Mary and Tweeters,

Your caterpillars sound like the Western Tent Caterpillar (Malacosoma
californicum), a common pest of deciduous trees, including fruit
trees, in western Washington and nearby areas. Here is a helpful
factsheet about tent caterpillars and their control (if considered
necessary):

http://gardening.wsu.edu/library/inse003/inse003.htm


The following webpage has good photos of the caterpillars, tents, and
adult moths:

http://facweb.furman.edu/~snyderjohn/tatum/260-265.htm

Most of what people have said so far is accurate. Western tent
caterpillars rarely kill or seriously harm the trees they infest, but
they can be unsightly. Some birds may eat them, but bird predation
will almost surely not control the outbreak. Mechanical control
(removing tents) may be adequate; if stronger control is needed,
Bacillus thuringiensis (Bt) is usually recommended. Bt kills all
species of caterpillars, but has little effect on other insects. There
have been some concerns about reductions in butterfly and moth
populations where Bt is used in aerial spraying (e.g. for gypsy moth
control), but if used only on 4 trees, such effects would be
negligible. At any rate, Bt is much preferable to chemical pesticides,
because of its specificity.

I hope this helps!

Wayne C. Weber
Delta, BC
contopus at telus.net