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Tomato Hornworms

This article was originally published on June 11, 2012 and expired on June 18, 2012. It is provided here for archival purposes and may contain dated information.

Dry weather has hit area crops as well as the natural controls that keep pests in check. That scenario probably means more problems with caterpillars, including the tomato hornworm. The tomato hornworm, Manduca quinquemaculata, is a close relative to Manduca sexta, the tobacco hornworm.

Damage is usually the first readily observable sign of the hornworm for most growers, since it takes only a few of these hungry, green, three-inch long feeders to strip the plant of its leaves and their color matches that of the plant, effectively camouflaging the pest from the most keen observers.

Capable of growing up to three inches in length, hornworms are larvae of moths that emerge from the ground in the spring and feed upon the nectar of garden flowers. Eggs are laid by the adult moth on the underside of tomato leaves where they hatch about a week later. The larvae feed for three to four weeks, going through approximately five to six growth stages, and severely damage the plant and its fruit. They then drop off the plant, burrowing several inches into the ground. Here the larvae pupate and overwinter, emerging as an adult moth the following summer. There is one generation per year.

Since the pest is destined to bother at least a few gardens this summer, one definitely needs to know how to control this pest. Handpicking may be an option and usually provides adequate control. However, nature can also play a role in hornworm control.

A naturally present parasitic wasp, known as apanteles congregatus, seeks out hornworms and lays its eggs within the worms' bodies. These eggs hatch within several days and the newly- emerged grubs feed on the internal organs of the worm. Within one to two weeks, the grubs migrate to the skin of the worm and emerge within cocoons. It is not uncommon for gardeners to come across hornworms that are literally plastered with these cocoons. Such infected worms should be left alone since the internal feeding of the wasp larvae weakens the pest and results in its eventual death. Cocoons hatch into new wasps within 3-4 days.

Source: Matt Montgomery, Extension Educator, Local Food Systems and Small Farms, mpmontgo@illinois.edu

Pull date: June 18, 2012

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