Dance of the Fairy Moth

Ocean spray fairy moth. photo by oxalismtp via iNaturalist, some rights reserved (CC BY-NC)

One morning while exploring the yard deep into May, I spot an insect no longer than a quarter of an inch in length, if that. Perched near the tip of a glossy green and leathery salal (Gaultheria shallon) leaf, it might have gone unnoticed had the sun’s lower angle not cast a beam of light just so, bathing the tiny creature in a kind of solar spotlight.

I lean in to take a closer look. Those extravagantly long, white antennae are impossible to miss! Easily twice the length of the insect’s forewings, they sway ever so slightly in a whisper of a breeze. I also notice that the blackish-purplish, metallic forewings sport two thin crossbands of white. My eyes then travel to the tip of its head, where I’m struck by a rather strange-looking “hairdo” that is a dark, bushy tuft.

I take some photos. Hmmm . . . what is this insect? I had never seen it before in the yard. Some day-flying moth is all I can surmise.

Later, on the iNaturalist site, I learned its genus and species name: Adela septentrionella. The common name is ocean spray fairy moth, one of nine species of fairy moths that live in the Pacific Northwest. This one was male, distinguished by its dark upright hairs and the length of the antennae. The female’s “hairdo” is orange, and their antennae are typically shorter than those of the males.

Putting a name to a face is certainly satisfying, but I’m intrigued by what I read about the insect’s plant preference and behavior as it moves through its lifecycle. To attract the female fairy moths, the males “dance” in swarms above and on the flowers of our native ocean spray (Holodiscus discolor) shrub. The fertilized females then lay their eggs on the buds of its flowers. This means that the ocean spray is a host plant for the fairy moth larvae, which overwinter in their cases on the ground under the shrub.

Sparkly blossoms and buds of the native ocean spray plant. photo by Mary Johnson

Over the last several years I’d planted around the yard bare-root plants of these tenacious shrubs that feed and shelter pollinators, birds, and other wild organisms. Now the oldest of these plants have grown from short, spindly sticks sporting just a hint of attached roots, into flourishing, multi-stemmed shrubs mature enough to bloom and to attract the ocean spray fairy moth.

Making this unexpected connection between a native plant and an insect further deepened my interest. I began to actively search for fairy moths, finding them on the leaves and flowers of the ocean spray and sometimes nectaring from non-native plants. But witnessing the aerial dance of the male fairy moth eluded me. Until . . .

A year later on a sunny and calm day in early June, I pause to admire the ocean spray shrubs planted near the top of the slope that rises sharply from the driveway. Just coming into full, glorious bloom, their long, arching branches sag from the pendant, creamy white clusters of blossoms, each tiny starburst of a flower seeming to twinkle as the angle of the sunlight dipped.

Just then, my eyes glimpse movement in the air just above the branches. A swarm of fairy moths moving up and down as if being pulled by invisible puppet strings, a movement that looks irregular and jerky, yet also seems to possess rhythm and lilt. Here was the male’s aerial mating display — the dance of the fairy moths — just as I had read. Watching for a while, I find myself once again entranced, as I was on that morning when I first discovered the lone fairy moth on the salal leaf.

Male ocean spray fairy moths swarming above an ocean spray plant. video by Dr. Paul J. Watson

Dr. Paul J. Watson, an evolutionary behavioral ecologist, has extensively studied the aerial mating display of the ocean spray fairy moth at the Flathead Lake Biological Station in Montana. In a recent phone conversation with him, I learned the surprising details of these flight displays.

Many butterfly and moth species have tibial spurs — spike-like appendages on their legs — that they might use, for example, to clean their antennae. In the case of the ocean spray fairy moths, these spurs are used for an entirely different purpose, one that, as far as Dr. Watson knows, hasn’t been observed in male individuals of other species.

What he observed was that, during the swarming, the male fairy moths use the long tibial spurs on their hind legs to slash and fray the wings of other males, attempting to disable or kill their competitors. Dr. Watson hypothesized that, while this combative activity is taking place, the female fairy moths are watching upwind, 15 or 20 feet away, from nearby leaves. They release a pheromone, letting the males know they are being observed, which intensifies the combat. He has observed that the prevailing males work their way to the middle of the swarm, which affords the greatest protection. Eventually, the females fly through the middle of the swarm. They engage in evasive flight so that only the fittest males with the most energy can catch and mate with a female.

Ocean spray fairy moth. photo by oxalismtp via iNaturalist, some rights reserved (CC BY-NC)

If you’re out walking the trails on a sunny, late-spring day, look for the ocean spray plants in bloom beside the trails. You’ll find these native plants along coastal bluffs and in open woods at moderate elevations in the nearby mountains. The ocean spray fairy moths are weak fliers and don’t tend to fly on windy days, so you’ll have better luck observing them when the air is still or has just a hint of a breeze. To observe moth activity, check the leaves and blossoms on the ocean spray plant or other nearby plants, and look just above the plants to observe any active swarming.

Bugguide.net. https://bugguide.net/node/view/26749

iNaturalist. https://www.inaturalist.org/home

Pacific Northwest Insects, Merrill A. Peterson, Seattle Audubon Society, first edition, 2018.

Plants of the Pacific Northwest Coast, Jim Pojar and Andy MacKinnon, Lone Pine Publishing, 1994.

Mary Johnson is a former freelance technical writer and editor, who now writes stories about paying attention to the natural world and its inhabitants. Through her work, she hopes to inspire readers to do the same. She is particularly interested in the interrelationships between native plant species and insects.

Table of Contents, Issue #26, Winter 2024-25

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