Heading out on a favorite hiking trail north of Missoula, the colors of springtime are vibrant. In the low, shaded areas near the trailhead, slender petals of trillium spatter the woods in white. Farther up the gulch, glacier lilies wash the slopes in yellow and the indigo lanterns of wild clematis dangle from their climbing vines.
The canopy opens where a shallow stream crosses the path, revealing two small, silvery blue butterflies flitting about in slow and bouncy patterns. As I find my footing rock-by-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 will be few herbivore competitors for the hatchling blue caterpillar’s food source.
In her search for an ideal lupine on which to lay her egg, the mother blue identifies a plant located near a colony of small black ants. The 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.
Immediately upon hatching, the caterpillar uses these glands to emit chemical signals that broadcast its location to the nearby ant colony. It might seem that the slow-moving, soft-bodied caterpillar would 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 they have located the caterpillar, the ants set about patrolling the plant, 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 another chemical signal, this time mimicking the ants’ alarm pheromone, and 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. Silvery blue caterpillars that were isolated from ant colonies were twice as likely to be attacked by parasites, such as the braconid wasp. This wasp, if given the chance, will lay its eggs inside the living caterpillar, allowing its hatchling larva to dine on the flesh of the silvery blue as their first meal – and leaving behind an empty husk.
In exchange for their protection, the silvery blue caterpillar supplies its attending ant colony with a nutrient-dense ‘honeydew.’ The ants harvest this thick, carbohydrate and amino-acid-rich honeydew from a gland in the caterpillar’s abdomen, called the dorsal nectory organ or ‘honey gland.’
Under the ants’ protection, the 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.
This is Krys Standley with the Field Notes writing workshop at the MNHC, and recorded outside at the Fort Missoula Native Plant Garden. Fieldnotes is brought to you by the Montana Natural History Center, providing natural history education for schools and the public throughout Montana. For information about upcoming events and programs at the Center, call 327.0405, or visit our website at MontanaNaturalist.org