When NOT To Kill a Tomato Hornworm

August 4, 2011

RECENTLY, while pruning a row of paste tomatoes, I noticed a strange-looking worm (above; click to enlarge) on one of the plants. What this worm is, why it has white projections on its body, and why — if you ever see such a thing in your own garden — you should leave it alone:

A quick Google search revealed the odd fellow is the Tomato Hornworm. This Manduca quinquemaculata feeds on the leaves and stems of tomato plants, as well as other plants in the Nightshade family, like eggplant and potatoes. It can devour an entire plant in a matter of days. Worms should be picked from plants, and dealt with as your conscience permits.

However, if the worm has white ovals on its body, you should take no action at all. The white projections are the larvae of the braconid wasp, Cotesia congregatus. Larvae that hatch from the wasp’s eggs, which are laid on the hornworm, feed on the inside of the hornworm until the wasp is ready to pupate. Such “host” hornworms should be left in the garden in order to conserve the beneficial parasites. The wasps will kill the hornworms when they emerge from their cocoons. They will also seek out other hornworms to feed upon and kill.

Want to attract this hornworm-destroying wasp to your garden? Then plant, and preferably near your tomatoes, such things as parsley, dill, yarrow, and mustard. Adult wasps feed on the nectar of these plants. Also, provide a source of water. A birdbath will suffice.

And finally, I hope you won’t scream if you notice a host-hornworm on your tomato vines. For it means your garden is ecologically balanced, thanks to your non-use of pesticides.

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Related Posts:
Classic Tomato Pie
Slow-Roasted Cherry Tomatoes
Angelic Zucchini Fritters

Comments

  1. Lisa F says:

    Wow, good to know! We have been freaking out with the hornworms on the tomatoes. Thanks!

  2. Lisa F – Hornworms are strangely-beautiful. But they are also very destructive. Maybe try to encourage the braconid wasp to visit your garden?

  3. Very interesting. I'm going to have to see if hornworms are what my hubby calls a tobacco worm.

  4. The Japanese Redneck – Tomato and tobacco hornworms are very similar. Both are preyed upon by the braconid wasp.

  5. Anonymous says:

    What happens when the wasps hatch? Are they the same stinging wasps?

  6. Anonymous – Yes, they are the same wasps which will go on to kill other tomato hornworms. The wasps, by the way, are not known to attack humans.

  7. Very cool post. No hornworms here but I'm glad I know now to watch for the larvae. It makes me cringe to think how many times in the past we SPRAYED the wasps nesting behind our shutters. We were ignorant about their benefits :(

  8. BBI – Not all wasps are the parastic braconid-type. The braconids are tiny, and — I've just discovered from further research — they are harmless to humans and pets. So probably not what you have behind your shutters (I have the nasty stinging-types behind shutters here, too).

  9. Anonymous says:

    Thanks Kevin! I found 4 in my 2 cherry tomato plants! I've never seen one before, all 4 have the parasites though :-)

  10. Marilyn Wilkie says:

    Hi Kevin,
    We have picked maybe 16 Tobacco hornworms off of our 6 tomato plants. I know they are tobacco hornworms because their stinger is red. None have the wasp larva unfortunately. They are hard to spot..blend right in. Look for their scat which are little black ball s. They eventually turn into beautiful sphinx moths. I've seen a few this year around my garden along with hummingbird moths and bumblebee moths. The worms remind me of Alice in Wonderland…tobacco..hookah??? LOL

  11. Yaxue says:

    Thanks Kevin for this edifying tip! I haven't been taking close look at my tomato plants, and now I know what not to do with it if I see one. And it's comforting to know it's actually a sign of goodness….smile.

  12. Anonymous – Four out of four had the parasites? You are blessed indeed!

    Marilyn Wilkie – LOL – yes, the hookah-smoking caterpillar!

    Hornworms — tobacco or otherwise — are really hard to spot. I only noticed mine because of the white cocoons the wasps had spun.

    Yaxue – As you said, definitely a sign of goodness!

  13. Tammy says:

    Kevin, I reached in to pick some tomatoes this afternoon and came nose to nose with a hornworm. Then I saw several more near it. Most, but not all, had the white larvae, so I'll leave those be. But what of the ones going about their merry munching way? I'm trying to figure out if they too will be visited by the parasitic wasp soon or whether waiting could give them too much time to wreak damage. Any rule of thumb you use to decide how long to give the healthy suckers a chance to become larva hosts? Thanks!

  14. Tammy – When one parasitized hornworm is present, I leave the others alone. The young wasps hatch quickly, and SHOULD immediately attack the other worms.

  15. Tammy says:

    Thanks, Kevin, that sounds like a good plan.

  16. Anonymous says:

    How long does it take the wasp to hatch?

