Tomato Hornworm

Ah, the tomato hornworm, ‘Manduca quinquemaculata’ … if you’ve seen one of these 4 inch, fat, green, silken-skinned beauties on your tomatoes, you’ve no doubt had the same deep-seated reaction I have… _barf_.

While the hornworm does end up making a fairly cute moth, they ravage tomato plants, stripping them overnight sometimes. What's a gardener to do? There are a number of tactics that practiced together can reduce the population.

It’s important to note, in pest management a 50% reduction each year is an aggressive goal. Most problem pests can be reduced over time to a 10% population that is manageable. The following represent a multi-pronged offense that together can help reduce their population.

Cultural Practices
Mature worms drop to the soil, burrow about 4 inches down, and form a red brown pupal cell. Good tillage has been shown to help break up more than 90% of these in a large infestation, cutting down on the number of eggs that are laid the following year.

An important cultural practice is one of doing nothing… in a specific circumstance. There’s a parasitic braconid wasp, ‘Cotesia,’ that lays its eggs to feed on the horn worm. (The paper wasp 'Polistes' also preys directly on the adult hornworm itself - delicious.)

If you should find a hornworm that is covered in little, rice-like cocoons, do not remove this worm or harm it. Its survival (until the wasp larvae have eaten it from the inside out – there’s some poetic satisfaction there, eh?) will ensure the survival of more parasitic wasps, who will hatch in the same season, lay more eggs, devour more hornworms… well you get the picture. Circle-of-life, miracle-of-birth -- all that.

Manual Removal
As icky as it is, hand picking is the most successful control in the home garden. It is unfortunate that they are hard see until there is a lot of destruction. Hornworms are very small when young and at all larval stages will avoid the heat of the day, sticking to the inside of the tomato plant.

They’re most easily spotted around dawn and dusk. Picking these guys off and (eesh) squishing them in whatever method you find least nauseating… is your key to success.

Hornworms are in their egg stage for too short a period to pick off or smash as one might with the eggs of other garden pests, usually 5 days or less. The green to white eggs are also very small (1.5 mm) and hard to find.

Biological Controls
Bacterial insecticide containing ‘Bacillus thuringiensis’ or BT (e.g., Dipel, Thuricide) can be applied in the very early stages when the worms are small (always follow label directions, even with 'safer' biological controls like BT).

This is a good preventative to use in gardens that experience regular infestations. Apply early in the season before there are any signs of the worm. BT is a bacteria that harms only certain insects like the caterpillars, mosquitos, and others; it has little or no effect on humans, wildlife, pollinators, and most other beneficial insects.

Unfortunatly, BT will also do little to harm the adult worms; treat early!

Chemical Treatment
Hornworms can be controlled with limited success using carbaryl, permethrin, spinosad insecticides. Read the label carefully before using any insecticide.

Icky? Yes. (i got all yicked-out just writing about them.) Picking them off icky-er? *shudder* Definitely. But gardeners who love tomatoes will brave these grodies. Think of them as… dragons of the garden, and you the St. George of Tomatoland.

Or maybe try not to think about it at all. Just squish and get it over with.