If you grow tomatoes and your leaves seem to have disappeared overnight, you might wonder what is munching its way through your tomatoes.
You might blame some four-legged critter, but you might be harboring the Tomato Hornworm (Manduca quinquemaculata). A large tomato hornworm can eat a tomato plant in a few days if not found and stopped.
The hornworm caterpillar is a large caterpillar reaching up to 4 inches long when mature. They are a pale green with black and white markings.
Mature hornworms will have eight V-shaped white marks running down each side.There is a brown form which is not common. A small, black “horn” can be found on the last segment, which is where the name comes from.
Hornworms can be found throughout most of the United States feeding on plants in the nightshade family, including tomatoes, potatoes, peppers and eggplants. Nightshade, jimsonweed and horse nettle are a few of the weeds that can be used as a host by tomato hornworm.
You’ll probably find signs of this big, green caterpillar before you actually see it. They are well camouflaged and blend right in.
Caterpillars will attach themselves to the undersides of leaves. They also don’t like the heat, so they tend to stay in the shaded part of the plant during the day.
They move to the outside of the plant to feed during the night. They will strip a plant of its leaves and larger caterpillars will even eat green tomatoes.
The final caterpillar stage eats almost as much as its previous stages combined. Look for their black or dark green droppings — frass — on lower leaves and the ground.
The caterpillars will feed until late summer or early fall.
Caterpillars go through five or six stages before reaching full size in three to four weeks. Then they drop to the ground and burrow into the soil where they make a small chamber. They pupate and spend the winter there.
In some cases, under the right weather conditions, moths might emerge in two to four weeks creating a second generation. In late spring the adult moth emerges.
The adult is one of the sphinx moths, which is called the five-spotted hawkmoth.
The five-spotted hawkmoth is a rather beautiful moth with a wingspan of 4 to 5 inches. It has five yellow-orangish spots on each side of the thick brown and white abdomen.
When its wings are spread you can see the two blue “eyes” on the back of the thorax. The narrow front wings are mottled brown and gray while the hindwings are banded with white and brown.
They also have two zigzag bands going across the hindwings.
The adults feed on nectar from flowers like phlox, petunias and honeysuckle. They even act as pollinators for some plants.
Like other moths, they fly at dusk. The moths hover over flowers and use their long proboscis to suck up nectar.
They are also food for bats, birds and small animals.
The female moths lay eggs singly on selected host plants. Sometimes they pick a tomato plant and leave an egg on the underside of a leaf.
Eggs are about a millimeter in diameter, round to oval, and off-white to a pale green. Eggs hatch in about a week.
Home gardeners should scout their plants for the caterpillars — and other problems — at least twice a week. An easy solution is to handpick the caterpillars off of the plants.
If you don’t want to handle them, just pick off the leaf they are on. You can drop them into soapy water or feed them to your chickens.
To reduce overwintering pupae, cultivate the soil after you clean up the garden. Also remove any solanaceous weeds from around the garden area.
Usually pesticides are not necessary but if they are, the caterpillars are easier to control when small. Bacillus thuringiensis or Bt is an organic insecticide that only affects caterpillars.
It does not affect other insects or bees.
Caterpillars have to ingest the Bt for it work. Rain can wash Bt off of plants so several applications could be needed.
Follow the label instructions for proper application. The label will also tell you how many days you should wait before picking vegetables after spraying.
Beneficial insects like green lacewings and ladybugs will prey on the eggs and small caterpillars.
Paper wasps (Polistes spp.) are also important predators of the caterpillars. They feed caterpillars of all kinds to their larvae.
The eggs are also attacked by tiny Trichogramma parasitic wasps. These tiny wasps lay their eggs inside the hornworm eggs preventing it from hatching.
The eggs turn black as the wasps mature.
If you happen to find a hornworm caterpillar that appears not to be feeding, leave it alone. It’s possible that it’s been parasitized by a parasitic braconid wasp.
If there are tiny white cocoons attached to the caterpillar that’s a good thing.
The cocoons contain the wasp larvae. They feed inside the caterpillar, and it dies as they emerge.
The adult wasps fly off looking for other hornworm caterpillars.
If you notice a large moth visiting your flowers in the evening you might want to check on your tomatoes.
Have a gardening question?
Master Gardener volunteers are normally in the office 10 a.m. to noon weekdays. You can stop in at the CCE office at 420 East Main Street, Batavia, call (585) 343-3040, ext. 127, or e-mail them at email@example.com.
Join us for Garden Talk at noon on Aug. 5 via Zoom. Our topic will be “Introduction to Beneficial Insects.”
Most of the insects that live in your garden or landscape do little or no harm to you or your plants. Many of these good guys provide free pest control for you.
Who are these allies? Join us to learn about the beneficial insects that might be in your garden.
This program is free, but registration is required to get your Zoom link. Visit the events page at the CCE Genesee website: genesee.cce.cornell.edu/events.