The Lovely Luna Moth
by Claire Hagen Dole
How would you react to seeing a Luna moth (Actias luna) for the first time, especially if you lived in an improbable setting like Maine? Spectacular and unmistakable, the moth looks as if it belongs in the tropics. Indeed, it closely resembles the Indian moon moth of Asia, the continent where most moths in the genus Actias are found.
Early American colonists werent slow to notice the Luna. It was the first giant silk moth (family Saturniidae) to be mentioned in the scientific literature of North America. Detailed accounts and illustrations had been published decades before it was described by Linnaeus in 1758.
Medium-sized by Saturniid standards (its wingspan is 3-4), the Luna gains length from its delicately curved tails. Each pale-green, translucent wing bears a small eyespot; the forewings top edge is pink-to-purple. If a second generation occurs in the same year, these moths will be more yellowish-green in color, with a yellow edge. All have dark-pink legs on a stocky body that is covered with white hairs. Broad, feathery antennae are wider on males than on females.
Named after the Roman moon goddess, the Luna occurs in the eastern half of the U.S. and from southern Saskatchewan through Newfoundland. Northern populations produce one brood per year, with adults active in late spring/early summer. A mild-winter year may cause adults to eclose (emerge) from their cocoons earlier than usual, allowing time for a second brood during late summer. In the South, two to three broods are usual.
The Very Hungry Caterpillar
Female Luna moths lay grayish-brown, cylindrical eggs singly or in small groups on the underside of host-plant leaves. White birch is the favored host plant in the North. Black walnut, butternut, hickory, persimmon, sweetgum, alder, beech, willow, wild cherry or sumac may be chosen in other locations.
In Connecticut, John Himmelman finds black walnut to be the favorite food plant of Lunas. He reared Lunas while writing and illustrating A Luna Moth's Life (Grollier Publishers, 1998), part of his series of childrens nature books. In North Carolina and points south, persimmon is often the first choice of ovipositing females.
Larvae hatch in a week to twelve days. They begin a three-to-four-week eating binge (up to six weeks in the cooler North) that increases their weight by over 4,000 times. To accommodate this phenomenal growth, they molt four times to their final size, about three inches long. Light-green larvae have a dark head, a yellow stripe along each side and rows of red knobs (tubercles), some bearing short hairs.
Luna larvae who feed on walnut, butternut and hickory ingest high amounts of quinone, a toxic plant compound. Does this make them unpalatable to predators, like the Monarchs who ingest cardiac glycosides from milkweed? According to Dr. Richard Lindroth, professor of chemical ecology at the University of Wisconsin, Monarchs are able to sequester toxins in the body without being poisoned by them. Lunas use the more common insect strategy of detoxifying and excreting the toxins, so they arent distasteful. In fact, the larvae are eagerly sought out by birds and other predators.
Even though the caterpillars are veritable eating machines, they probably wont defoliate your walnut tree. The adult females spend time dispersing their eggs to ensure adequate food for the hatching larvae. Some trees, like the sweetgum, dont attract any significant insect pests, and Lunas arent numerous enough to do much harm.
When the final-instar (last molt) larva is ready to pupate, it leaves its perch in the tree and crawls or drops to the ground below. There it spins a cocoon inside a loose wrapping of leaves, which will be well-hidden in the leaf litter. Some larvae pupate in an attached leaf on the tree and then fall to the ground in autumn.
Bill Oehlke, who rears Lunas on Prince Edward Island, observes that larvae turn dark amber or burgundy-brown just before pupating if they will overwinter in the pupal stage. In areas where there are multiple broods, only the final brood darkens in color. Luna cocoons are papery thin; the active pupa can be seen if held to the light. Cocoons of individuals that will eclose the same summer are usually nearly white in color; those that will overwinter are tan or brown.
The adult moth normally ecloses from its cocoon in mid-morning. Says Oehlke, "The moth makes quite a racket as it heaves against one end of the cocoon, tearing at the silk with hornlike projections near the base of the forewing. A secretion called cocoonase helps to break down the sericin binding the silk."
Newly-eclosed Lunas climb to a perch where they can inflate and harden their wings. Males usually eclose first, and mating is top priority when females arrive on the scene a day or two later. The females "call" males by emitting potent pheromones, and they have often mated before they take flight. During their short lifespan of a week or so, they are occupied with seeking host plants and laying eggs for the next generation.
Since the larvas digestive tract disintegrates during pupation, an adult Luna moth does not feed at flowers. It survives on stored fat from the larval stage. Males are attracted to lights, however, as are some females who have finished egg-laying.
The Lunas Future
Despite the public perception that such a fragile and seldom-seen insect must be rare, Lunas are generally considered a common species. Have Luna moth populations declined in the past few decades? Many naturalists believe that street lights and pesticide use have caused a decline in populations of giant silk moths and, in turn, their predators (such as the whippoorwill).
I asked Dave Winter, co-author with his wife, Jo Brewer, of Butterflies and Moths: A Companion to Your Field Guide, about a 1980 field-guide reference to the Luna as an endangered species. He replied, "In the Northeast at that time, there was an impression that Lunas and other giant silk moths had declined significantly--perhaps due to pesticides or to distraction of male moths to mercury-vapor lights along highways and in parking lots. This was never quantified, and at the same time various rural observers saw no significant decline. These concerns seem to have subsided. Pesticide use is down, and more night lighting is being switched to sodium lights, which are less expensive to run and far less attractive to moths."
The New York City Department of Parks has undertaken a program to reintroduce the Luna moth to Central Park. Park ranger Bob DiCandido says, "Lunas were common in the city during the thirties and forties. Theyre still present in the Greenbrook Sanctuary, across the Hudson River, so we know they're in the area." DiCandido is also involved in reintroducing screech owls to Central Park, and the Baltimore checkerspot to Kissena Park in Queens. For information on the Luna project, contact the NYC Department of Parks, Urban Park Rangers, 1234 Fifth Avenue, New York, NY 10029; (212) 360-2771.
Article by Claire Hagen Dole, Publisher/editor of Butterfly Gardeners' Quarterly. #17, Summer 1998.