Tuesday, April 30, 2013

Phenology: Eastern Tent Caterpillars

Malacosoma americanum become visible around the Cat Sanctuary around this time of year. Here's what they currently look like:

Unmistakable. Nevertheless, I'm aware of these little animals having been confused with half a dozen other species: Forest Tent Caterpillars, Gypsy Moth caterpillars, Bagworms, Webworms, Stingingworms, Walnut Caterpillars, and even Cutworms.

I would say that confusing them with Cutworms is just plain silly, except that it was done by a foreigner who hadn't been here long enough to recognize American insects. He'd read that cutworms are likely to attack young tomato plants, so he asked me whether the caterpillars crawling on his tomato plants were cutworms. They were tent caterpillars. After reaching their full size, before they pupate and turn into moths, tent caterpillars spend a few days exploring the world and crawling on all sorts of things they can't eat and don't try to eat, including humans.

Tent caterpillars hatch out of egg masses that look like globs of dried foam extruded around twigs of Prunus--usually cherry trees, but sometimes other fruit trees or rosebushes. It's possible to see and remove the egg masses in winter, but it's seldom necessary. Although the caterpillars will eat a lot of leaves, fruit trees usually have reserve buds from which they can re-cover themselves in leaves without even seriously affecting the fruit crop. Only if the trees have been badly frostbitten are the caterpillars an economic problem. It's after they stop eating that they become a nuisance.

Not all the caterpillars hatch at once. As shown in the picture above, some of the siblings in this family are much bigger than others. The bigger ones are older by about a week. They hatch around the time leaves form on the trees, and immediately begin crawling up and down the tree, leaving trails of silk behind them. Afterward they can tell by tasting the silk where the best food supply was. When caterpillar and tree populations are well balanced these trails don't become noticeable beyond the nest, but in "plague years" the tree may be completely defoliated and covered in webs of silk. (Horrible though it looks, the tree will probably look normal in a month and bear fruit later in summer.)

Tent caterpillars are among the very few species of Lepidopterae that seem to know their families. (They seem to notice more about the world in general than many caterpillars do; they're quite sensitive to touch and temperature, and probably even see about three feet ahead of their faces.) Younger siblings follow older siblings up and down the tree in search of good food.

In the evening they all come back to the nest, where they groom the dust out of one another's thick fur. Vincent Dethier reported that when tent caterpillars are isolated and prevented from grooming each other, they die; "of depression and ennui," he suggested, though probably also of suffocation, since they breathe through pores in their skins, underneath the fur.

Tent caterpillar hatchlings look like very small scraps of black yarn. In a few days they molt and have black skins with white fur and solid white stripes down their backs, like the smaller caterpillars in the picture. They keep this coloring for most of the caterpillar phase of their lives, but the last caterpillar skins they have are beautifully patterned in black, white, and blue with ginger fur, like the bigger caterpillars in the picture.

The distinctive blue color comes from cyanide. The leaves these caterpillars eat contain significant traces of cyanide, which seems to do the caterpillars no harm but discourages most birds from eating them. They would be poisonous to humans if swallowed. Wikipedia reports that they are sometimes inadvertently swallowed by horses, and can have poisonous effects on pregnant mares:


Though not especially warm (being cold-blooded animals) these caterpillars are certainly fuzzy. Because they seem to see and feel more than most caterpillars do, while still unable to see how large and dangerous humans are, they can briefly seem to be "pets." Most of their "cuddly" habits are probably illusory--their tendency to climb up your finger if you touch them is an instinct based in their way of defending themselves against small parasitic wasps--but they may enjoy being stroked, if you have a light and steady touch. (Unfortunately they're also fragile and easily crushed.)

There are no venomous "stings" in their soft fur; the caterpillars seldom even try to bite, and they're not strong enough to do any harm if they do want to bite. There is no valid reason to confuse tent caterpillars with stingingworms, which hatch later in the year. However, about one out of four humans may be mildly allergic to tent caterpillars' fur. I tend to suspect that part of the allergy reaction has to be psychosomatic, because watching tent caterpillars makes most humans feel "crawly" in any case. It is not to be compared with the reaction even non-allergic people have to contact with stingingworms. When reactions are reported, as here...


...contributing factors seem to include heat, length of contact, and contact between hairs and skin more sensitive than the skin on humans' hands and feet.

I've heard people say, "If it's not trying to bite or sting me, why is it running after me and trying to crawl up my leg?" Tent caterpillars do follow people, and occasionally jump out of trees to land on people, in an unmistakably purposeful way. Their sensitivity to temperatures, humidity, and scent probably has a lot to do with this. They're definitely most likely to run after humans when crawling across hot pavement. Curiosity could be a factor; in order to see things clearly caterpillars probably do need to get close enough to touch them. The wandering stage of their lives also has something to do with separating from their siblings--they often pupate close to their eventual mates--so there might even be some motivation to impress their friends.

