The Karner blue (Plebejus samuelis) is a small blue butterfly native to the Great Lakes region and northeastern North America. It is closely associated with oak savanna and pine barrens habitats because its larvae feed exclusively on wild lupine (Lupinus perennis). The species once occurred in a narrow, nearly continuous band across twelve U.S. states and southern Ontario, Canada. The United States Fish and Wildlife Service currently identifies surviving populations in portions of Wisconsin, Michigan, New York, New Hampshire, and Ohio.
The butterfly has undergone extensive range contraction caused principally by habitat loss, fragmentation, and the ecological succession of formerly open savanna and barrens habitat. Fire suppression has contributed to this process by allowing shrubs and trees to shade out wild lupine and flowering plants required by the butterfly.
The Karner blue was listed as endangered under the United States Endangered Species Act on 14 December 1992. It is the official state butterfly of New Hampshire, designated the same year under RSA 3:18.
In Canada, the butterfly has not been reported since 1991 and is considered extirpated. The Committee on the Status of Endangered Wildlife in Canada designated it extirpated in April 1997, and confirmed that status in 2000, 2010, and 2019.
Taxonomy and naming
Vladimir Nabokov, better known as a novelist but also an accomplished lepidopterist, described the Karner blue in 1944. It was traditionally treated as a subspecies of the Melissa blue, under the name Lycaeides melissa samuelis.
Nabokov himself came to regard the butterfly as a distinct species, noting that the male genitalia of the Karner blue were remarkably constant across its range while those of the Melissa blue varied geographically, and that the Karner blue used a single larval host plant where the Melissa blue used many. He never completed the taxonomic work required to elevate it, and subspecific status remained the accepted treatment for decades.
Population genetic research published by Forister and colleagues found that gene flow between the Karner blue and the Melissa blue was low, comparable to gene flow between the Melissa blue and Lycaeides idas, which are treated as distinct taxa. The authors concluded that subspecific rank was inappropriate and that the butterfly should be recognised at species level. Subsequent Canadian assessments adopted the species-level classification Plebejus samuelis. COSEWIC noted in 2019 that the placement was also supported by morphological differences and by the geographic separation of the Karner blue in Ontario from the Melissa blue in western Canada.
The legal listing of the butterfly under the U.S. Endangered Species Act continues to use the older name Lycaeides melissa samuelis. Ontario changed the scientific name used on its Species at Risk list to Plebejus samuelis on January 26, 2022, following the taxonomy used by NatureServe. The province stated that the taxonomic change did not alter its legal protection.
The common name refers to Karner, New York, a hamlet between Albany and Schenectady near the Albany Pine Bush, where the butterfly was first described. Nabokov later described a group of the butterflies in his novel Pnin, although he did not identify them by name.
Description
The Karner blue is a small butterfly with a wingspan of approximately . Forewing length is 1.2 to 1.4 cm in males and 1.4 to 1.6 cm in females. The sexes differ in dorsal coloration. Males are usually silvery blue to deep blue above, with narrow black margins and a white outer fringe. Females are darker and may range from blue to purple-brown or grayish brown. They have a row of dark spots and orange crescents along the outer portions of the wings.
The underside of both sexes is pale gray or silver-gray, with black spots and orange crescents near the outer margins. These underside markings help separate the Karner blue from similar small blues within its range: the eastern tailed-blue, spring azure, silvery blue, and several other species lack the orange crescents beneath, while the eastern tailed-blue also bears small tails on the hindwing. The Melissa blue can be distinguished by orange banding on the upper surface of the female forewing, and Nabokov's blue (Lycaeides idas nabokovi) by its different habitat and host plant.
Eggs are about 0.7 mm across, flattened and pale greenish-white. Larvae are pea-green with a dark head capsule, pass through four instars, and bear glandular structures that mediate their interactions with ants. Pupae are bright green, turning light tan shortly before emergence.
Life cycle
The Karner blue normally produces two generations, or broods, each year. Its life cycle is closely linked to the seasonal development of wild lupine.
Eggs laid by the second brood remain dormant through winter and hatch in mid- to late April. The larvae feed exclusively on the leaves of wild lupine for about three to four weeks before pupating in late May or early June. First-brood adults fly from late May through late June. Eggs produced by these adults develop during the same year and give rise to a second brood, whose adults appear from early or mid-July until mid- to late August, and in some years into early September. The second brood is typically three to four times the size of the first, although in some years it is not larger; high overwintering mortality of eggs is thought to account for the usual difference.
In historical Ontario populations, adults were recorded from approximately May 25 to June 30 for the first generation and from July 12 to August 18 for the second. Weather caused substantial year-to-year variation in these dates.
Adults are short-lived. Reported average lifespans range from under 3.5 days at Indiana Dunes National Park to four or five days elsewhere, with individuals occasionally surviving two to three weeks. Canadian studies summarized by COSEWIC reported an average of approximately five days.
