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

8 thoughts on “Dealing with Hornworms on Tomatoes”

  1. Pam, Thanks for the details. Braconid wasps are our friends!
    Have you considered the life history of these wasps, specifically how they overwinter? (Does the last generation overwinter as adult wasps or as pupae in cocoons?) Could you implement a strategy in your high tunnel for them to overwinter there, so that their emergence would be in sync with the early tomatoes and hornworms?

    Separate, related: In the past, we handpicked “green worms” from summer brassicas (fish and chick food), until seeing the first ones with braconid cocoons, then let them be. Ongoing damage was tolerable in our leafy brassicas.
    My kids learned to recognize the worms that, so full of wasp larva, had stopped feeding and became immobile.
    Do hornworms display similar behavior, becoming immobile just before braconid cocoons appear? I seem to recall that they stop feeding, or feed much less, once festooned.

    1. Thanks Richard, That’s a good suggestion, to look into the braconid wasp lifecycle. I was just hoping we’d now got a strain of braconids that liked living in the hoophouse! I think one of the links I put in (I hope I did!) has some more info. Yes, the “festooned” hornworms, as you call them, do stop feeding, growing, thriving, and then shrivel and die,but I don’t know if they stop feeding before you can actually see any cocoons. Time is short, hornworms are plenty, I have not got to know them individually!

      We do also get those white cocoons on brassicas. I have heard of some people storing some leaves with cocoons in their fridge over the winter, but I’ve never tried that! We also have what Barbara Pleasant calls “good wasp coverage” – the wasps haul off the brassica caterpillars. So we try to welcome the wasps too. Pam

      1. Pam, I presume paper wasps feed on young hornworm larvae, up to a certain size. Do you know?
        My youngest son specifically grew a few collard plants last year to raise green ‘cabbageworms’ (Pieris) to feed the paper wasps (vespids). I used to knock down the paper wasps nests around our farm and feed the larvae to fish, until he advocated for them. Now he feeds the paper wasps green cabbage worms by hand. Yes, they fly down from the nest and take them out of his hand, then fly back up. Or walk off the nests he can reach, onto his hand.

        When I expressed alarm and asked how he keeps from getting stung, as a farm boy he retorts “They’re just like any other animal, learn not to threaten them”.
        We destroy perceived threats in ignorance, to our peril.

        As for keeping a few leaves in the refrig, with parasitized Pieris, sounds like an idea worth trying if humidity is adequate. I recall a recommendation to leave a few brassica plants in the field all winter, same reason. In our push to break disease cycles (complete plant removal), we interrupt other life cycles as well…

    2. Richard, I reread Hadley, Debbie. “What Are Braconid Wasps?” ThoughtCo, Jun. 18, 2018, thoughtco.com/what-are-braconid-wasps-1967998. That’s the link I put in the post. She doesn’t say where the adult braconid wasps spend the rest of the year. Maybe I’ll try keeping some of the last parasitized caterpillars of the season overwinter in the fridge (won’t my housemates love that?) Pam

    1. Oh amazing! But I’m too tired at night to go hunting, even for such gorgeous creatures as these in their disco finery! Pam

  2. This is fascinating. I grew up on a tobacco farm so I’ve always called these “tobacco worms,” even now that we grow tomatoes and not tobacco. I never knew there were 2 different kinds of hornworms!

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