Under normal circumstances, tent caterpillars' defensive behaviors consist of (1) climbing or crawling on top of things that touch them unexpectedly from behind, (2) squirming (they can bend almost double in any direction, and do this rapidly to scare off some attackers), and (3) being unpalatable to most birds--hairy, full of cyanide, and sometimes able to spit up drops of leaf juices. Biting is a last-resort behavior that seems to emerge only when the caterpillar is in pain.

Controlling tent caterpillar populations is easy. Nature does it for you. In approximately ten-year cycles, tent caterpillars give us object lessons of what happens to creatures that reproduce too efficiently. Local populations become overcrowded and vulnerable to infections. These are "plague years" for humans who don't appreciate having large numbers of tent caterpillars in their gardens, and for the caterpillars, who suffer and die from a variety of strange insect diseases. After one or two plague years, the population thins out and the caterpillars are no longer much of a nuisance...until population congestion recurs.

If there is some special reason to persecute these mostly harmless insects, there's no need to buy any expensive, dangerous chemicals. If you're due for a plague or near-plague year and don't want to be plagued by the caterpillars, pick off the twigs containing egg masses before your trees bud, when the eggs are easy to see. If a late frost has killed the early buds, so the tree is already using its reserves, and the late frost has somehow failed to kill the caterpillars, use a stick (a long stick--don't be like that visitor to Dave's Garden who got them down his neck!) to remove the nest from the tree. (Many caterpillars will escape and regroup, or die, somewhere else, but you can burn the nest; it will be full of cast-off skins and caterpillar droppings, which are called frass, and will have a distinct odor and burn with a blue flame.)

A heavy stick is also an effective way to get rid of random, wandering caterpillars if, for example, you have had a long thaw and a severe late freeze, such that, instead of having a chance to destroy the trees' reserve leaves, the caterpillars starve to death and congregate on your porch. I did that in 2011; after a few childhood years when it seemed necessary to kill the caterpillars to protect our baby fruit trees, followed by many years of living in peace with them, I started euthanizing them because they kept trying to crawl up me--I think in a last desperate hope that I might be some sort of tree. Probably what the cats objected to was the mess and odor in the yard, but I consciously resisted a tendency to think that the cats were trying to say that these pitiful little things had come to the Cat Sanctuary in search of, well, sanctuary, and I was being mean. I felt mean. I have certain instinctive reactions to physical contact with insects dead or alive, but that should, ethically, be my problem. But I kept sweeping the caterpillars off the porch and bashing them, anyway. So I can't judge readers whose approach to Malacosoma americanum involves a stick.

Poisoning these harmless animals, and who knows what-all else along with them, seems ethically beyond the pale. I would even beware of spraying BT to control other caterpillars, e.g. Gypsy Moth caterpillars, that hatch when the tent caterpillars are theoretically old enough to be immune (according to corporate spokesmen). We tried that in Takoma Park, back when I lived there. The tent caterpillars were not immune. It takes a caterpillar about a week to die from BT; during that time the caterpillar is obviously suffering, and the tent caterpillars crawled out onto hot pavement and convulsed and bit things and generally seemed to be trying to hasten the end. Not a pleasant sight--especially in view of the fact that most of the gyps hatched a little later than expected and suffered no noticeable effects from BT, and during the next winter Takoma Park lost several of its century-old oak trees.

Tent caterpillars wander for less than a week, although they don't all reach this stage at once, so you see them wandering about for two or three weeks. Then they find a sheltered place to spin light cocoons and turn into moths, which doesn't take long. When the caterpillars wander in early May, as they usually do in Virginia, the moths fly in June.

Adult Malacosoma americanum aren't conspicuous, although the Cat Sanctuary, being in an orchard, sees a few each year. The medium-sized, light brown moths are only occasionally attracted to light, and don't eat; they have just time to pair off, mate, and lay eggs before they die. Then the cycle begins again next year.

People who actually look at the animals can't confuse Eastern tent caterpillars with any of the other species mentioned above. Confusion with webworms and bagworms is due to the literal meaning of the names--these are completely different species. Confusion with gyps is, I suspect, due mainly to having pored over Herbert Zim's Golden Book field guides, in which the drawings aren't very lifelike and can be confusing. Confusion with Forest Tent Caterpillars and Walnut Caterpillars is a little more reasonable--they do look a bit alike, but not enough to be confused when you look closely.

Mainly because the computer reports that readers are fascinated by caterpillar stories, but also because I always learn something new when reworking this material, I'll try to post something about each of the other species in time for readers in my part of the world to look for them in your gardens.

Meanwhile, although I'm not exactly familiar with the Western Tent Caterpillars (some classify them as several different species), I will acknowledge their existence...

Malacosoma californicum is not found at the Cat Sanctuary. As shown, he's obviously in the Tent Caterpillar family, but not an Eastern Tent Caterpillar. Here is his Wikipedia page:


There's also a European tent caterpillar, nicknamed the Lackey...is that short for Malacosoma, or a play on the idea of their grooming one another, or both?


If this article has not completely exceeded your interest in tent caterpillars, you may want to read Vincent Dethier's book, The World of the Tent-Makers...and you may also want to consider a major in entomology!