Tomato Pests (Hornworms) – and THEIR Pests (Braconid wasps)

(female) luna moth, Actias luna

A Luna moth (Actias luna). Not the enemy, even though its children are very large and green.

I don’t have a picture of a hawkmoth, aka sphinx moth or hummingbird moth (so named for its ability to hover and its very long tongue). But if you see one of these gray-brown creatures, almost big enough to pass for a small bird, you’re seeing disaster on the wing. The Hawkmoth’s very large green children are hornworms.

Manduca quinquemaculata or Manduca sexta, tomato or tobacco hornworm

Tomato – or more likely tobacco - hornworm (Manduca quinquemaculata or M. sexta), both voracious consumers of tomato, pepper, petunia, tobacco and other plants in the nightshade family.

In our New York and Maine gardens, hornworms usually show up in late July or August. But I’m thinking about them early this year because a Facebook friend in Virginia is already beset.

“Hornworms are eating my tomato plants,” she wrote, “anyone have advice on how to get rid of them?”

But of course!

Try the tips on Hornworm eradication at the end of this post, I replied, and if you get the chance, employ these two major organic defenses:

1. Bt (Bacillus thuringiensis), kurstaki strain, a widely-sold bacterial insecticide that kills caterpillars but is otherwise more or less harmless. It must be eaten to do its work, so baby butterflies are safe as long as you spray it only on the  hornworms’ dinner.  Unfortunately, Bt is most effective on hornworms when they’re still small. By the time they’re big enough to notice – or do damage that’s noticeable – Bt is no match for them.

2. The Braconid wasp Cotesia congregatus. These tiny, deadly parasites have evolved to prey only on hornworms and are generally available anywhere hornworms are found. To get some, simply notice which hornworms have white bumps and leave those alone.

pupa of braconid wasp Cotesia congregatus, on hornworm

This hornworm is (inadvertently) an organic gardener’s dear friend.

I used to think the white bumps were wasp eggs, which would on hatching enter the hormworms and eat ‘em up, but that was literally backwards. The bumps are cocoons, spun by the wasp pupa after they’ve eaten the hornworm and emerged, ready to transform themselves into adult wasps.

Same difference in the control department except you don’t have to worry about how much Mr. Bumpy might eat before the wasps get busy. By the time you see the cocoons, he’s on his last legs and no longer very hungry.

PS. Wondering what the Luna moth has to do with anything? Nothing, except that it’s prettier than a hornworm. To view the Luna moth’s children, check out this series of photos.

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  • Marci Said,

    Or you could recruit a couple 5-10-yr-old boys (my grandsons, for instance) who know the joys of a hornworm battle–picking and splattering them on each other.
    Oh, the wonderful green slime!

    Welcome, Marci
    Nothing like a sense of humor to get you through hornworm season – although it does sound as though the grandsons help. Here’s hoping the general joy of slime doesn’t wear off too soon.

  • I picked 9 hornworms off the tomato plants this morning and fed them to the chickens. Alas, none of them were wearing wasp cocoons but I remain hopeful. I didn’t know they like peppers, petunias, tobacco…I’ll have to go check those plants. (Weirdly, I picked one off of a weigela last week! very odd).

    Hi Michelle,

    My condolences on the hornworms – no nightshade is safe and as you point out, the wasps don’t always show up when wanted. Strange indeed about the weigela; I’ve never heard of hornworms eating anything BUT nightshades. (Weigela is in the honeysuckle family.) Hope they’ve slacked off by now!

  • Sarah Braik Said,

    It is now August 9 and I have yet to see a hornworm, which is very unusual. My tomato plants are HUGE, strong, and covered with tomatoes. Might it be too late in the season for the hornworms to arrive? Does anyone know. I’d be surprised to get that lucky.

    Hi Sarah,

    What can I say but “may you be surprised;” good luck in the hornworm department is something to treasure. Whether it’s late enough to let you off the hook is another matter. There can be multiple generations in a single season, even in the north. Your best guide may be the hawkmoths, or absence of same. Their eggs take only a short while to hatch, so you’re not really in the clear until frost is on the horizon

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