Your tomato plants have been growing well. Then one day you notice that a lot of foliage has disappeared! You may have hornworms in your garden.
The hornworms blend in very well with the green tomato foliage and usually go unnoticed until the damage is done or they get too large to easily hide. Damage usually begins in midsummer and continues throughout the remainder of the growing season.
Hornworms are large, pale green caterpillars with white and black markings that reach 3 to 4 inches long when mature. The name "hornworm" comes from the spike on one of the caterpillar's last abdominal segments.
There are two types of hornworms that both feast on tomatoes--the tobacco hornworm and the tomato hornworm. The easiest way to tell the difference between these two tomato worms is by examining the adult caterpillar's stripes.
Tomato Hornworm
The tomato hornworm has eight angled stripes that point toward its head. The "horn" is usually black.
Tobacco Hornworm
The tobacco hornworm (which also feasts on tomato plants) is actually more common in gardens than the tomato hornworm. That's because its range is from southern Canada to Argentina. In contrast, the range of the tomato hornworm extends only from southern Canada to the southern U.S.
The tobacco hornworm can be distinguished from the tomato hornworm by its seven diagonal white stripes and its "horn" is usually red. Some plant hosts of the tobacco hornworm include tobacco, tomato, eggplant, pepper, and some weedy plants.
Life cycle of the hornworm
In late spring, the adult moth lays eggs on the undersides of tomato leaves. The eggs hatch in 6 to 8 days, and the larva pass through 5 or 6 stages, reaching full growth in 3 to 4 weeks. The full-grown larva then burrows into the soil and transforms into the pupal stage.
The pupa may remain in the soil all winter, with the adult moth emerging the following spring. Or, if the weather conditions are suitable, the moth may emerge in 2 to 4 weeks. The emerging moth makes its way to the soil surface and mates. The female then deposits eggs on the tomato plants for the next brood.
Sphinx Moth
The adult tomato hornworm is called a sphinx moth or hawk moth. It has a mottled gray-brown color with yellow spots on the sides of its abdomen. Its wing spread is 4 to 5 inches. It is a medium to large heavy-bodied moth, with narrow front wings. The spindle-shaped body is tapered at both ends, with fairly thick antennae.
The tobacco hornworm moth has 6 orange spots on each side of the abdomen, whereas tomato hornworms have 5 similar, but less distinct, spots on each side. The tobacco hornworm moth has less distinct and jagged lines on the hind wings compared to the wings on the tomato hornworm moth.
How the hornworms damage tomato plants
The tobacco and tomato hornworm caterpillars feed on the leaves and new stems of the tomato plant. They strip tomato leaves from the vines. If a heavy population of hornworms develops, they will also feed on the tomato fruit. Rather than boring into the fruit, they feed from the outside of the fruit and leave open scars. However, defoliation is a much more common problem than fruit damage.
Controlling infestations of hornworms
There are a number of natural factors that help to control the hornworm. One of the most common parasites in home gardens is the small braconid wasp. If you see hornworms with a number of white projections protruding from their bodies, these are the cocoons of the wasps. You should leave these affected tomato worms in the garden so the emerging wasps can attack other hornworms. The wasp larvae feed inside the caterpillar and kill the host upon emergence during mid- to late-summer.
Hand picking the hornworms off of infested plants in the garden is the most effective control method short of chemicals, and is the recommended method for small gardens. One method used to make them more visible is to plant dill near the tomato plants--hornworms on dill are more easily discovered. A natural repellent to hornworms is basil or borage planted near the tomato plants.
For chemical control of tomato worms I suggest you contact your county agricultural extension office for recommended sprays and dusts.
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