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On the Trails: Wooly bears and moths

Posted: April 6, 2012 - 12:00am
A woolly bear caterpillar in Juneau. It is similar to the famous Isabella caterpillar, but this one has, in addition, those elegant long white hairs. Almost nothing is known about woolly bears in Southeast, so this was a real find.  Photo courtesy of Lynn Armstrong
Photo courtesy of Lynn Armstrong
A woolly bear caterpillar in Juneau. It is similar to the famous Isabella caterpillar, but this one has, in addition, those elegant long white hairs. Almost nothing is known about woolly bears in Southeast, so this was a real find.

The woolly mammoths are long gone, but woolly bears are still with us. They are common up north and have been seen in Juneau too. They are so interesting, I just wanted to share with readers just some of the amazing things they do.


Woolly bears are the fuzzy caterpillars of tiger moths and some of their relatives. There are thousands of species of tiger moths in the world, exhibiting an astounding array of color, both as adult moths and as caterpillars. Although some adults show the quiet browns and grays that we commonly associate with moths, many tiger moths are arrayed in bold patterns of black and white or gaudy red, orange, yellow and blue. The fuzzy caterpillars are equally varied.


The striking colors serve as warnings to would-be predators that the caterpillar or adult moth deems distasteful or poisonous. When a caterpillar chews the leaves of its food plant, it sequesters defensive chemicals from the plant that render it unpalatable, and predators learn to avoid the warning colors. In addition, adults of some species suck up plant juices that contain particular alkaloids (or sometimes other chemicals, such as cardenolides) that have a defensive function, for both the plant and the moth.


But it doesn’t stop there — in some species, these defensive compounds can also be used as mate-attractants and in courtship. For example, in at least one North American species, the defensive chemicals collected by the caterpillar are converted into sexual pheromones (hormone-like chemicals, usually volatile, that commonly work at some distance from the animal that produces them). Female caterpillars exude pheromones to call in males. As a male approaches a calling female, he expands special organs from his abdomen, waving them around and eventually brushing them against the female, transferring to her his own pheromones. Females can detect the male pheromones and use them to help choose the best mate. Choosing the right male is important, because the defensive compounds are transferred to the female during copulation, and females then use these compounds to defend themselves and their eggs. Males that did not consume the necessary alkaloids as caterpillars were smaller and possess less of the critical defensive compounds, and females avoid mating with these lesser males.


In the Lower 48, tiger moths can have several generations per year, going from egg to adult to egg again, several times. At higher latitudes, with shorter growing seasons, there are fewer generations per year. In the Far North, a woolly bear that is related to tiger moths takes this latitudinal trend the next step: it overwinters as a caterpillar, under rocks or plant debris. And not just one winter! These caterpillars are known to spend up to 14 winters — and that many freeze-thaw cycles — before transforming into adults. It can take that many short summers to get food enough to allow it to reach mature size.


It has long been known that many kinds of adult moths can hear — they can detect the ultrasonic calls of an approaching bat that is zeroing in on a possible snack. When some kinds of moths hear an incoming bat, they take evasive action, dodging and dropping suddenly to elude pursuit. Tiger moths detect the high frequencies too, but instead of flying erratically, they talk back! And they talk in ultrasound, made by a "clicker" on the thorax. Some researchers have suggested that talking back is a way of jamming the bat’s transmission. But increasing evidence indicates that the back-talk may be a warning that the bat’s intended prey is unpalatable and not worth chasing. So, in addition to visual warning signals that work in the light, there may be auditory warning signals that work in the dark.


Less well-publicized is the fact that the caterpillars can hear too. Some of those fuzzy hairs detect vibrations — specifically the low frequency vibrations made by the wings of a wasp that could attack the caterpillar, either to take the caterpillar back to its own larvae or to lay a parasitic egg on the caterpillar. In either case, the caterpillar would be dead. However, what the caterpillar does in response to detecting a wasp is not recorded.


I should also mention, albeit in passing, that some tiger moths use acoustic signaling in courtship, in addition to or, in some cases, instead of pheromones. This is a lively arena for intense research.


A common species of woolly bear is the larva of the Isabella moth, a rather plain, yellowish moth that is widespread in North America. The caterpillar has broad orange and black bands around its body. Folklore says that the width of the bands predicts the severity of the coming winter — but there are contradictory stories. Some say narrow orange bands indicate a severe winter, while others say that wide orange bands do so. In any case, even within a single brood of caterpillars, there is much variation, and band widths may also change with age. So, winter weather forecasting should probably be based on more scientific information.


The black-and-orange banded Isabella caterpillars are so common in some places that they are known to most of the people in the community. They are so popular that towns in at least four states stage annual festivals featuring these woolly bears, complete with caterpillar races and costume competitions.


Woolly bears have it all: popularity among kids and townsfolk, fascination among scientists. We don’t have the Isabella moth in our area, but we have other tiger moths. They are not common — Southeast is not a friendly place for most butterflies and moths. But if you see one, you have had a successful "treasure hunt"!


• Mary F. Willson is a retired professor of ecology.

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