Heinileucanuttali, Family Saturnidae

Our species profile for this issue is a large, attractive moth, the Nuttall’s Sheepmoth.  There are 18 species of Sheepmoths (genus Hemileuca) in the United States. Three of these are closely related and widespread in Washington.  A fourth species, more distantly related and different in appearance, occurs along our southeast border with Oregon.  Sheepmoths are large, attractive Saturnid (Giant Silk) moths, which fly diurnally (during the day). In flight they are easily confused with butterflies, but can be distinguished by their rapid erratic flight, and their “crash landings”.

The Nuttall’s Sheepmoth occurs in southeastern British Columbia and much of the western United States, east to the Rocky Mountains, south to northern Arizona and west to the Sierra Nevada and Cascade Mountains. In Washington, nuttali is found only east of the Cascade Mountains. Nuttall’s Sheepmoth flies in arid portions of Washington in sagebrush steppe, pinyon-juniper woodland and sagebrush meadows. It may occasionally be found flying with its two closest relatives.

The host plants for Nuttall’s Sheepmoth larvae are bitterbrush, snowberry and currant.  These host plants are also used by the Western (Elegant) Sheepmoth, Hemileuca eglanterina, but its other close relative, the Hera Buckmoth (Hemileuca hera) feeds on sagebrush.

The  following life cycle information is taken from the USGS Moth web page:

“Adults emerge in the morning and mate in the early afternoon. Females lay eggs in rings around host plant stems. Eggs overwinter and hatch in May and June. Young caterpillars feed together in groups and when older they feed alone. Fully-grown caterpillars pupate in loose cocoons in the leaf litter or in burrows in soft soil, and adults emerge from July-September. At high elevations and northern latitudes, 2 years are needed to complete development. Cocoons overwinter and in the spring the adults emerge.”

The larvae of all of the Hemileuca species are initially black and spiny, and can be found in tight clusters of numerous individuals on the host plants. The older larvae of hera are quite variable, sometimes entirely black, other times bearing bright yellow clusters of dorsal spines, and sometimes with white side stripes. All Hemileuca larvae have urticating spines, which means that they sting like nettles if handled roughly. The branched spines make the Hemileuca larvae rather similar in appearance to some brushfoot butterfly larvae, but they can be definitively separated by gently rubbing a larva on the tender skin of the back of one’s hand. If a stinging (urticating) sensation results, the larva is a Hemileuca.

Nuttall’s Sheepmoth is single-brooded, with adults flying from July to September, depending on elevation. Adults of all sheepmoths do not feed and thus are short-lived.

The adults of our three most widespread sheepmoths are quite similar in appearance, and all three are quite variable among their subspecies. Within Washington, the three can be separated fairly reliably by the amount of color on the dorsal (top) wing surfaces.  Hemileuca hera, the Hera Buckmoth, is entirely black and white and can be reliably identified by its lack of yellow or pink coloration. Hemileuca nuttali, the Nuttall’s Sheepmoth, has a yellow hindwing and generally a black and white forewing, although the forewing may have a small amount of yellow. Hemileuca eglanterina, the Western Sheepmoth, has yellow hindwings and pink forewings.  When the colors seem intermediate, nuttali and eglanterina can always be separated by the shape of the bold black line across the middle of the hindwing. In nuttali, this line flares away from the body at the back end, giving a sinuous recurved appearance, but in eglanterina this line is always straight at the back end, or even convex, curving slightly toward the body.  Occasional hybrids do occur, however, with intermediate characteristics.

Photos by Dave Nunnallee

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