Saturday, April 25, 2009

Caterpillars on the march

Southwestern tent caterpillars (Malacosoma incurvum) are maturing and dropping out of the trees along the San Pedro River, providing a boon for northbound migrants. As Tom likes to say, these fuzzy eating machines are nature's way of turning cottonwood leaves into warblers. As unpleasant as it can be to have creepy crawlers and their poop raining down on you on a bird walk, it's peculiarly satisfying to watch a tiny Wilson's Warbler slamming a python-sized caterpillar against the ground to subdue and tenderize it before squishing out the good parts like toothpaste from a fur-covered tube.

Not everyone is as appreciative of southwestern tent caterpillars as we and the migrating birds are, though. When they emerge in spring and start stripping the cottonwoods and willows of their tender new leaves, some people freak out. We've gotten panicked calls asking why someone isn't doing something to halt the destruction before the caterpillars kill the trees. Well, the trees are in danger, but it's not from the caterpillars. The defoliation happens every spring, and healthy trees can recover quickly once the cats stop eating and go into the pupal stage. Stressed trees whose water supply is being sapped away may not be able to recover, though, and the annual caterpillar invasion may push more and more trees beyond the point of no return as more and more residential wells drain the aquifers that feed the San Pedro River. —SW

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