Deb Babcock: Caterpillars in our gardens


Deb Babcock

Deb Babcock's gardening column appears Thursdays in Steamboat Today.

Find more gardening columns here.

One of the favorite speakers at our master gardener classes is the entomologist from Colorado State University, Whitney Cranshaw. He tells us about bugs in our gardens. According to Cranshaw, all bugs have a purpose here on Earth. It's just that some bugs tend to - well, bug us more than others.

One of the bugs we're finding in our yards and gardens this month is the painted lady/thistle caterpillar. It is a black caterpillar with some light flecks and spiny thorn-like appendages that are harmless. The most common host plant for this caterpillar is the Canada thistle. That's good because this noxious weed is a real problem in Colorado. However, this caterpillar will inch toward garden plants such as sunflower, hollyhock, basil, artemisia and geraniums if there are no thistles to latch onto. These caterpillars usually produce a little webbing as they feed on the leaves of plants.

When this insect metamorphes, it becomes a beautiful orange, black and white spotted painted lady butterfly.

Many gardeners in Steamboat also have noted hornworm caterpillars in their yards. The whitelined sphinx is the most common. It matures into the hummingbird moth that so many of us are delighted to see in our gardens. These caterpillars are green with light striping and tend to live on aspen, cottonwood, poplar and willow trees. They rarely are found in large enough numbers to cause injury to your trees.

Two other species of hornworms found in Routt County vegetable gardens are the tomato hornworm and the tobacco hornworm. These caterpillars can be quite a surprise to come across in your garden with the scary-looking horn on their hind end. The horn apparently has no known function except to scare gardeners, says Cranshaw.

You can tell the difference between these two caterpillars by the color of the "horn" they sport. Tomato hornworms have a dark green horn, while that of the tobacco hornworms are red. These caterpillars tend to tunnel into the flower buds, preventing or limiting the flowering of the plant, tomatoes as well as elm, ash and poplar trees and weeds such as purslane.

The alfalfa caterpillar, present in large numbers this year in alfalfa fields or meadows where alfalfa may once have grown, has a green body with a narrow white stripe on each side. Hairs grow out of small dark spots on this insect, which is about 1 1/4 inches long at maturity. Alfalfa caterpillars consume the leaf midrib of foliage. Natural predators are spiders and parasitic wasps. When yellow and white butterflies appear in your fields, it might be a good time to cut your alfalfa before they lay eggs and the larvae begin consuming your crop.

So if the caterpillars are not causing too much damage in your garden, enjoy watching them turn into beautiful butterflies later this season.

Deb Babcock is a master gardener through the CSU Extension Routt County. Questions? Call 879-0825 or visit


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