Plebejus [Icaricia] acmon
Acmon Blue

Family Description:
Alternate Common Names:
Emerald-Studded Blue, Silver-Studded Blue.
Note: The distinctions between this species and the Lupine Blue, Plebejus [Icaricia] lupinus, are unclear and debated by scientists.

The caterpillar is brownish yellow, striped lengthwise with dark green, and marked on the sides with a band of green or other variable markings. It is covered with fine white hair. The average, full-grown length is inch.
Adult: The butterfly is small, with a wingspan of to 1 inch. The male is blue on the upperside while the female is more brownish, but may often have blue where the wings attach to the body. Both sexes are outlined in black, fringed with white, and marked on the hindwing with a curved band of orange followed by a row of black dots. Underneath is dingy white, dotted with black. The hindwing has a curved row of orange, followed by a row of black dots with a green metallic sheen.

A widespread species, it ranges from southern British Columbia east to southwestern Saskatchewan, and south through the western half of the U.S. to Mexico. It occurs throughout Idaho. Isolated populations may occur in Minnesota and New Jersey.


It utilizes a wide variety of habitats, including fields, prairies, open woodlands, and deserts.


Caterpillar: Caterpillars feed on the leaves, flowers, and fruits of wild buckwheat (Eriogonum spp.) and legumes such as trefoils (Lotus spp.), milkvetches (Astragalus spp.), and lupines (Lupinus spp.).
Butterflies drink flower nectar.

Caterpillars often can be found inside host plant pods. Each caterpillar is equipped with a honey gland, also known as a dorsal nectary organ, which emits a sugary solution agreeable to ants. The ants feed on the solution and in turn protect the caterpillar from predators. Also for protection, the caterpillar bears a pair of everscible tubercles or tentacles on the eighth segment. These tubercles are usually housed within the body, but when the caterpillar feels threatened by the approach of a potential predator, they can be pushed out to release a chemical which mimics an ant alarm pheromone. This scent causes the ants to become frenzied and aggressive, and the potential predator takes leave or is eaten by the ants. There can be several generations of caterpillars each summer in most of the range, but only one in the north. Each caterpillar undergoes four stages of growth, or instars. Young caterpillars overwinter in a physiological state called diapause. Adults generally fly from February to october. Butterflies may disperse to some degree, as the isolated populations in Minnesota demonstrate. The New Jersey population was probably introduced. This is one of the most common Blues in the west.

Males actively patrol in search of receptive females. Females lay light green eggs singly on the leaves and flowers of host plants.

Idaho Status: Unprotected nongame species.
Global Rank: G5; populations are widespread, abundant, and secure.

Ballmer, G. R. and G. F. Pratt. 1988. A survey of the last instar larvae of the Lycaenidae of California. Journal of Research on the Lepidoptera 27:1-81.

Ferris, C. D. and F. M. Brown. (eds.) 1981. Butterflies of the Rocky Mountain States. Univ. of Oklahoma Press, Norman, Oklahoma, USA, 442 pp.

Opler, P. A., H. Pavulaan, and R. E. Stanford. 1995. Butterflies of North America. Jamestown, North Dakota, USA: Northern Prairie Wildlife Research Center Home Page. (Version 05Nov98).

Opler, P. A. and A. B.Wright. 1999. A Field Guide to the Western Butterflies.   Second Edition.  Peterson Field Guide Series. Houghton Mifflin Company, New York, New York, USA, 540 pp.

Pyle, R. M. 1981. National Audubon Society Field Guide to North American Butterflies. Alfred A. Knopf, Inc., New York, New York, USA, 924 pp.

Scott, J. A. 1986. The Butterflies of North America. Stanford University Press, Stanford, California, USA, 583 pp.

Stanford, R. E. and P. A. Opler. 1993. Atlas of Western U.S.A. Butterflies (Including Adjacent Parts of Canada and Mexico). Published by authors, Denver, Colorado, USA, 275 pp.