Hornworms are among the largest caterpillars found in Colorado, some reaching lengths of three inches or more. The most widely recognized hornworms are those that feed on tomatoes--the tomato hornworm and the more common tobacco hornworm. Although these species are considered garden pests, most of the two dozen hornworm species are rarely observed and cause no significant plant problems. Adult stages of hornworms turn into large, heavy-bodied, strong flying insects known as sphinx or hawk moths. Most fly at night and are rarely observed except occasionally near porch lights. Characteristically they sport a flexible spine ("horn") on the hind end, although in some species this is lost and replaced with an eyespot marking. Full-grown hornworm larvae migrate from their host plant and dig in loose soil where they pupate in a small chamber of packed earth. Pupae are typically brown, two inches or more in length, and many have a pronounced "snout" on the head end. A few species, however, fly during cloudy afternoons and at dusk when they feed at flowers in a manner similar to that of a hummingbird.
The whitelined sphinx is the species most commonly observed with this habit and is usually most active in late afternoon and at dusk. Caterpillars of this insect feed on various weeds and only rarely occur on garden plants. The great ash sphinx and twinspot sphinx are other species with "hummingbird moth" behavior. Tomato hornworms are easily controlled by most available garden insecticides (carbaryl, permethrin, etc.). A biological control organism that is also highly effective and sold commonly is Bacillus thuringiensis (Bt--Dipel, Thuricide, etc.). Hornworm larvae may also be hand picked although they can be surprisingly difficult to detect due to their cryptic coloration. Larvae tend to feed on the exterior parts of the plant during shadier periods, near dusk and dawn, when they may also be more readily observed and destroyed. For more information see the fact sheet Hornworms and "Hummingbird" Moths.

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