How to Fight Hornworms

Reader Contribution by Pam Dawling
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A large tobacco hornworm on the ground 

Two summers ago, I wrote on my website SustainableMarketFarming.com about Hunting Hornworms on Tomato Plants and this summer I wrote Dealing with Hornworms on Tomatoes. Here’s the short version.

Know Your Hornworms (Tomato and Tobacco Hornworms)

Hornworms are large caterpillars capable of doing serious damage to tomato crops. In our photos, you might notice our hornworms are not the same as yours. Ours aretobacco hornworms, not tomato hornworms, but both are bad news and both attack tomato plants. Before Twin Oaks Community started here in 1967, the land was a tobacco farm. Tobacco hornworms have a red (not black) horn, and diagonal white lines, not arrowhead vees.

One July day, in two 80 ft (24.4 m) rows of tomatoes in our hoophouse, I found 42 hornworms varying in length from 1” (2.5 cm) to 4” (10 cm) and totaling 85” (2.2m)! They were stripping leaves and munching on the green fruit.

Hornworms hatch from eggs laid by the night-flying Carolina sphinx moth or Tobacco hawk moth. This year I caught one of the moths, and killed it, but we still have plenty of caterpillars. The moths hatch from strange coppery pupae with pipes or spouts attached, which overwinter in the soil. Even our most vigilant caterpillar-hunting seems to miss some, which then drop to the ground to pupate. Another way to break the lifecycle is to close the hoophouse at dusk every night (and open it promptly every morning before it gets too hot), but we’ve decided not to go that route.

This hornworm pupa overwinters underground.

Know Your Friends (Braconid Wasps)

Hornworms often get parasitized by a tiny braconid wasp that lays eggs in the backs of the caterpillars. The larvae develop inside the caterpillar and then the pupae develop as white rice-grain-like cocoons sticking out of the back of the hornworm. Usually our friend the parasitic wasp doesn’t come inside the hoophouse and to get parasites into the hoophouse hornworms, and so we have to bring in parasitized hornworms from outdoors. This doesn’t work so well, because the hoophouse tomatoes are a month earlier than the outdoor ones, and the hornworm cycle is well underway in the hoophouse by the time the parasitic wasps are in action outdoors. This year we’ve found several parasitized hornworms indoors, and we are very happy. If you see parasitized hornworms, leave them be. They will stop eating, shrivel and die, and meanwhile the wasps (harmless to us) will hatch and go forth and multiply.

A hornworm parasitized by braconid wasps.

Signs of Hornworm Trouble

We conduct hunting raids every morning. To find where the hornworms are working, first look at the upper leaves of the tomatoes. If they are stripped bare down to the ribs, that’s a good place to look. Hornworms only like the tender upper leaves. If there are intact newer younger leaves, it might mean there was a hornworm, but it’s been removed already, and the plant is recovering. Another sign of hornworms in the area is chewed fruit. Another sign is “pineapple poop” – miniature brown pineapples or hand grenades. If you see fresh poop, look directly upwards – remember the law of gravity. The size of the poop is, naturally enough, in proportion to the size of the hornworm.

Having determined there is a hornworm in the vicinity, the next task is to find it. You’d think it would be easy – a big striped caterpillar like that. Not so! They are the exact same shade of green as tomato leaves. Hornworms can look remarkably similar to curled tomato leaves. The white stripes mimic the veins on the undersides of the leaves.

How to Hunt Hornworms

I do my hornworm hunting when it’s warm but not too hot, on the theory that then the caterpillars are more likely to be active, rather than snoozing in a sheltered spot. I walk along the row looking for damaged leaves. When I find some, I gaze at the area, looking for discrepancies in the pattern – bare stems with lumps on them. Usually the caterpillars are on the underside of a chewed stem, and often (but not always) they have their heads raised.

If I still can’t see the worm, I stand still and sway a bit from side to side, viewing the plant from different perspectives. It helps if the top of the plant is back-lit, but I do always check both sides of the row, no matter where the sun is. Knowing the signs of hornworm grazing can save you time looking everywhere. Focus your attention on where you are most likely to find them, and you will get the most success in the least time.

When you find one, get a firm grip, pull it off the plant (they have strong legs which hold on tight), drop it on the ground and stomp on it. The skins are quite thick.

Bt is an organically-approved pesticide spray that kills small caterpillars, without killing other insects. I don’t expect it to work on big hornworms. Hunting seems to be the way to go!

Pam Dawling works in the vegetable gardens at Twin Oaks Community in central Virginia. She often presents workshops at MEN Fairs, as well as sustainable agriculture conferences. Pam also writes for Growing for Market and other magazines. Her books, Sustainable Market Farming, and The Year-Round Hoophouse are available atwww.sustainablemarketfarming.com. Her blog is on her website and also on facebook.com/SustainableMarketFarming.


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