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).
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.