Plebejus idas nabokovi (Masters, 1972)
Lycaeides argyrognomon nabokovi, Lycaeides idas nabokovi
Basis for Listing
Nabokov's blue was originally described from Minnesota specimens. This subspecies of Lycaeides idas is restricted to northern Minnesota, southeastern Manitoba, southwestern Ontario, northern Wisconsin, and northern Michigan. Other subspecies range across Canada and southward in the mountains into the western United States. Throughout its limited range, Nabokov's blue is uncommon to rare and highly local (Masters 1972; Klassen et al. 1989).
Nabokov's blue is a small butterfly with an average wingspan of 2.75 cm (1.08 in.) in both sexes. The upper side of males is an intense, bright blue; females are somewhat variable, but generally they are mostly brownish gray, with blue limited to the basal third at most. A submarginal row of dark spots is usually present on the hind wings, sometimes with a hint of orange along their inner edge. Beneath, the 2 sexes are very similar: irregular rows of white-ringed black dots on a pale grayish ground (females tend towards a warmer, slightly darker shade) and a more regular submarginal band of paired black crescents enclosing orange, especially on the hind wings. The outermost row of dark crescents is enlarged into silvery-blue spots on the hind wings.
The Nabokov's blue inhabits various upland openings in the northern forest with low vegetation and an abundance of the larval host plant, dwarf bilberry, a diminutive rhizomatous shrub that forms mat-like colonies. This plant occurs on sandy soils and on rock outcrops. In Minnesota, all known colonies of this butterfly occur at sandy sites, but it is recorded from rock outcrops in Michigan (Nielsen 1999).
Biology / Life History
The Nabokov's blue has a single annual generation. Larvae hatch from overwintering eggs in early spring and complete growth and pupate by mid-June to early July. Adults emerge after about 10 days in the pupal stage. Females mate shortly after emergence and begin egg laying right away. Eggs are affixed singly to stems of the larval host plant, nearby plants, or debris and remain dormant until the following spring. Adults can live for at least three weeks, but average survival is probably not more than one week (Wolf 1993). They feed on nectar from a variety of available plants, native and non-native; males also feed on dung, urine-soaked soil, or even just damp soil (mud-puddling). Adults are almost always encountered in the close vicinity of host plant patches, giving the impression that this butterfly is highly sedentary. The transient nature of suitable habitat requires that emigration from patches occur to colonize new patches created by disturbance, but this aspect of adult behavior is poorly known. In one small 30-day study, no marked butterfly moved farther than 260 m (853 ft.) from the point of marking, but two unmarked individuals were encountered more than 2 km (1.2 mi.) from a known colony (Wolf 1993).
Conservation / Management
Reforestation of habitat openings will eliminate this butterfly (although dwarf bilberry, a long-lived perennial, can persist in the understory). All known Nabokov's blue sites in Minnesota are capable of supporting forest, and therefore it appears that the presence of this butterfly in the state historically depended upon periodic habitat-opening disturbances and upon the butterfly's ability to find and colonize new openings as old ones became unsuitable. Occurrence of the host plant is controlled primarily by substrate characteristics; therefore forest disturbances influenced butterfly distribution by making the already present host plant available to the butterflies. Fire was presumably the primary disturbance that created habitat for this butterfly in the pre settlement landscape. Today, after decades of fire suppression, Nabokov's blue is dependent on human induced mechanical disturbances, including timber harvest. However, silvicultural practices that reduce or eliminate the host plant, such as rock raking and herbicide application, will negate the potential of timber harvest to create habitat. Rapid reestablishment of tree cover through intensive management will also negatively affect this butterfly, as it shortens the time available for colonists to find the habitat. If the butterfly does colonize, intensive management can shorten the time interval to produce emigrants to colonize other patches. Forest is a barrier to dispersal for Nabokov's blue, so colonization requires connectivity via natural openings such as streams, or via human created openings, such as roads or utility corridors. Managing a few good sites specifically to sustain strong Nabokov's blue populations and otherwise restricting only those silvicultural activities that would harm dwarf bilberry, would probably be an adequate conservation strategy. The use of prescribed burning to maintain the suitability of occupied habitat, while probably the most effective management approach, requires caution as all non-adult life stages of the butterfly are highly vulnerable to incineration. Subdivision of occupied sites and burning the units in a rotation that allows for recolonization after burning will minimize the risk of local extirpation.
Conservation Efforts in Minnesota
In 1986, The Nature Conservancy (TNC) purchased a significant portion of the McNair site from a timber company. This site is the type locality for the Nabokov's blue (the source of the specimens from which the subspecies was originally described). TNC transferred the property to the U.S. Forest Service in 1991, which designated it as a Special Use Area in the Superior National Forest. Forest Service biologists, with support from the Minnesota DNR, developed a management plan for the site and began cutting planted and invading trees and shrubs and burning small subdivisions of the site in 1998. Monitoring of Nabokov's blue numbers at the site, also begun in 1998, is an ongoing project. Nabokov's blue is included on the U.S. Forest Service Region 9 Regional Forester Sensitive Species List, which will encourage attention to the needs of this butterfly in forest planning for the Superior National Forest. Both the Forest Service and the Minnesota DNR have funded inventory efforts in northeastern Minnesota for the butterfly. In 2002, the eastern region of the U.S. Forest Service prepared a conservation assessment (Wolf and Brzeskiewicz 2002) for the Nabokov's blue and dwarf bilberry to aid in developing a plan for conserving these species on U.S. Forest Service lands.