The Strange Case of the Catalpa Caterpillar

Caterpillars decimate a tree that doesn’t seem to care? On the latest You Bet Your Garden, host Mike McGrath will investigate the surprising symbiosis of the catalpa tree and its resident ‘worm.’ Plus, all your wormy questions!

Question of the Week:

We have a number of Northern Catalpa trees on our property; three very large ones that are a focal point of our yard and some smaller ones that I suspect came from seed shed by the large trees. We have caterpillars on the trees every summer, but they have previously never been overly destructive—and I’ve noticed signs that tachinid wasps have been laying their eggs on the caterpillars in the past, so I pretty much ignored them. This year, however, the caterpillars have completely defoliated a few of the small trees and are everywhere. I also don’t see any sign of the wasps’ eggs on them. There are so many caterpillars that I was considering using some Bt on them. But I’ve read that despite defoliation, the caterpillars aren’t really harming the trees. What would be your recommendation about these creatures?
—Laura “in Pennsylvania, near Phoenixville”

Are these caterpillars harmless? »