Creature Feature – Appalachian Azure

You wouldn’t necessarily think of a caterpillar as a particularly gluttonous creature – although, Eric Carle’s very hungry caterpillar certainly could be described as such. Typically, when we think of the hungry heavy hitters of the animal kingdom, we think of bears, elephants, lions, or even snakes fitting something seemingly much too large down their gullets with ease. But, a caterpillar’s sole job as the larval form of a butterfly or moth is to eat! Pupating and metamorphosing into their adult forms requires a great deal of energy, and in some moth species, such as the giant silk moths, the adult moths have no functioning mouthparts, so their fat stores are extremely important. This particular species pictured above is a butterfly and will possess the ability to eat, but it’s doing a stellar job consuming calories as a larva, too.

The Appalachian azure, as an adult, is small – about the size of a thumbnail – with wings so pale blue on the outside, they almost look white. Dark brown flecks mare them. Like other members of the Blue family (Lycaenidae), they rarely rest with their wings open, but if you catch one in flight, you might see flashes of a more saturated blue inside. The caterpillar varies in color but is often pale green to blend in with the unopened floral buds of its only host plant, black cohosh, with brown splotches that appear with age.

The head of this caterpillar is not commonly seen, as feeding activity is concealed within the flower bud. Their long, extendable neck allows this, and their thick, fleshy prothorax forms around their neck to protect them during this vulnerable time.

While this caterpillar is a voracious eater, it is also a picky eater. Not only does it solely feed on black cohosh, but it also prefers the pollen sacs inside the plant’s unopened flowers. It will chew through the sepal concealing the tightly folded petals to reach the pollen. However, much like us, the caterpillar expands its palette with age, eventually feeding on the rest of the flower bud, and, even the foliage, if, and only if, they must – when no parts of the flowers remain. Black cohosh produces steroid-like molecules, a defense mechanism best avoided by would-be plant munchers, that are more concentrated in the green tissues of the plant. In laboratory studies, when confined to close quarters, early instar larvae would resort to cannibalism.

But the curiosities surrounding this caterpillar don’t stop there. Also on this plant, and often on the caterpillars themselves, are ants, and they’re there for a reason.

The ants and the Appalachian azures have a symbiotic relationship. A very small percentage of all caterpillars reach adulthood; most are picked off by birds at some point during their larval stage or parasitized by a number of wasp species. Caterpillars that are tended by ants, though, may be defended from parasitoid wasps, flies, and/or other predators. The ants do this in exchange for honeydew, a sweet substance which is secreted by the caterpillars. They obtain it by tapping their antennae on a fourth or occasionally third instar caterpillar, prompting it to release an amount I couldn’t begin to quantify. Younger caterpillars haven’t yet fully developed the organ that releases the honeydew, so the ants pay little attention to them. Once mature enough, a drumming sensation from the caterpillars also signals to the ants that the caterpillars require tending.

Other lycaenids use chemicals to “encourage” protection via ants; they may release a pheromone or store a pheromone in their waxy coat that mimics the pheromone of an ant larva, tricking the ants into caring for the larvae as their own. The ants will carry the caterpillars into their colony where the caterpillars will be fed like the ants’ larvae, or perhaps even the queen ant if the caterpillar produces a noise similar to hers (acoustic mimicry), or where they may feed on the ant larvae themselves. This is known as brood parasitism. Research by David Nash at the University of Copenhagen, Denmark suggests that the ants are somewhat aware they’re being duped, and are constantly evolving different pheromone signatures in defense.

Still, another lycaenid, the Japanese oakblue, appears somewhat sinister – it secretes a substance that, while sugary and nutritious, when consumed by an ant, actually causes the ant to abandon the rest of their kind to guard the caterpillar. Researchers found that the secretion contained chemicals that affected the dopamine levels in ant brains, so much so that when a caterpillar inflated defensive, tentacle-like organs, the ants would suddenly become aggressive and search for any nearby threat!

Our sweet little Appalachian azures engage in no such trickery. Their relationship truly is mutually beneficial… right?

– Naturalist Leah