Monday, October 18, 2010

Leave Them Alone!

They march across sidewalks and appear on the trunks of trees. Plump, fuzzy, caterpillars, often appearing sluggish, confused, and far from anything they could possibly eat. Along with spiders, changing leaves and ripe pumpkins, the sudden appearance of caterpillars seems to be a sign of fall in Seattle.

Pacific Science Center’s Life Sciences department gets its share of calls about these larvae. Most callers are at once charmed by their cuteness and anxious about their fate, as they seem to have lost all interest in taking care of themselves.

Much of the worry is unnecessary. Most caterpillars that become obvious in the fall belong to species that overwinter as larvae. Banded woollybears, Pyrrharctia isabella, are a famous example. These rust-red and black larvae have been fattening up during the summer. As the days grow shorter, they lose interest in eating and become restless. They leave the plants that feed them, and wander about, hunting for safe places to hide. Once they find the right spot, they go into diapause - delayed or suspended development.

Their bodies are amply prepared for the cold weather ahead. The liquid in their body is able to cool below freezing. Their instincts help them burrow into soil and dead leaves, where they will be further protected from weather extremes.

In late winter, they will resume development, pupate, and emerge as grown moths. One of the concerns about climate change is that the timing for these sensitive developments may become confused, as temperatures warm sooner but day lengths are still short in spring. In general, adapting to changes in climate is easier for species that have multiple generations per year, and have cycles that are less strongly linked to the changes in season.

In past articles, this blog recommends against raising caterpillars found in the fall. At best it is frustrating, and at worst, can lead to serious timing problems for the insect, which may complete its life cycle at a time of year that is inappropriate for feeding or finding mates. Not only that, but their soft-looking coats can be irritating to the skin and eyes, and are best not touched! It may seem like a cold, cruel world for such a tiny animal, but trust these little bears’ instincts and let them fend for themselves this winter.


  1. Is it true that you can predict how cold the winter will be by how wide a woolly bear's orange stripe is? Are the caterpillars predicting a cold one like the weather guys do?

  2. I'm not sure if any meteorological conclusions can be gathered from the above subjects. Photos of the four caterpillars were taken between October 3 - October 11, 2010 around Westport, WA.