Butterflies, Lupines, and Ants, Oh My!
by Krys Standley
Broadcast 6.8 & 6.11.2021
It’s a good spring for wildflowers. Heading out from a favorite trailhead north of Missoula, trillium spatters the woods in white. Farther up the gulch, glacier lilies swathe the slopes in yellow and the indigo lanterns of wild clematis dangle from their climbing vines.
A shallow stream crosses the path and a pair of small blue butterflies flit about in a slow and bouncy flight pattern, adding to the color palette. As I find my footing rock-to-rock across the stream, I recall an unusual aspect of the life cycle of these butterflies.
In addition to the normal progression – from egg, to caterpillar, to chrysalis, to winged adult – their story contains another chapter. These butterflies, commonly referred to as “blues,” are myrmecophilous. Literally translated from Latin as “ant-loving,” this term applies to species frequently associated with ants, a phenomenon that is widespread within the Lycaenoid family of butterflies to which blues belong.
Silvery blues are one of twenty species of blues in Montana. With a wingspan reaching just over an inch, they are among our smallest butterflies. Males often congregate around streams and puddles and can be recognized by the iridescent light blue of their upper wing surfaces. Females, more subdued in color, are brown with just a dusting of blue.
Silvery blues are one of the first butterflies to emerge in the spring. After mating, the silvery blue mother lays her eggs singly on the undersides of flower buds in the pea family, of which lupine is a favorite. Her plant selection is no chance occurrence: All parts of the lupine, from its palmate leaves to its tall blue flower stalks, contain toxic alkaloids, to which blues are evolutionarily adapted. This specialization means there are few herbivore competitors for the growing blue caterpillar’s food source.
In her search for an ideal lupine on which to deposit her egg, the mother blue identifies a plant located near a colony of small black ants. Her egg hatches in four to six days, revealing a rugged green caterpillar covered in short white hairs and marked with a rust-colored stripe running lengthwise along its back. If you were to view the caterpillar through a magnifying glass, you would see dark, single-celled glands dotting its skin.
After hatching, the caterpillar emits chemical signals from these glands. These signals broadcast its location to the nearby ant colony. The slow-moving, soft-bodied caterpillar could be an easy meal for the ants. However, the silvery blue caterpillar has evolved to avoid this fate: Its chemical secretions pacify and appease the arriving colony.
Once recruited to the caterpillar’s service, the ant colony patrols the lupine on which their charge feeds, guarding against predators and parasites. If you’ve ever noticed a lupine covered with ants, they were surely engaged in such a task. The caterpillar actively participates in its guardianship: When threatened, it emits a different chemical signal that mimics the ants’ alarm pheromone, spurring the colony to a heightened defensive state.
Association with ants greatly increases the caterpillar’s chances of survival. In one study, researchers manipulated the presence of ants to determine the effectiveness of their protection. When isolated from ant attendants, silvery blue caterpillars were twice as likely to be attacked by parasites, such as the braconid wasp. (If given the chance, this wasp will lay its eggs inside the caterpillar. And, the hatchling wasp larvae will eat the living flesh of the silvery blue as their first meal – leaving behind an empty husk.)
In exchange for their protection, the blue caterpillar supplies its attending ant colony with a nutrient-dense “honeydew.” The ants harvest this thick, carbohydrate- and amino-acid-rich liquid from a gland in the caterpillar’s abdomen, called the dorsal nectory organ or “honey gland.” Honeydew production is supported by the high protein content of the lupine. Its harvest is a boon to the colony’s growth.
Under the ants’ protection, the silvery blue caterpillar will shed its skin seven times, finally reaching its full-grown length of three quarters of an inch. It is now ready to pupate. The silvery blue will overwinter in a halted state of development called diapause, under rocks and leaf litter, before emerging as an adult the following spring to begin the cycle anew.
Every week since 1991, Field Notes has inquired about Montana’s natural history. Field Notes are written by naturalists, students, and listeners about the puzzle-tree bark, eagle talons, woolly aphids, and giant puffballs of Western, Central and Southwestern Montana and aired weekly on Montana Public Radio.
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