Brephidium exile
Western Pygmy-Blue

Family Description:
Alternate Common Name: Pygmy Blue.
Note: This species is referred to as Brephidium exilis by some authors.

The coloring of the caterpillar can vary, but generally it is yellowish green to tan and shaded or striped with yellow on the back and sides. Alternatively, it may be green, shaded or striped with dark green or dark pink, or it may be green with a dark colored head and lacking stripes altogether. Often it is covered with brownish or whitish bumps. Its average, full-grown length is 7/16 inch.
Adult: This is the world’s smallest butterfly, with a wingspan of 2/5 to 3/4 inch. The upperside is an orangey-brown, with blue shading on the bases of the wings. Underneath, the wings are grayish white towards the base. The forewing is shaded with orange on the outermost half; the hindwing is marked with brown patches in the middle and is edged with a row of dark blue spots lined in metallic gold.

This species ranges from southeastern Oregon and southern Idaho south to South America, and east through the southwestern quarter of the U.S. to Nebraska, Arkansas, and Missouri.

It is typically found in lowland areas, often those that have been recently disturbed or are alkaline, such as salt marshes, deserts, or prairies.

Caterpillars eat the leaves, flowers, and fruits of members of the goosefoot family (Chenopodiaceae), including pigweed (Chenopodium spp.), saltbush (Atriplex spp.), and pickleweed (Salicornia spp.).

Adult: Butterflies use flower nectar for food.

The 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. Each caterpillar undergoes four stages of growth, or instars. There are multiple generations of caterpillars each year in most of its range. The pupal stage can overwinter in a physiological state called diapause. Adults are yearlong residents in the southern part of its range and migrate each year into the north where they generally fly from June through November. This is the smallest butterfly in the world.

Males actively patrol for receptive females. Females lay eggs singly on all parts of the host plant with most placed on the topsides of leaves and near flowering stems.

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.