August 02, 2002

Tomato: One Hornworm: Zero

Tomato Horn Worm having a bad day. He has been parasitized by a Braconid wasp.

The Rest of the Story: When I saw this in the garden this morning, I whooped like I had won the lottery. In the eye of the beholder, this is beautiful. I think I can explain.

This picture I took in the garden this morning illustrates that plants, contrary to the thinking of the average college biology student, are not defenseless creatures. I'm not even talking about thorns, waxy cuticles, noxious tastes and poisonous substances that prevent being eaten by insects or mammals. Plants 'know' when they are being eaten, even apparently WHO is eating them, and can chemically signal an army to protect them from attack.

Pardon the anthropomorphic language, but plants really are smarter than we typically give them credit for being. And here is a great example:

The Tomato Horn Worm pictured here is a weeny. Full grown, they can be 4 inches long and almost an inch thick. Hiding among the dense tomato leaves, they are all but invisible until you finally see that half the plant has been skeletonized, or see their droppings on a leaf below where one is feeding.

The tomato plant responds to the Horn Worm by exuding a chemical much like a pheromone in mammals. This chemical signal is detected by small, inconspicuous Braconid wasps who then come to the scent, and find the caterpillars hiding deep in the plant foliage. The female wasp then uses her sharp ovipositor to inject a couple of dozen or more fertilized eggs into the soft body of the worm. They hatch and eat the caterpillar from the inside. The worm, understandably, loses its appetite, and pretty much just stays fixed on the plant without feeding, itself a Happy Meal.

At the stage you see here, the hatched eggs have become larvae which have pupated into silky white cases on the exterior of the hornworm. Soon they will mature and cut their way out of the pupa case, and will head off to repeat the process, hopefully, elsewhere in our garden. So far, I have found three hornworms, and each was stopped in its tracks, covered with white worm-eating wasp babies.

After all our hard work nurturing these tomato plants along, it is truly a thing of beauty to see these wormy green garden gargoyles become "the eaten" instead of our produce!

Posted by fred1st at August 2, 2002 06:26 AM

Thanks for the enlightening article. I've known since my father told me as a child that the white larva on the backs of these monsters were parasites that would eventually kill the host. What I didn't realize was the importance of allowing nature to take it's course so that the larva could reach adulthood and reproduce!

It seems there are more of these than usual this year.

Posted by: Scott at August 13, 2003 05:57 PM

I pulled 12 of these beasts off one plant. I'm curious-left to its own course, what does the horn worm turn into--moth, butterfly...? ms

Posted by: Martha Sayblack at August 21, 2003 08:56 AM

I'm so glad to find your photo and article about the white wasp eggs on the horn tomato worm. I have seen some massive and ugly tomato worms in my life and so has my father. Today (8/24/2003) we discovered 3 dozen of the buggers on one of my fathers tomato plants! I don't think I have ever seen that many on one plant either. He wasn't very happy. He knew they were there about 3 days ago but at that time must have been to small for him to see or hiding very well. About 4 of them had the wasp eggs and we had no clue what they were until I discovered your page. THANKS! I did notice that they were not as active as the ones without and even noticed that they were a different (darker) shade of green. I just phoned my father to read your explaination...he was facinated. THANKS AGAIN! Jennifer, Chapel Hill, NC

Posted by: Jennifer at August 24, 2003 07:41 PM

Posted by: josh at August 29, 2003 04:42 PM


Posted by: FLIP WALKER at September 10, 2003 06:12 PM

The tomato horn worm turns into the beautiful Hummingbird Moth. I don't mind the Horn worms at all. My plants have many leaves that I don't eat, so why not share! The horn worms in my garden didn't get the white larva on them. The worms finally darkened and dried up.

Posted by: susan at September 11, 2003 11:00 PM

What do the cocoons look like? I have found many on the tomato stakes that look to me like praying mantis overwintering cocoons. Do hornworm's look like that? Tan, 1 1/2" by 1/2"; texture like a light fluffy styrofome peanut?

Posted by: Laura Trent at December 6, 2003 08:08 PM

When I was a child a lady next to our house grew the most gorgeous white flowers imaginable. They drew large beautiful moths. I just found out what they were. I intend to grow those flowers for their beauty and attract the moths for their beauty. The flower is datura inoxia and the moth is the product of the tomatoe horn worm.

Posted by: jjjjjjj at February 9, 2004 11:11 AM

If you love them, I can deliver them to your garden instead of mine. I hate these things- they not only eat leaves, but also chew on the tomatoes themselves, of course just enough of a nibble to ruin them.

Posted by: toe at August 4, 2004 07:44 PM

I am sorry you are mistaken about what the tomato horn worn turns into. It does not turn into the Humming bird moth. It turns into a thick bodied narrow winged gray sphinx moth commonly called the "five spotted hawk moth". The Humming Bird Moth is indeed a sphinx moth, but only a relative of the tomato horn worm. There are about 100 species of sphinx moths. The five spotted hawk moth (tomato horn worm) and the humming bird moth are two differnt ones.
They do not spin a cocoon. They form a hard black pupae cases in the soil.
Below is a link to a picture of the adult of the tomato horn worm.
So don't feel bad about killing tomato horm worms they do not form pretty moths.
BTW I also LOVE the humming bird moth

Posted by: nancy at August 5, 2004 11:28 AM

today a friend found the most wonderful worm like thing I have ever seen....I was about 5 inches long and about 1 inch thick...It was a brilliant turquoise color and had orangish brown long spikes on it's upper body and small black ones on it's body...the spikes on upper body are about 1 inch long. I took wonderful pictures of the thing and would love to share them and find out what it is.

Posted by: Terry at September 7, 2004 08:42 PM

When I was a kid and my dad was growing a garden, we hated tomato hornworms. My dad would toss them onto the patio, whereapon the local blue jay would swoop down and carry them off. We even --gross alert -- trained our dog to play with them until she tired of them, and then eat them.

Now I'm the adult with the garden and no dog. Our pet toad died last week and the day after I cleaned out her home, I discovered my first -- quite large -- hornworm. To allow my kids to observe it, I plucked a few sprigs of tomato plant, inserted them into a tiny vase, anchored it in the terrarium, and set the hornworm up for controlled eating and observation.

Guess who our pets are now? Two huge tomato hornworms, named Wasabe and Tomato -- and if we ever find a third one, "Leafy". I have to refresh their food supply twice a day. I find it amusing that I'm now caring for creatures I hated, but then again -- this way I can appreciate God's creation without God's creation eating my tomatoes.

If they pupate underground, I guess I won't be able to observe the rest of their lifecycle -- not sure if I'm willing to have my terrarium tied-up that long. I'm assuming a hard-shelled underground stage would be significantly longer than a cocooned above-ground stage. Any thoughts on this?

Posted by: Paula at September 8, 2004 10:16 AM

I foung a 3 in beautiful green worm, I think it is a tomato worm, and I wonder if it turns into a butterfly. I don`t have any tomatoes, I found it on a tree. I put it into a large container with grass, leaves, and sticks. It seems to have started to enclose itself in a cacoon around the stick. Should I open the container and let it go back to nature?

Posted by: Cindy Hart at September 12, 2004 02:34 PM

does anyone know what to use to keep these worms off of tomato plants?

Posted by: carol at October 13, 2004 03:36 PM

Carol: Next season try companion plantings, marigolds (the tall old-fashioned orange ones, not the dwarf border varieties) and/or elephant garlic both work very well...or spray reguarly with a BT solution.

Posted by: feste at October 13, 2004 04:57 PM

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