Butterfly milkweed opens for the hairstreak

The butterfly milkweed is in flower along the open slopes near Boulder right now, its flat-topped clusters of small orange blooms catching the full length of early summer light. This is one of the drier-adapted milkweeds, rooted in rocky or sandy soils where other forbs thin out, and it holds its flowers open for weeks. If you are standing near a patch, look closely at the blooms. Something small and green may be working across them.

The western green hairstreak is one of the more easily overlooked butterflies in this landscape. It has a wingspan just over an inch, and when it closes its wings, the undersides show a flat, dusty green that disappears against lichen-covered rock or leaf litter. The upperside is a plain gray-brown. It is not a butterfly that announces itself. What draws attention, if anything, is the quick, low flight between plants, and the way it pauses to probe individual flowers. This is a threatened species in Colorado, present in low numbers across its range, and the population near Boulder is one of the places it still reliably appears. Its presence here in early summer is not incidental. The butterfly milkweed is part of why it is here.

Female western green hairstreaks lay their eggs directly on milkweed plants, and butterfly milkweed is one of their primary host plants along the Front Range. The eggs are tiny, flattened discs pressed against the stem or the base of a flower cluster. When the caterpillars hatch, they feed on the plant's flowers and developing seed pods. This is a narrow dependency. The hairstreak is not a generalist that will switch to another host if milkweed is scarce; it needs this plant, at this stage of flowering, to complete a generation. The early summer bloom period is the window. The flowers open, the adults arrive, eggs are laid, and the next generation begins feeding before the pods harden.

The milkweed itself is doing more than hosting one butterfly. Its flowers produce nectar in substantial quantities from structures called horns, small curved projections that hold nectar above the petals. Insects landing on the flowers contact a pollen mass called a pollinium, which adheres to their legs and transfers to the next flower. Milkweed pollination requires a visitor strong enough to pull the pollinium free, which is why larger bees and butterflies are the effective pollinators rather than small flies or beetles. The Taxiles skipper is working these flowers too right now, and the Aphrodite fritillary has been seen on the open slopes nearby. Each visit contributes to seed set, and butterfly milkweed produces its characteristic long, tapered pods through late summer, splitting open in fall to release seeds on silky white fibers.

Not everything in this landscape is native. Broad-leaved sweet pea, an invasive plant, is spreading through disturbed areas near the trail edges, and musk thistle, also invasive, has pushed into the open ground between native forbs. Both compete for the dry, open habitat that butterfly milkweed depends on. Neither is a host for the hairstreak. The more these invasive plants establish in rocky, sunny patches, the less suitable ground remains for the milkweed and the butterfly tied to it.

The orange of the butterfly milkweed flowers is distinct from almost everything else blooming at this elevation right now. Stand near a plant and give it a few minutes. The hairstreak moves quickly but lands with deliberate stillness, wings folded, green against green.