Dressed in orange, yellow, black, and brown with a few sprinkles of blue and white Milbert’s Tortoiseshell (Aglais milberti) is perfectly colored for October. Its colors remind me of autumn leaves and the embers of a fire.
Milbert’s Tortoiseshell butterflies and their relatives the Mourning Cloak (Nymphalis antiopa), Question Mark (Polygonia interrogationis), Satyr Anglewing (P. satyrus), European Peacock (Aglais io), and small tortoiseshell (A. urticae) overwinter as adults and sometimes appear on warm days in early spring.
The forewing and hindwing have a dark brown triangular section at the base which is bordered by a broad orange submarginal band grading to yellow along its inner edge. In the dark area on the forewing are two orange and two black patches. There is narrow black marginal border grading into dark gray on both wings. In the upper corner of the forewing is a small white patch. The hindwing is bordered with blue spots. Wing edges are shallowly scalloped. The lower wing surfaces are patterned with wavy brown and black markings that resemble bark. Wingspan is 4.2 to 6.3 cm. The body is dark brown and black.
Class: Insecta (Insects)
Order: Lepidoptera (Butterflies and Moths)
Superfamily: Papilionoidea (Butterflies)
Family: Nymphalidae (Brushfooted Butterflies)
Subfamily: Nymphalinae (Crescents, Checkerspots, Buckeyes, Tortoiseshells, Anglewings)
Tribe: Nymphalini (Tortoiseshells, Admirals, Anglewings)
Genus/species: Aglais milberti
Caterpillars of Milbert’s Tortoiseshell feed exclusively on stinging nettle (Urtica spp.). Eggs are laid on the lower surface of the host plant and hatch in about six days. From 400 to 500 eggs are laid at one time, although as many as 900 have been recorded, in scattered heaps two to four layers thick. Upon hatching the young larva ascend as a group to the top of the plant and spins a silken web around the leaves where they feed. The young larva feed together at first but by the fourth molt begin feeding singly or in small groups of two to four. The larvae are dark-colored and covered with soft spines. The chrysalis is light brown, angular with spiny projections, and suspended by a silk thread from vegetation. There are three broods a year in most areas. Adults of the last brood hibernate during the winter in hollow trees, between boards in buildings, rock crevices, stone piles, caves and other similar places.
Adults feed on flower nectar from a variety of species including lilac (Syringa), asters (Symphyotrichum, Canadanthus, Doellingeria), goldenrod (Solidago), and joe-pye-weed (Eupatorium). New England Aster (Symphyotrichum novae–angliae) is a good late-season flower to plant for Milbert’s Tortoiseshell. They also drink tree sap and fluids from rotting fruit. You can make feeding stations with mashed over-ripe fruit and some stale beer. This will attract Milbert’s Tortoiseshell and other butterflies, too, like the Mourning Cloak.
Moist fields, riparian areas, woodland edges are preferred habitat for the adult butterflies. Egg-laying sites must have large vigorous growths of stinging nettle (Urtica dioica and other Urtica species). Along my gardens and at the edges of shrub rows I have planted clumps of stinging nettle to provide egg-laying sites for Milbert’s Tortoiseshell.
Range and Distribution
Milbert’s Tortoiseshell occurs from the Boreal Forest of Canada and Alaska to California, Nevada, and New Mexico and as far east as Newfoundland and West Virginia.
Habitat for Milbert’s Tortoiseshell can be improved by leaving old hollow trees in the woods, building rock or log piles, dense piles of brush, planting the host plant stinging nettles, and planting a variety of nectar-rich flowering plants that will bloom from early spring to late fall during the insect’s active season.
Dvořák, L.; Belicek, J.; and Fric, Z. (2009). Observations of overwintering nymphalid butterflies in underground shelters in SW and W Bohemia (Czech Republic) (Lepidoptera: Nymphalidae: Nymphalini). Journal of Research on the Lepidoptera 41: 45-52.
Opler, P. and V. Malikul. 1992. Eastern Butterflies: A Peterson Field Guide. Houghton Miflin Company, New York.
Scudder, S. H. (1899). The Butterflies of the Eastern United States and Canada with Special Reference to New England, Vol. 1, Nymphalidae. Published by the Author, Cambridge 1899.
Taylor, S. J.; Krejca, J. K.; Slay, M. E.; and Harrison, T. L. (2009). Milbert’s Tortoiseshell, Aglais milberti (Lepidoptera: Nymphalidae): A facultative trogloxene in alpine caves. Speleobiology Notes 1: 20-23.