Distribution and habitat
The historical range of the Karner blue formed a narrow, nearly continuous band extending from the western Great Lakes region eastward to New England and southern Ontario. The U.S. Fish and Wildlife Service states that it formerly occurred across twelve U.S. states and Ontario, but is now known from portions of only five states: Wisconsin, Michigan, New York, New Hampshire, and Ohio.
NatureServe regards the species as critically imperiled globally and notes that populations have disappeared from much of the historical range.
Wisconsin and Michigan support the greatest number of sites and individuals. In New York, the Albany Pine Bush, from which the species was described, had been reduced from roughly 25,000 acres to about 2,500 acres by 1988, and the largest New York population is at Saratoga Airport, where open habitat is maintained by regular mowing.
Canada
In Canada, the Karner blue was known only from southern Ontario. The most recent confirmed Canadian populations were at the Port Franks and Pinery Provincial Park area in Lambton County and at the St. Williams Conservation Reserve in Norfolk County. Historical records also exist from Toronto, London, and Sarnia. A possible historical occurrence near Cobourg in the Rice Lake Plains has been reported, although no associated specimen is known.
By the early 1980s, the butterfly had been reduced from six known Ontario localities to two surviving sites. The Port Franks population still contained more than 1,000 butterflies in 1984 and was considered relatively secure, while only seven second-brood individuals were recorded at St. Williams that year.
The remaining populations collapsed during the exceptionally hot and dry summers of 1988 and 1989. Wild lupine at the sites dried before the second brood could lay its eggs successfully. COSEWIC reported no confirmed observations at either site after that period. The federal recovery strategy identifies the last confirmed Canadian observations as occurring in 1988 at Port Franks, Pinery Provincial Park, and St. Williams. Unconfirmed reports continued between 1988 and 1991, but no Karner blue has been reported in Ontario since 1991 despite searches at historical sites and other areas containing wild lupine.
COSEWIC designated the species extirpated in Canada in April 1997. The status was re-examined and confirmed in May 2000, April 2010, and May 2019. It is listed as extirpated under Schedule 1 of the Canadian Species at Risk Act.
Ontario initially listed the butterfly as endangered when the province's Endangered Species Act, 2007 came into force in 2008. It was reclassified as extirpated on February 18, 2009.
Ontario's natural heritage databases follow the NatureServe element occurrence methodology, under which an occurrence record documents an area in which a species is, or was, present. Historical records for the Karner blue therefore remain mapped at Ontario sites where the butterfly no longer occurs, and the presence of such a record on a distribution map does not indicate that a population was extant on the date the map was produced.
Habitat
The Karner blue is associated principally with dry, sandy ecosystems that support wild lupine, including oak savannas, pine barrens, sandy woodland openings, dunes, and related early-successional habitats.
Suitable habitat requires disturbance to prevent ecological succession from producing a closed tree and shrub canopy. Historically, wildfire helped maintain these open conditions. Modern Karner blue populations can also occur in human-maintained habitats such as utility rights-of-way, roadsides, airports, old fields, young forest stands, and military training areas where mowing, clearing, fire, or other disturbance prevents canopy closure. At Fort McCoy in Wisconsin, lupine stems disturbed by military vehicle traffic carried significantly more larval feeding damage than undisturbed stems.
Canopy cover and habitat structure
A mixture of open and partially shaded habitat is important, because the requirements of adults, larvae, the two broods, and the two sexes differ.
Adults use open, sunny areas for nectaring, roosting, and mate location; males spend the large majority of their activity in open habitat. Temperature is part of the explanation. Adult activity falls off below about , and modelling of flight thresholds indicated that first-flight females had on average 10.5 hours of activity available per day in open habitat, compared with one to two hours in partial or closed canopy.
Ovipositing females are more evenly spread across canopy conditions than males, and at Indiana Dunes National Park the average canopy cover measured at oviposition sites was about 55 per cent. Larval survival is generally highest under partial to closed canopy, principally because shade-grown lupine is a better larval food and senesces later. Lupine is nevertheless more abundant in open areas, so a trade-off exists between the quantity of lupine in openings and its quality in shade. This trade-off is the principal reason that a heterogeneous landscape supports larger and more stable populations than a uniform one.
Populations can persist in structurally uniform habitat where management substitutes for that heterogeneity, as at Saratoga Airport in New York. Patch size and spacing also matter: occupied lupine patches tend to be larger and closer to other occupied patches than unoccupied ones, and the recovery plan treats subpopulations in patches smaller than as vulnerable to extirpation.
Ecology
Wild lupine
Wild lupine (Lupinus perennis) is the only known larval food plant of the Karner blue. Consequently, the abundance, distribution, condition, and seasonal persistence of lupine are fundamental limits on Karner blue habitat.