Those Awesome Hornworms

December 6, 2013

Nature Notes by Bill Dunson


Young butterflies and moths are of course caterpillars which eat and grow rapidly, pupate, and then emerge as adults. This total metamorphosis is very familiar to us and we sometimes fail to realize how remarkable it is. It enables two life stages of one creature to minimize competition with members of its own species by eating different food and inhabiting mostly separate habitats. The adults can be very obvious whereas the caterpillars are often camouflaged and rarely seen unless they eat our crops.

When I am on my daily hikes I try hard to set up a search image in my mind to see caterpillars but they can be very difficult to find unless they are brightly colored types that are protected by toxins. I was at Curry Creek Preserve recently and happened to notice the caterpillar of the Tersa sphinx moth on a false buttonweed plant (Spermacoce). This is in the group called hornworms since they have a spine at the end of the body which is not poisonous. In general sphinx moths are remarkable since they fly almost like hummingbirds with very rapid wing beats and are able to hover in front of flowers. Most are noturnal but there are some of the clearwings that forage at flowers during the day. The caterpillars are apparently very tasty to birds and thus are well camouflaged; but many also have a secondary line of defense as eye spots to intimidate birds that do find them. Note that the Tersa sphinx hornworm has a series of spots along the body with two large eyespots near the anterior end just behind the head. These appear to be mimicking a snake. The details of the eye are remarkable with a fake iris and pupil. This indicates that birds examine them very closely before deciding to eat them or not, selecting for the very best snake eye mimics.

The catalpa sphinx caterpillar or bean worm is beloved among fishermen as bait when it is turned inside out. The normal caterpillar at top is here compared with the lower hornworm which has been parasitized by a braconid wasp which has left many cocoons after pupating and flying away. The caterpillar may have few ways to protect itself against such an attack. People often plant the catalpa tree just to get the hornworms to use as fish bait. This is a reversal of the situation with tobacco or tomatoes where the valuable crop is eaten by hornworms. Tomato hornworms are remarkably difficult to find by eye even if you search for them carefully because they are so well camouflaged. I tried to grow tomatoes once in the FL Keys and finally gave up since all I was doing was growing caterpillars. Actually the hornworm shown in the photo which is eating a tomato plant is actually a tobacco hornworm (note red horn); both feed on members of the nightshade family.

One of the largest and most spectacular hornworms, the banded sphinx moth, is found on primrose willow along the edges of wetlands in southern FL. Despite what seems to be a colorful pattern it is also very well camouflaged and I am rarely able to find them at Wildflower.

So watch for the remarkable hornworms, if you can find them, and be amazed by their diverse strategies for survival.

Bill Dunson, Englewood, FL & Galax, VA

Bill Dunson

About Bill Dunson

Bill Dunson, born in rural Georgia, skipped 12th grade and went directly to Yale. Bill subsequent-ly received a PhD in Zoology from the University of Michigan, studying softshell turtles. Bill is Professor Emeritus of Pennsylvania State University thanks to a career spent entirely at that institution, teaching and doing research on the physio-logical ecology and ecotoxiciology of reptiles, amphibians and fish. Always curious about nature, Bill has dedicated his life to learning and sharing his knowledge with others. He has served on many advisory boards here in Southwest Florida to preserve the water that gives life to our region.

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