Plebejus samuelis

Description

The Karner Blue (Plebejus samuelis) is a small, critically imperiled butterfly with a wingspan of approximately 2.5 cm. The species exhibits pronounced sexual dimorphism: males are silvery to dark blue above with narrow black margins, while females are greyish-brown to blue with irregular bands of orange crescents inside the black wing borders. The underside of both sexes is pale grey with a continuous band of orange crescents along the wing edges and scattered black spots ringed with white. The butterfly was first described as a subspecies by novelist and lepidopterist Vladimir Nabokov in 1944, and the species was later elevated.

The Karner Blue is globally ranked Critically Imperiled (G1) by NatureServe and is listed as federally endangered in the United States. It was listed as extirpated from Canada under the Species at Risk Act in 2003, and was also extirpated from Ontario's Species at Risk list. The Necedah National Wildlife Refuge in central Wisconsin supports the world's largest remaining population. In 2012, the population at Indiana Dunes National Park was extirpated following an extreme heat and drought event.

Lifecycle

Two broods are produced annually, closely synchronized with the phenology of its sole host plant, Wild Lupine (Lupinus perennis). Eggs laid by the second brood in late summer overwinter and hatch in mid to late April. Larvae progress through four instars over 25-60 days. The caterpillars engage in a facultative mutualism with ants from the subfamilies Formicinae, Myrmicinae, and Dolichoderinae — tended larvae have significantly higher survival rates (67% vs. 38% for untended larvae).

The first adult flight occurs from mid-May to mid-June, with males typically emerging before females. The second flight, typically two to four times larger than the first, occurs in July through August. Adult lifespan averages only 3-5 days. Females preferentially oviposit on lupine near nectar sources, and both broods require a mosaic of sun and shade — adults prefer open areas for nectaring and mating, while larvae benefit from the higher-quality foliage of shade-grown lupine.

Ecology

The Karner Blue has the most specialized larval diet of any butterfly in eastern North America: caterpillars feed exclusively on Wild Lupine (Lupinus perennis). This absolute dependency means the butterfly's fate is entirely tied to the availability and quality of lupine populations across its range. The species requires heterogeneous habitat mosaics — open sunny areas for adults and partially shaded areas where lupine remains succulent later into the season for larvae.

Adults nectar on a wide variety of flowers, with at least 41 species documented at a single Wisconsin site. Preferred nectar sources include Butterfly Weed (Asclepias tuberosa), Spotted Beebalm (Monarda punctata), Common Cinquefoil (Potentilla simplex), New Jersey Tea (Ceanothus americanus), and various goldenrods (Solidago spp.). Both broods depend on nectar availability throughout the flight season.

The primary causes of decline are habitat loss through development and fire suppression. Wild Lupine is a fire-dependent species, and the suppression of natural fire cycles has allowed woody succession to shade out lupine populations across much of the butterfly's historic range. Climate change poses an additional threat — the 2012 extirpation at Indiana Dunes demonstrates the species' vulnerability to extreme weather events. Spongy Moth spraying programs, habitat fragmentation, and the introduced parasitoid Compsilura concinnata also contribute to population declines.

Host Plants

Larvae feed exclusively on a single plant species:

• Wild Lupine (Lupinus perennis) — sole obligate host

Habitat

Karner Blues are characteristic of oak savannas and pine barrens — open, fire-maintained ecosystems on dry, sandy soils. They also persist in frequently disturbed anthropogenic habitats including utility rights-of-way, old fields, and road margins, provided Wild Lupine is present. The species requires a fine-scale mosaic of open sunny areas (for adult nectaring and mating) and partially shaded areas (for higher-quality larval forage). Canopy openings of at least 25 m in diameter are recommended, with habitat patches no more than 300 m apart to facilitate dispersal.