Monday, May 13, 2013

Phenology: Gypsy Moth Caterpillars

(This is part two of a series that began here: This article may be less of a gross-out to read, because although Morguefile had plenty of images of Gypsies, as of today, it had no pictures of gypsy moths.)

Malacosoma americanum and Lymantria dispar aren't easy to confuse when you actually look at them, but there are similarities. Both are dark, furry caterpillars that can get about as long as my little finger; both are active and hardy, waste lots of silk, and roam around making nuisances of themselves in spring; and both mature into short-lived, mid-sized, drab-colored moths just after the summer solstice. That's why, in 1869, Etienne Troubelot imported the Gypsy Moth, Lymantria dispar, into Massachusetts, hoping that it could be crossbred with Malacosoma americanum to produce a hardier kind of silkworm.

Short answer: it couldn't, although gene splicing may reopen the possibility. In Europe Lymantria dispar fits into an ecological niche where its population is well controlled by predators, and is not a major "destroyer" (although that's the translation of Lymantria). In North America, lacking predators, the population exploded and the species became a serious pest.

Frank Lutz wrote that "the best excuse that can be made for Professor Troubelot is that he did not know the gun was loaded." Paul Villard, taking a brighter view, wrote in Moths and How to Rear Them that bringing a few gyps indoors for science projects is an ideal way to find out whether rearing moths and butterflies is something you want to do: the animals are hard to kill, and nobody will miss them, anyway, whatever you do with them, as long as you don't release them. He recommended "just...pick[ing] a few thousand" to experiment on. Even P.E.T.A. are unlikely to complain.

In the suburban neighborhoods where I've encountered gypsy moth caterpillars, it was illegal to "harbor" the creatures, meaning that it was mandatory to kill them on sight. There's a valid reason for this: since they need predators, and in North America they lack predators, humans need to fill the ecological niche of predators on gypsy moths, even though we can't eat them.

So, how to recognize which caterpillars you're supposed to kill? Gyps usually hatch about the same time Eastern Tent Caterpillars stop eating and wander about. Since gyps do most of their "Gypsy-like" wandering about during the first two weeks of their lives, this means both species are most annoying to humans at the same time, typically in May. When you see them side by side, the most obvious difference will be size. The tent caterpillars have reached their full size and are four or five times as long as the gypsy moth hatchlings.

Here's a page with good clear photos of typical mature caterpillars, for comparison...

...but the problems with it are: (1) they don't mature at the same time; (2) most of the gyps we see are not mature; and (3) even mature gyps don't necessarily look like the typical female on this page. Male gyps pupate earlier than females, go through fewer skins, and are much smaller than females when they start turning into moths. And during the active part of their lives, they're all less than an inch long; new hatchlings are black with barely noticeable clumps of white hair, and the second and third skins are also black but show those distinct colored warts.

Dispar refers to the pronounced disparity between male and female gypsy moths, whose life cycles are so different that they seem like different species. Male caterpillars stop eating and pupate before they're much more than one inch long. Male moths are likewise a little more than half the size of the females, slimmer, darker, and more active. Female caterpillars reach almost twice the size males reach before they stop eating, but most of their extra bulk turns into eggs rather than muscle.

A web search for images of the young, small, light gyps people actually meet is disappointing. This informative page contains photos of hatchlings and slightly older caterpillars, in their second skins, bungee-jumping down on silk threads...

...but where's a clear picture of the caterpillars themselves? Maybe they're just too familiar to the people who photographed those final-stage gyps to be interesting. Since I don't have a digital camera, all I can tell you is that they progress from looking like scraps of black yarn to gradually looking more colorful and more like the pictures shown on the Internet. The dots of color on the skin, around the identifying pairs of red and blue warts, can vary. Mature gyps can look gray, tan, brown, or greenish from a distance; on closer observation they're mottled, and the base color of the skin is still black. The caterpillars in a given area won't all hatch at once, and if you look carefully you might find specimens with the first, second, and third skins all in one place, as shown here:

The ones that are more than an inch and a half long are female, and usually stay in the treetops, having grown too big to float around on strands of silk.

