Tobacco/Tomato Hornworm - Vegetables

 Tobacco hornworm larva

Tobacco hornworm larva - Manduca sexta (photo by M. Raupp)

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Appearance Eggs: Smooth, round (resemble small pearls), vary in color from yellow/light green to white, hundreds laid singly on undersides of leaves.
Larvae: Large, cylindrical caterpillar with a black “horn” located dorsally on last abdominal segment (tobacco hornworms have a red "horn"). First of 5 instars (growth stages) is yellow-white; others are green; dark brown or blackish forms can occur. Along each side there are 7-8 white, side-ways Vs pointing towards the head (tobacco hornworms have 7 diagonal white stripes along the side). Fully grown caterpillar may reach 3-4" inches in length.). 
Pupae: Large, elongate-oval, hard, reddish brown case about 2" long, pointed at posterior end, with a distinctive loop attached anteriorly and extending ¼ to 1/3 the body length.
Adult: Large, gray-brown hawk moth with wingspan of 4-5". Front wings are much longer than back wings, which have dark and white uneven bands. Abdomen has 5 orange-yellow spots (6 in tobacco hornworm).
Life Cycle/Habits

Overwinter in pupal cases 4-6” deep in the soil. Adult moths emerge over a long period beginning in May and extending as late as early August. Moths are most active at sunset and again just before dawn, feeding on nectar from flowers.  Eggs are laid singly on leaf underside, rarely on top. The horns of newly hatched larvae are nearly the same length as the body, but become proportionately smaller as larvae mature. Larvae feed voraciously on leaves and sometimes fruit. There are 4-5 instars (growth stages) over about one month. Mature larvae drop to the ground and burrow up to 6" deep into the soil to form pupal case. One to two generations a year.

Video: Larva feeding
Host Plants

Solanaceous plants: particularly tomato; less commonly on eggplant, pepper, potato.

Signs/Symptoms

Caterpillar feeding ordinarily begins at the top of the plant.  Caterpillars usually consume entire leaves, rather than chewing holes in them, and can rapidly defoliate plants. Damage is usually unnoticed until the final caterpillar instar, when 90% of defoliation occurs. Hornworms may attack green fruit, and feeding in ripe tomatoes makes large gouges (especially during dry weather).

Monitoring

Look under leaves for egg clusters. Green larval color is an effective camouflage, making the caterpillars difficult to detect. Be alert for light feeding at plant tops. Later, entire leaves and stems are eaten, leaving only stubs. Mostly interior leaves are eaten (vs. deer that browse on one side of plant). Dark, BB-sized, cube shape droppings on leaf surfaces indicate late instar caterpillars feeding above.

Prevention/Control
  1. Spraying water on plants agitates the hornworms and makes them easier to spot.
  2. Handpick caterpillars and drop into a container of soapy water.
  3. Large hornworms are often parasitized by wasps. The cocoons of Braconid wasps look like grains of rice attached to the hornworm's back. Do not kill parasitized hornworms! Let the wasps complete their lifecycle so they can multiply. A parasitized hornworm stops eating and eventually dies.
  4. Hornworms rarely warrant the use of an insecticide.
  5. Tilling garden soil in spring or fall may expose and kill the pupae.
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