Overview
Boisduval’s Blue (Plebejus icarioides) is an enchanting, jewel-toned butterfly that graces sunny montane meadows, coastal dunes, and sagebrush flats across western North America. Named in honor of the famed 19th-century French entomologist Jean Alphonse Boisduval, this small gossamer-winged butterfly is intimately tied to native wild lupines, upon which its velvety green caterpillars feed and develop intricate mutualisms with attending ants.
Biology and Physical Characteristics
Plebejus icarioides exhibits pronounced sexual dimorphism. The dorsal wings of males glow with an intense, iridescent silvery-blue or violet-blue sheen bordered by a crisp dark margin and white fringe, while females are warm chocolate-brown, often dusted with variable amounts of blue scaling near the body. The underside of the wings is pale silver-grey to buff, marked with a distinctive pattern of rounded black spots ringed in white.
Behavior and Ecology
Active during warm summer days, adult Boisduval’s Blues flit lightly between wildflower blossoms to sip nectar. Females seek out native perennial lupine plants (Lupinus spp.) to lay individual eggs. The hatched caterpillars secrete sweet, amino-acid-rich honeydew from specialized abdominal glands to entice native ants, which in return fiercely defend the caterpillars from predatory wasps and spiders.
Conservation and Human Impact
While the species as a whole is assessed as Least Concern by the IUCN due to its vast western range, several localized coastal subspecies have experienced severe endangerment, most notably the Mission Blue butterfly (P. i. missionensis) of the San Francisco Bay Area, which was among the first insects protected under the US Endangered Species Act.
Interesting Factoid
Caterpillars of Boisduval’s Blue hire their own ant bodyguards: they produce sugary droplets from specialized ‘honey glands’ on their backs, paying off aggressive ants to stand guard and defend them from predators.