  17. chickory says:

    I am so glad I found this and read it – i’ve seen this in my garden every year and yet i am so woefully ignorant of garden pests I fed these worms to my chickens not knowing I was killing a garden helpmate. But i was stoked to read my garden is sound because they are present. I look forward to seeing it this year. thank you!

  18. chickory – Nice to meet you. I was stoked, too, to discover I had beneficial insects here. They work in strange ways, huh? Hope to see you again.

  19. Eica says:

    I hope I see some, as last year we had a bunch of hornworms! Is that bloodthirsty, or what!? Haha, still, I hope to see those white ovals sometime soon. I always have dill, and other herbs around the garden. I wonder what keeps the wasps away? Are there any plantings that discourage them (so I won’t plant that IN the garden)?

  20. Eica – Nice to meet you. Bloodthirsty, yes, but in the very best way! Only the presence of pesticides will keep the beneficial braconid wasp away.

  21. Tina says:

    Hi there,
    I had hookworms several years ago. I researched the Worm. ( It was a great learning experience for my daycare children) I read that if you plant baseil near your tomatoes it will keep the hookworms away. I have been planting basil near my tomatoes for the past 3 years. I have not seen a hookworm since!

  22. Lisa says:

    Had big infestation of hornworms last summer. Tried to pick them off & give them to the chickens, they weren’t interested. Then I picked of a branch that had the worm on it. Dangled branch with worm in front of the chicken & she snatched it right up. After the other 2 got sight of the other chicken eating a nice juicy worm they all 3 would go through the tomato patch snatching the horn worms right off the plants. No more horn worms!! (no the chickens did not start laying green eggs :P

  23. Pat Knoyyd says:

    How can I get rid of squash bugs?

  24. CATT says:

    The only thing is, by the time the wasp larva hatch the tomato plants may have been destroyed. Actually, I have never seen the larva of wasps on any horned catepillars, but I had these these dratted things attack my 8 heirloom tomato plants, growing in pots in my courtyard. It was pitiful, because I now had the plants protected from the critters (rabbits. gophers, squirrels)(we live in the rurals) and then to have them destroyed by, these critters, was heartbreaking. I’m now planning an enclosed greenhouse and going with either hydroponics or aquaponics.

  25. Grace says:

    “Larvae that hatch from the wasp’s eggs, which are laid on the hornworm, feed on the inside of the hornworm until the wasp is ready to pupate.”

    The eggs are actually laid *under the skin* of the hornworm, not *on* the hornworm. The female wasp uses her ovipositor (the tube through which eggs are laid) to deposit the eggs just under the hornworm’s skin.

    The white things you see on the hornworms’ backs are not eggs: they the cocoons of larvae that have fed on the hornworm’s innards, then emerged through the skin when they are ready to pupate. When the “lid” of the cocoons have been popped open, you know that the wasps have emerged. By the time you see the white cocoons, the hornworm can no longer do any more major damage. It is very close to dying.

  26. Reba says:

    I think here in Texas we call them “cut worms.” I had them a lot this summer. I put in organic beds last year and they seemed to moved in this year. I hated killing them. Now I know what to do with them. Thanks.

  27. Excellent Article!! Thank you Much!!

  28. Diana DeJane says:

    very cool article, although I already knew this as I saw some of what you described and googled it. I find hornworms fascinating in a weird way, they creep me out but they remind me of little aliens, like the one that came out of that guys stomach on Alien..kind of cool looking! They are hard to spot on the green leaves for sure but I leave the larvae worms alone but pick the others off, they can really do damage but you have to really search for them. Actually I used to have chickens but they were as destructive as the worms cause they LOVED to eat the tomatoes! Darn chickens! LOL

  29. Tracey Santi says:

    I am new to veggie gardening and happened upon an unfortunate sight in my garden this morning. Relieved to have made it through this horrible heat wave to go outside and find that something has eaten a lot of my sun gold tomatoes and the tops of the plant! Google to the rescue and I stumbled across your article here. I found it so interesting that I went outside and sure enough…found one of those stinking hornworms chilling on my plant. Sweet justice though, it is COVERED in braconid wasp pupae. I felt such a sense of victory. I battled and picked beetles off of my plants and wet the whiteflies and I had not seen any “cavalry” til today. I love what you said about having a good balance. I bought insecticidal spray but couldn’t bring myself to use it. This morning, an assasin bug waiting on my basil for leaf hoppers. Ladybugs on my peppers and eggplants with no sign of the few aphids that were out there. And now these hornworms are being attacked. Thanks so much for your article. I would have tossed the HW in the road thinking it was carrying around its own eggs. It really is all about balance.

  30. sb says:

    What about keeping them in a jar untill the larva hatch????

  31. Sharon Shade says:

    My first attempt at a real garden since I’ve moved to TN and while I never saw any worms on my tomatoes, I did see the above fellow on my red pepper plant complete with the white larvae. My garden had nothing but dirt and compost. So everything was totally organic. Happy it left my Roma tomatoes alone because they were amazing.

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