Catalpa worms are actually caterpillars that are attracted to the catalpa tree, according to forester Steve Nix for About.com. These catalpa worms are often used for fishing bait and can be detected by their yellow color and dark black lines.
Catalpa worms can strip a tree of leaves, but the tree seems to have no adverse affects from it, Nix says. The catalpa worm comes from larva of the catalpa sphinx moth. These catalpa worms can grow to 2 inches long and eat the southern catalpa and the northern catalpa tree. Pale when young, the catalpa worm darkens as it ages.