Wild Ideas: Strange ‘cats’ are eating my plants

The cavalcade of bugs around my yard have been a joy to observe, photograph and wonder about this year. With fall now here, many will soon die or find shelter for the winter, so I’ve been trying to enjoy the remaining ones as often as I can. Some of my favorites have been the larvae of butterflies and moths, better known as caterpillars — or “cats,” as Lepidoptera enthusiasts call them.

Although some butterflies were prolific this year, including spicebush swallowtails, I haven’t found any of their larvae on the abundant spicebush along a forest trail near the house. What I have found a lot of, starting in late summer, is moth caterpillars.

Although putting a sheet under a host plant and shaking it is the best strategy for finding caterpillars, I prefer not to disturb them just to satisfy my own curiosity. I take a less invasive approach — looking for leaf damage and inspecting the undersides of leaves. Most cats prefer to eat sheltered from the sun and out of sight of predators, particularly birds.

Most of the caterpillars I’ve found are of tiger moths, including the ubiquitous woolly bear (Isabella tiger moth, or Pyrrharctica isabella), the yellow bear (Virginian tiger moth, or Grammia virgo) pale tiger moth or banded tussock (Halysidota tessellaris) and the fall webworm moth (Hyphantria cunea).

The yellow bear caterpillar, despite its name, can come in colors ranging from yellow to brown to gray. In the brown form, the caterpillar can be confused with a woolly bear, but the yellow bear’s setae are longer and softer than that of its better-known cousin. Photos by Pam Owen.
The yellow bear caterpillar, despite its name, can come in colors ranging from yellow to brown to gray. In the brown form, the caterpillar can be confused with a woolly bear, but the yellow bear’s setae are longer and softer than that of its better-known cousin. Photos by Pam Owen.

I also spotted an io moth caterpillar (Automeris io), a cat that looks like a hornworm but with bright-green setae that look like carrot tops growing out of it, and a tussock cat lookalike, that of the white-marked tussock moth (Orgyia leucostigma). Despite its appearance and common name, the latter is not a tiger moth but a member of the Lymantriidae family, thought to be closely related to tiger moths. It’s easy to identify because of its bright-red head.

All of these cats look like they have fur coats, which are actually tufts of hair called setae. The setae can be long and short, narrow or plump. While the woolly bear has short, bristly setae, the others have longer, softer setae. With the pale tiger moth and the white-marked tussock, some of the setae are longer, stiffer and differently hued, making them look more like tiny Yorkshire terriers with fancy hairdos than insects.

I was dismayed to find, while trying to maneuver one of these cats into a better position to photograph it, that the setae come off quite easily. Touching them can also trigger an allergic reaction in some people, according to David L. Wagner’s “Caterpillars of Eastern North America,” an excellent caterpillar reference. While I don’t have an allergic reaction to this cat, I don’t want to denude any, so I avoid touching them. A few caterpillars, like the saddleback, can actually sting, causing considerable pain or worse.

All of the cats I found have a wide range of hosts. The yellow bears have been enjoying my sunflower garden but have also denuded a variety of native “weeds” nearby. According to Wagner, webworms enjoy more than 400 species of woody plants (trees and shrubs), along with many herbaceous (nonwoody) plants, and the white-marked tussock cat can be found on pretty much any woody plant.

The white-marked tussock moth is easily distinguished from similar-looking caterpillars by its red head. The setae on this caterpillar take various shapes, including ones on its back that look like cotton balls.
The white-marked tussock moth is easily distinguished from similar-looking caterpillars by its red head. The setae on this caterpillar take various shapes, including ones on its back that look like cotton balls.

On what might be considered the opposite end of the spectrum from these hairy cats is the tobacco hornworm, a.k.a. the goliath worm (Manduca sexta), which lack setae and can grow to almost four inches. I recently discovered several feeding on a tomato plant in one of my flower gardens. This big, fat caterpillar is bright green with white lines running diagonally down its sides. What looks like an eye but is actual a respiratory opening is near the base of each line. The tobacco hornworm is often confused with the tomato hornworm (five-spotted hawk moth, or Manduca quiquemaculata), which has white chevrons down its sides, and is in the same family, sphinx or hawk moths (Sphingidae).

Both species of hornworm feed on plants in the nightshade family, including tobacco and tomato. Even with their large size, their coloring blends in so well to their host plants that I had to look really closely to find the hornworms on my tomatoes. I also found that they would stop eating and become motionless if I made the plant move or came too close, making them even harder to spot. Species in the Sphingidae family also “possess the most acute color vision of any animals, discriminating floral colors at light intensities that would appear pitch black to the human eye,” Wagner writes.

It’s hard to tell which end is which on the large tobacco hornworm at left, but the red horn is on the rear end. The first of eight spiricals (openings used for respiration) on its side look like eyes.
It’s hard to tell which end is which on the large tobacco hornworm at left, but the red horn is on the rear end. The first of eight spiricals (openings used for respiration) on its side look like eyes.

While anyone growing tomatoes is likely to be appalled to find hornworms devouring their crop, I was overjoyed to see them. I love tomatoes, but my two tomato plants are volunteers and came up late. With the limited light up here on the mountain, they’re unlikely to produce ripe tomatoes before frost kills them. While I managed to grab a few very green tomatoes before the hornworms finished eating the leaves and started on the fruit, I was happy to have fewer green tomatoes for frying in exchange for being able to watch and photograph these magnificent caterpillars.

Hornworms, like many caterpillars, are often parasitized by wasp larvae. An infested hornworm looks like it has grains of white rice stuck to its body. These “grains” are actually the cocoons of braconid wasp larvae, which have been feeding on the cat and are now pupating. I’ve somewhat ghoulishly been watching to see if this would happen to my hornworms, but so far they’ve escaped this slow death. Anyone concerned about hornworm infestation should leave a parasitized one alone, since the wasps that will hatch out will help control the hornworm population.

(For more photos of moth caterpillars, see the slideshow at NighthawkCommunications.net.)

Pam Owen
About Pam Owen 283 Articles
Writer, editor, photographer, and passionate nature conservationist living in Rappahannock County, in the Blue Ridge Mountains of Virginia. Two favorite quotes: By E.O. Wilson, who coined the term "biodiversity," "Nature holds the key to our aesthetic, intellectual, cognitive and even spiritual satisfaction”; by Douglas Adams, “I love deadlines. I love the whooshing sound they make as they pass by.”