Although whacking any gyps you see with a stick may be required by local law, it's not the most efficient way to control the species. The most efficient way is to look for the egg clusters, which look like clumps of tan fur (as the eggs ooze out of Mama Moth most of her body hair adheres to the eggs), and burn those before the caterpillars hatch. Here's a serious infestation (if allowed to hatch, the caterpillars might defoliate the tree)...this legitimate link to a California government photo page is too long to fit into a Blogspot page...the tan-colored blobs are egg masses, the white blurs are female moths, and yes, the egg masses do seem to expand in the air and look bigger than the moth out of which they came; this effect is partly due to their being covered in fur, but so was Mama Moth.

After the caterpillars have hatched, squirting water on them can knock the smallest ones out of trees, but you still have to go and kill them. Burlap traps can effectively catch them, during the middle of the day, when they like to hide in snug shady spots for an afternoon nap: scroll down to Figure 14 on this page:

...but, again, someone has to go out and kill the caterpillars every single day, and it's not a pleasant chore. (They don't "sting," technically, but their stiff hair can irritate most people's skin. The people who apparently Google "gypsy moth caterpillar sting" regularly don't seem to realize that half-inch lengths of human hair also irritate most people's skin.) This web site does not recommend poisoning the burlap. This web site does recommend wrapping bands of duct tape around a tree and painting them with glue, as discussed here...

Although it costs more than burlap, the position of this web site is that not having to handle gyps is worth the money; you can leave a sticky band on the tree until it fills up, burn it, and apply a fresh one.

Pupae are also variable; some caterpillars go to the trouble of spinning a cocoon, some don't. Adult moths are variable too; males can look tan or gray, females are bigger and paler but can look white, pale gray, or pale beige.

Can the female moths fly? The answer seems to depend on genes. Some female gypsy moths seem to have no use of their wings, some can fly a few feet if disturbed, and some are seen flying about, but not very far.

By the time gypsy moths get their wings, they are old, tired animals. Males have reserved enough energy to fly about for a few days, looking for females, and may even have the strength to fly away after mating. Females usually mate with the first male they find (not very enthusiastically or imaginatively--barely spreading their wings), extrude the frothy egg mass that took up most of the space inside their bodies, and collapse, often within a yard or even a foot of their cocoons (or pupal shells).

As discussed further down the page at, the male moths can be trapped using chemicals that smell like a female moth. A problem not discussed on this page, but often discussed by people who've bought the traps, is that humans have no way to tell whether they've got the scent on themselves--humans don't smell it--until they're mobbed by male moths. This phenomenon is often inaccurately described as being raped by moths. More accurately, the humans are being searched by moths, not that that's exactly fun either.

This web site does not recommend poisoning gypsy moths. The more you try to poison insects, the more predators you poison, and the more unwelcome insects you will have. A poisoned gyp could potentially harm a "bluejay, blackbird, catbird, black-capped chickadee, crow, grackle, red-winged blackbird, nuthatch, oriole, chipping sparrow, robin, tanager, vireo, [or] woodpecker" (list at Although these birds have not evolved much of a taste for gyps, each of them will eat enough other undesirable insects to make up for the inconvenience of humans having to do most of the predation necessary to control Lymantria dispar.

However, species-specific germ warfare can be tempting...until you think about the more lovable native species who are also likely to be infected.

Scroll down to see what baculovirus does to a harmless little baby monarch butterfly. Not something I'd want to have on my conscience, for sure. I don't feel personally responsible for the BT bacteria attack, meant to kill gyps, that caused so much unmistakable misery among tent caterpillars in Takoma Park when I lived there, but it wasn't a very pleasant season.

This web site recommends sticking with good oldfashioned predation. Do as much of it as possible in late summer, autumn, and winter, when (in most of North America) Lymantria dispar are easy to find and destroy in the form of egg clumps.

It's interesting to note that in Europe, where gypsy moths belong, they're not a major pest. In fact they went extinct in England for a while, and some naturalists tried to reestablish the species...

...but the caterpillars aren't exactly lovable, and currently Londoners seem to be trying to render them extinct again.