Dealing with Hornworms on Tomatoes

 

A large tobacco hornworm. Note the fake eyes near the horn at the tail end.
Photo Pam Dawling

Two summers ago, I wrote about Hunting Hornworms on Tomato Plants.

Here we are in July again, and here are the hornworms again! Yesterday, in two 80 ft (24.4 m) rows of tomatoes, I found 53 inches (1.35m) of hornworms! There were 24, varying in length from 1” (2.5 cm) to 4” (10 cm). Today I found even more: 42 caterpillars totaling 85” (2.2m)! They are stripping leaves and munching on the green fruit.

In our hornworm photos, you might notice ours are not the same as yours. Ours are tobacco 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.

Hawk moth (mother of hornworms) caught in a web of a zipper spider. Photo Pam Dawling

Hornworms hatch from eggs laid by the night-flying Carolina sphinx moth or  Tobacco hawk moth. This year I did catch one of the moths, and kill 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.

Hornworm with parasitic wasp pupae.
Photo Pam Dawling

Hornworms often get parasitized outdoors 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 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 in the past few days, and we are very happy.

Hornworms have stripped these tomato leaves.
Photo Pam Dawling

Meanwhile, we are conducting 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.

Hornworm poop on a tomato.
Photo Pam Dawling
Evidence of hornworms.
Photo Pam Dawling

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.

When I find some signs, I gaze at the area, looking for discrepancies in the pattern – bare stems with lumps on them. Usually the caterpillar is on the underside of a chewed stem, and often (but not always) they have their heads raised. 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.

Found it! Hornworm on a tomato leaf.
Photo Pam Dawling

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.

A large hornworm eating a tomato.
Photo Pam Dawling

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!

Our goal – tasty ripe tomatoes! These are Five Star Grape.
Photo Pam Dawling

Hunting Hornworms on Tomato Plants

Tobacco Hornworm eating a tomato plant
Photo Pam Dawling

Perhaps you also have hornworms eating your tomato plants? The upper leaves stripped to stems, the fruit munched, and big fat caterpillars getting bigger and fatter? Ours are tobacco hornworms, not tomato hornworms, but both are bad news. Fifty years ago, the Twin Oaks land was a tobacco farm. Tobacco hornworms have a red (not black) horn, and diagonal white lines, not arrowhead vees.

Outdoors our hornworms often get parasitized by a tiny braconid wasp whose larvae develop white rice-grain-like cocoons sticking out of the back of the hornworm. But the parasitic wasps don’t usually fly into the hoophouse, so we have to provide the pest control ourselves. We could prevent the night-flying hornworm-mother Carolina sphinx moth or  Tobacco hawk moth from entering our hoophuse by closing it up every night, but we don’t want to do that, as it means we have to reliably open it every morning before it gets too hot.

Hornworms strip the upper tomato leaves until only stems remain.
Photo Pam Dawling

When I find these beasties, I pull them off the plants (it can take quite a tug, their legs are strong), drop them on the ground and stomp on them. They can grow to be 4″ caterpillars. One day when I’d finished my safari along the first tomato bed and was working my way along the back of the second bed, I saw a cardinal fly in twice and fly off with a dead hornworm in its beak. I guess they can’t tackle live ones. (If only . . .)  I’m intrigued by how they knew I had provided dinner. Did they smell the hornworms? Or see the dead ones on the path? Sight seems more likely to me than smell.

So, why are these big caterpillars hard to find? You might imagine such big worms with such vivid stripes would be easy to see, but not so. They are, of course, the exact same shade of green as tomato leaves. Curled tomato leaves can look remarkably similar to hornworms. The stripes mimic the veins on the undersides of the leaves.

Tomato plant badly damaged by hornworms
Photo Pam Dawling

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 as in my second photo.

If I’m looking at damaged young leaves I’m pretty confidant that there’s a hornworm somewhere in the vicinity. If there are newer leaves that are intact, it might mean there was a hornworm, but it’s been removed already. Another sign that a hornworm is nearby is what safari hunters call fresh spoor – caterpillar poop. Hornworm poop looks like miniature brown pineapples or grenades (use whichever comparison you are more familiar with.)

Hornworm poop on tomato leaf.
Out-of-focus photo Pam Dawling

If I see damaged fruit, I redouble my efforts to find the culprit.

Tomato fruit damaged by hornworm
Photo Pam Dawling

If I still can’t see the worm, I sway a bit from side to side, viewing the plants from various perspectives. Sometimes I scrunch down a bit, so that the top of the plant is back-lit – that helps. Hunt frequently, every day or two. Knowing the signs of hornworm presence can save you time looking high and low. Instead you focus your attention on where you are most likely to find them.

ADDITION JULY 2019: I have a newer post Dealing with Hornworms on Tomatoes. Click that link for more info.