Satyrium sylvinus
Sylvan Hairstreak

Family Description:
Alternate Common Name:
Western Willow Hairstreak.
Note: This species is really a complex made up of several species or subspecies, including sylvinus, dryope, and an unnamed form.

Caterpillar: The caterpillar is yellowish green and marked lengthwise with two yellowish white lines along the back and a row of white diagonal dashes along the side. It reaches an average, full-grown length of 3/4 inches.
Adult: This is a fairly small butterfly, with a wingspan of 7/8 to 1 3/8 inches. The upperside is grayish to reddish brown with a bluish sheen on the darker sections of the wing and a yellowish sheen where it is lighter. The hindwing is marked with a patch of orange near the trailing edge. Underneath is grayish white and marked with a vertical row of black spots, possibly with a second, fainter row near the edge. The underside of the hindwing is marked with a large orange spot and a grayish blue spot at the rear. A thin tail originating near the spots may extend off the hindwing, although some individuals may be tailless.

This species ranges from southern British Columbia south to Baja California, and east to Montana south to northern Arizona and New Mexico. It occurs throughout most of Idaho.

It occurs in wet areas with willows (Salix spp.) such as streamsides, ditches and moist meadows.


Caterpillar: Caterpillars eat the leaves of various species of willow (Salix spp.).
Adult: Butterflies drink flower nectar, often from milkweeds (Asclepias spp.).

Eggs are laid in the fall, overwinter, and hatch in the spring. There is one generation of caterpillars each year. Each caterpillar undergoes four stages of growth, called instars. Adults generally fly from mid-May through mid-August. Butterflies tend to remain with a specific colony. The tails of the two hindwings resemble antennae and may act to fool predators into biting the wrong end of the butterfly allowing it to escape.

Males perch on short plants near willows to wait for receptive females from morning to early afternoon. Pale green eggs are laid singly on willow stems.

Idaho Status: Unprotected nongame species.
Global Rank: G4; population levels are secure, but may be of concern in the future.

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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).

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

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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.