The Western Pygmy Blue is a member of the family Lycaenidae, the large worldwide family of butterflies which includes all blues, coppers, hairstreaks and metalmarks. In Washington there are 46 species of lycaenids, 20 of which are blues. The Western Pygmy Blue is the only blue in our area in the genus Brephidium. Brephidium exilis is arguably the smallest butterfly in America.

This tiny blue is a year-round resident of arid regions in southern states but has been documented only once in Washington State, on 9-17-04 by Robert M. Pyle at the northern unit of the Umatilla NWR along the Columbia River. This species occurs most years in southern Oregon but may reach Washington only during invasion years. Beyond the Pacific Northwest the Western Pygmy Blue occurs throughout much of the southwestern USA from east Texas to the California coast, and from Oregon south as far as Venezuela. The preferred habitat is low altitude desert, salt marsh, alkaline prairie and waste areas. This is a species of warmer climates and is unlikely to survive northern winters, however climate warming may increase the chances of this species occurring more frequently in Washington.

Brephidium exilis reproduces quickly with continuous multiple broods, perhaps as many as eight or nine broods per year based on observed development rates. It is likely that the juvenile stages occur occasionally in Washington but until further records are obtained the species must be considered single-brooded here. In California eggs were observed laid on leaves and inert surfaces. Host plants include various species of Atriplex (Saltbush), Chenopodium (Pigweed), Salsola (Russian Thistle) and other chenopods which are more common in Washington than Atriplex. Kochia scoparia (Kochia) is another recorded host and is an abundant weed in the Columbia Basin where this species may be sought. Eggs hatch in three-four days at room temperature. The tiny newly hatched larvae (0.7mm) are very difficult to see let alone tend. Shortly after hatching the larvae bore entirely into Atriplex seeds (and likely those of other hosts) where they remain hidden, feeding on the seeds from the inside. The only evidence of larvae visible at this time is occasional frass around seeds with tiny entry holes. The larvae remain inside the seeds during the second instar, but thereafter feed from the exterior. One pupa found wild was anchored by a thin silk girdle within an Atriplex seed cluster. Larval development is rapid, pupation occurring about 26 days after oviposition. Adults eclose ten days later, a total of 36 days from newly laid egg to adult. The larvae do not construct nests at any point in their development other than the hollowed out seeds in which they initially live. This species apparently has no diapause stage in southern areas, with broods developing all year, however Scott (1986) recorded B. exilis overwintering as pupae in more northerly parts of its range. This species is reportedly myrmecophilous (ant-tended), thus the larvae may be found in the wild by watching for associated ants.

The Western Pygmy Blue is unique in our area. Ventrally the wing pattern is mottled white and gray on the hind wing, with a row of large dark peripheral spots, completely distinct among our fauna. Adults may be observed roosting communally at night on dead or relatively leafless shrubs near the host plants, with up to 8 or 9 adults in close proximity at such roosts. During the day adults frequent plants with fresh, soft clusters of seeds, at least in the case of Atriplex food plants. This species can be very common in disturbed or waste areas around its food plants, where the easily overlooked males fly endlessly in search of mates. The females are even less conspicuous, often remaining quietly in the interior of the plants or sipping nectar.

Photos by Dave Nunnallee

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