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Where’s Ripley When You Need Her

September 15, 2010

Developing inside this lovely, cream-colored chrysalid is a Bordered Patch, a common butterfly throughout the Southwest. The bristly, gregarious caterpillars feed on the many wild sunflowers and other composites that come up everywhere during southeastern Arizona’s summer rains and are not above munching their domesticated cousins as the opportunity arises. If all goes well, a mature Bordered Patch will emerge from this chrysalid within a couple of weeks.

This empty husk also started out as a Bordered Patch caterpillar, but it won’t produce a butterfly. Something had other plans for it. Like the chestburster from the movie Aliens, a parasite consumed it from the inside, emerging through the wall of the chrysalid and leaving its once-living nursery behind.

Unfortunately, I didn’t see what emerged. One day it was a normal-looking chrysalid, the next it was brown, empty, and dead. The usual suspects include a long list of parasitoid wasps (they’re not considered true parasites because they inevitably kill their hosts). A female wasp, possibly attracted by chemicals released when an insect munches on its host plant, lays her egg on a caterpillar. Her young hatch and burrow inside, feeding and waiting like a ticking time bomb ready to explode once the caterpillar forms into a chrysalid.

Parasitic wasps are considered valuable allies to agriculture because so many of their hosts are pests of crop plants. If I want to harvest a few sunflower heads to feed the birds this winter, I guess I should be grateful for the wasps’ assistance in keeping the patches in check. Still, I can’t help feeling a little sorry for the caterpillar, being eaten from the inside out.

So far chrysalid #1 seems normal and healthy, but, like the crew of the Nostromo, I’ll be keeping a close eye on it for any signs of “possession.” –SW

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