Gardening Harmful insects

Tomato Hornworms Are Easy to Control

20170804A Whitney Cranshaw, Colorado Ste University, WC
Although big and scary-looking, the tomato hornworm is harmless to people. Photo: Whitney Cranshaw, Colorado State University via Wikimedia Commons

You suddenly find a leaf on your tomato plant has been almost eaten off. And the next day another, then yet another? This is the usual symptom of the tomato hornworm (Manduca quinquemaculata), a menacing-looking (but harmless to people) huge green caterpillar 4 inches (10 cm) long. You can also find it on other plants in the nightshade family (eggplants, peppers, nicotianas, potatoes, daturas, etc.), but it does seem to prefer tomatoes. It has a similar-looking cousin, the tobacco hornworm (Manduca texta) with the same host range, but it prefers tobacco plants (Nicotiana).

Manduca quinquemaculatus a.k.a. Five-Spotted Hawkmoth a.k.a. tomato hornworm
The adult sphinx, almost the size of a small bird, is rarely seen. Photo: User:Lpm, Wikimedia Commons

The adult sphinx is a large, narrow-winged, dull-colored moth that beats its wings at high speed and pollinates many flowers … including, oddly enough, flowers of several nightshade relatives, like nicotiana and datura, the same plants its caterpillars eat! Because of its very nocturnal habit, the adult is rarely seen, although there are other smaller sphinx moths you may well see pollinating flowers during the daylight. The female lays its eggs underneath the leaves of the host plant and the larvae hatch in only 3 to 5 days. There are two or more generations per year.

This is an excellent example of a case where hand picking is the only “insecticide” necessary. That’s because there is usually only one hornworm per plant, rarely two. Why would you spray an entire plant with a possibly toxic pesticide just to kill just one bug? It’s so much easier to locate the caterpillar, remove it and “dispose” of it. (Usually, I knock it to the ground and squash it under my shoe, but the more squeamish could simply drop it into a cup of soapy water.)

But, despite its enormous size, this caterpillar is almost the same color as the foliage and can be hard to see. If you see chewed leaves but not the pest, spray your plants with a jet of cold water. The caterpillar will react by thrashing about … and is then easily spotted.

The tomato hornworm is found throughout the USA as well as in southern Canada and northern Mexico, but is most common in the northern part of its territory while the tobacco hornworm, which has a similar range, tends to be more common in the south.20170804A Whitney Cranshaw, Colorado Ste University, WC


4 comments on “Tomato Hornworms Are Easy to Control

  1. Cold water, hey? I’ll try that….also found some on my sweet mint this year- yipes, I almost stuck it in my cold drink. We’d both be thrashing around then. :{

  2. Pingback: The Moth That Flies Like a Hummingbird – Laidback Gardener

  3. Pat Hayward

    Or better yet, plant an extra “sacrifice” plant (or 2) to relocate the caterpillars on to. This year we successfully removed 21 hornworms from our garden onto 2 sacrifice plants. After a week or so of feeding, I’m sure they made it into the soil to pupate and eventually emerge to become those magnificent sphinx moths we’re all in awe of.

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