By TOM TURPIN
On Six Legs
More than once I have been asked some version of this question: Where have all the big moths gone? Almost always the questioner elaborates by saying something like: "Every summer when I was a kid there would be nights when several large moths would be fluttering around our porch light. I haven’t seen any of those giant moths for years."
As an entomologist I have learned not to jump to conclusions when responding to questions such as this, especially when the term "big, large or giant" is used. The size estimation of an insect is something that varies widely according to the eyes of the beholder.
Nonetheless, the mention of big moths and their diminishing numbers is a pretty good clue. The chances are that the moths in question are classified in the insect family saturniidae – known popularly as giant silkworms.
These moths are some of the largest moths in the world. Many found in North America have wingspreads of 5 or 6 inches. Some species of the genus Attacus have a 10-inch wingspread, but the largest is found in Southeast Asia, not the United States.
The largest of the saturniidae in North America are the polyphemus, promethea, luna, and cecropia moths. Unlike many animals, including insects, the species name is used as the common name for these insects. For example Calosamia promethea is known as the promethea moth and Actias luna as the luna moth.
In addition to their common names and size, these moths also have other things in common. Most moths are dull-colored, so they blend into the environment during the daylight hours. Giant silkworm moths, on the other hand, are brightly colored and many have conspicuous eye spots.
The coloration and the eye spots benefit the insect by providing the illusion of a larger animal. The eye spots and the shape of the wingtip of polyphemus and cecropia moths suggest the head of a snake.
Caterpillars of the giant silkworm moths, as you might have guessed, are also large in size. Many of these caterpillars have obvious spines or other protrusions on their bodies making them look dangerous to a potential predator. These caterpillars earn their name as silkworms because they spin silken cocoons in which they spend the winter.
Another interesting feature of the giant silkworm moths is that they have nonfunctional mouthparts and do not feed as adults. As is the case in many insects, the function of the caterpillars is feeding, and they store up enough food energy for the adults to live their lives without feeding.
Also, male moths have antennae that are larger and fuzzier than females. The male antennae function to detect the pheromones produced by the females in order to attract males for the purpose of mating. In some instances, male giant silkworm moths have been shown to fly miles upwind to the location of pheromone-producing females.
So are the giant silkworm moths less common today than they have been in the past? The answer to that question would seem to be yes. One of the reasons for this is that of decreasing habitat. In general, the caterpillars of these moths feed on hardwood trees. The luna caterpillars feed on walnut and hickory, the promethea on spice-bush and sassafras. As large tracts of eastern woodlands in the United States have been removed, the feeding sites for these caterpillars has diminished.
It has also been suggested that the presence of the Asian lady beetles might also have an impact on giant silkworm populations. The Asian lady beetles feed on aphids in trees and apparently will destroy eggs of these moths as well.
There probably aren’t as many big moths being attracted to our porch lights as there used to be. It is a sign of the times, I suppose.
However, when one of the giant silkworm moths or its caterpillar does show up, it is likely to attract attention. And that is the way it should be, for a moth that wants to be seen.
The views and opinions expressed in this column are those of the author and not necessarily those of Farm World. Readers with questions or comments for Tom Turpin may write to him in care of this publication.
