Western pygmy blue (Brephidium exile)

Western pygmy blue resting on flower
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Western pygmy blue fact file

Western pygmy blue description

GenusBrephidium (1)

The smallest butterfly in the USA, the tiny western pygmy blue (Brephidium exile) is notoriously difficult to find, but is certainly worth searching for as it is beautifully patterned with delicate markings (3). The wings of this alluring butterfly are copper brown, marked with dull blue on the upper-sides, and flecked with white on the underside of the hindwings. The wings are also fringed with white, and there is a row of black spots on both the outer margin and near to the base. The western pygmy blue begins life as a yellow-green caterpillar spotted with tiny brown tubercles (4) (5).

Wingspan: 1.2 - 2 cm (2)

Western pygmy blue biology

Between May and November, the male western pygmy blue defends a small territory, and spends much of its time searching for receptive females. After mating, the female western pygmy blue lays the eggs on the leaves of a saltbush species (Atriplex spp.) (2). The eggs hatch into caterpillars which form a symbiotic relationship with ants (Iridomyrmex species), for which the caterpillar provides food in the form of a nectar liquid whilst gaining protection from the ants in return (6) (7). Once the caterpillar is fully grown, it will pupate to form a loose cocoon, and then undergoes metamorphosis before emerging as an adult butterfly (8).

A weak flier, the western pygmy blue flies close to the ground, and is restricted by its short proboscis to collect the nectar from small flowers. The caterpillar feeds on all parts of the host plant (3).


Western pygmy blue range

Only occurring in North America, the western pygmy blue ranges from the Texas coast, northwards into the central states and to the western coast of the USA (2).


Western pygmy blue habitat

The western pygmy blue occurs in alkaline habitats such as deserts and salt marshes (2).


Western pygmy blue status

The western pygmy blue has yet to be classified by the IUCN.


Western pygmy blue threats

There are no known threats to the western pygmy blue.


Western pygmy blue conservation

There are no known conservation measures targeting the western pygmy blue.

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Having a pH greater than 7.0. Soil is regarded as alkaline if it has a pH between 8.0 and 10.0. Alkaline soils are usually rich in calcium ions.
A sheath of silk, which is spun around the pupae of some insects (a pupa is a stage in an insect’s development, when huge changes occur that reorganise the larval form into the adult form. In butterflies the pupa is also called a chrysalis).
An abrupt physical change from the larval to the adult form.
A tubular protrusion from the anterior of an animal (e.g. the trunk of an elephant).
The process of forming a pupa, the stage in an insect’s development when huge changes occur that reorganise the larval form into the adult form. In butterflies the pupa is also called a chrysalis.
Symbiotic relationship
Relationship in which two organisms form a close association. The term is now usually used only for associations that benefit both organisms (a mutualism).
An area occupied and defended by an animal, a pair of animals or a colony.
A small, rounded, wart-like bump on the skin or on a bone.


  1. ZipcodeZoo (May, 2009)
  2. Butterflies and Moths of North America - Western pygmy blue (December, 2010)
  3. Phillips, S.J. and Comus, P.W. (2000) A Natural History of the Sonoran Desert. University of California Press, California, USA.
  4. Tveten, J.L. and Tveten, G. (1996) Butterflies of Houston and Southeast Texas. University of Texas Press, Texas, USA.
  5. Opler, P.A. (1998) Eastern Butterflies. Houghton Mifflin Company, New York, USA. 
  6. Art Shapiro’s Butterfly Site - Western pygmy blue (December, 2010)
  7. Atsatt, P.R. (1981) Lycaenid Butterflies and Ants: Selection for Enemy-Free Space. The American Naturalist. 118(5): 638-654
  8. O’Toole, C. (2002) The New Encyclopedia of Insects and their Allies. Oxford University Press, Oxford, UK.

Image credit

Western pygmy blue resting on flower  
Western pygmy blue resting on flower

© Bill Bouton

Bill Bouton


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