a decline in skippers (Runquist and Heimpel 2017). A number of these insecticides were detected drifting into Dakota Skipper habitat from applications on adjacent cropland (Runquist 2017, Runquist and Heimpel 2017, USFWS 2018).