Gypsy moth pupae in white pine, black oak, and white oak.
A few days ago I first noticed a white pine with caterpillar pupae--I counted sixteen on one small branchlet. Today I found oaks and maples with leaves bunched and curled around many more pupae. That means we will see a lot of adult gypsy moths--smaller brown males, and large, white flightless females--in two weeks or so. (Probably not all the caterpillars around here pupate at the same time: my wife noticed numbers of moths flying around the high school.)
Adult gypsy moths live only about a week, and their sole aim is to mate. The large female will attract a male with a powerful pheromone. (One environmentally-friendly way to combat an infestation from year to year is to decoy the males with synthetic pheromone, which is specific to this one species.) A single female will lay perhaps five hundred eggs in a patch covered with peach-colored fuzz on tree trunks, rocks or other surfaces. The eggs overwinter and hatch in spring. The tiny caterpillars then disperse on silken threads like the baby spiders at the end of Charlotte's Web.
What signals the caterpillars to pupate? Do they need to reach a certain size, or begin to run short of food, or is there a certain triggering temperature, or a certain day length? Since the adults live such a short time, whatever the trigger is should be experienced at about the same time by all the caterpillars in one region-- anything more idiosyncratic would spread out pupation and leave early and late moths at risk of finding no one to mate with.
What will next year bring? I may invest in some BT (Bacillus thuringiensis) bacterial insecticide and a sprayer. I know from experience that a little hand-pump sprayer can't reach the tops of tall trees, but it's something to try.
UPDATE: my son spotted moths laying eggs right in our yard the next day.
The mate for one of the females is still there.
I wouldn't imagine white would make sense as protective coloration,
but it seems to work for these moths. (At least until they begin laying eggs.)