Hornworms – Control

Q: Until four years ago I always planted three tomato plants in a spot where they covered the entire side of my garage and bore fruit until frost. Then, the hornworms got my address. The first year they defoliated an entire plant overnight. Soon, I couldn’t pick them off fast enough. Is there anything I can do before planting?

A: Hornworms have a voracious appetite. When they grow large, usually by July, one can devour a foot long tomato leaf in one day. Small ones are almost impossible to spot. Their green coloration is excellent camouflage against human and animal predators.

Though we refer to them as “tomato hornworm”, the most common hornworm on tomatoes is actually the tobacco hornworm.

You can try using a UV (blacklight) source to find them at night. They glow a ghostly green.

Fortunately, there are two insect enemies that have no problem finding tomato hornworms: both are wasps.

The tiny braconid wasp lays her eggs on an unlucky hornworm. The immature wasps consume the caterpillar from the inside and then make numerous white cocoons on the creature’s skin. Additionally, the common paper wasp loves to eat small hornworm caterpillars.

To avoid hurting your insect friends, spray or dust an organic garden insecticide containing Bacillus thuringiensis (B.t.) click for sources) on your plants in mid-May and every week thereafter.

The B.t. is harmless to humans, animals and insects but is fatal to caterpillars.

tobacco hornworm

tobacco hornworm

tobacco hornworm eating tomato

tobacco hornworm eating tomato

tobacco hornworm with wasp cocoons

tomato hornworm with wasp cocoons

hornworm droppings

hornworm droppings

hornworm in UV light

hornworm in UV light

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