September Nature Almanac: Pine Whites Float on Late Summer Breezes

by Stephen Jones and Ruth Carol Cushman

September 2021

When out looking for butterflies in the foothills, we typically scan wildflowers blooming along either side of the trail, alert for flashes of orange or yellow. But during September, if you look straight overhead, you're likely to see a striking butterfly flitting through the conifers.

Males spend much of their time perching in or flitting among the pines searching for mates. Photo by Stephen Jones.

Pine whites (Neophasia menapia) are habitat specialists who breed in conifer forests, with a preference for ponderosa pines. They range from British Columbia and Alberta south to southern California, northern Arizona, and central New Mexico; and north through Colorado to western South Dakota. Our ponderosa pine woodlands offer ideal habitat for these unique conifer dwellers.

While distinguishing among Boulder County's more than a dozen white butterflies can prove challenging, pine whites are relatively easy to identify, with strong black veins on the underside of their hindwings and heavy black marking on the wingtips. A narrow band of red often lines the upper edge of the females’ hindwings. Most revealing is this bright white butterfly's habit of sailing among the treetops before floating down to the forest floor to sip nectar.

Females have thicker black veins and often display prominent red markings on the edges of their hindwings. Photo by Stephen Jones.

In late summer and early fall, pine whites mate and lay their oblong, watermelon-striped eggs in neat rows on conifer needles. The following summer, green caterpillars with black and white side stripes emerge, feeding in groups on fresh conifer needles. Once full-grown, the caterpillars descend on silk threads to the base of the host tree, where they pupate.

The numbers of emerging butterflies vary dramatically from summer to summer depending on the severity of the previous winter and the abundance of predatory forest birds. During "breakout years," the bright white adults resemble falling snowflakes as they float through the forest canopy. Groups of voracious caterpillars can defoliate entire conifer branches, sometimes leading to the death of host trees.

This past fall and winter, our foothills experienced a decline in forest birds due to a relative scarcity of ponderosa pine and Douglas-fir cones, part of a natural cycle that helps trees control numbers of conifer seed eaters. This decline may have been exacerbated by drought conditions throughout the West last year. This July, we began to see dozens of pine whites flitting through the pines of Gregory Canyon, Meyers Gulch, and Heil Ranch.

Though this may not qualify as a "breakout year" locally, it's been a good summer for observing and photographing these relatively slow-moving butterflies. Once they descend to the ground to sip nectar from a blooming goldenrod, it's possible to creep to within a few feet without their seeming to notice.

Pine whites occasionally descend to the forest floor to sip nectar from goldenrods and other late-blooming wildflowers. Photo by Stephen Jones.

Butterflies fear birds, not humans. So getting close simply involves moving slowly and giving them enough time to figure out who we are.

Stephen and Janet Chu's second edition of their field guide, Butterflies of the Colorado Front Range, offers profiles and photos of 100 local species and is available through the Boulder County Nature Association.

Other September Events

• Elk begin bugling, while mule deer bucks begin sparring.

• Dispersing Sage Thrashers, Pinyon Jays, and Bushtits sometimes gather in Doudy Draw and other foothills canyons.

• The last summer generation of monarch butterflies, slightly larger in size than the previous generations, emerges and sets off for Michoacán, Mexico. Individuals will overwinter in high mountain forests before initiating the return flight and breeding in early spring. Their descendants usually arrive back in Boulder County by early June next year.

Stephen Jones and Ruth Carol Cushman are authors of Wild Boulder County and The North American Prairie.