Monday, December 27, 2010

It's Hard to be a Hornworm


In Ohio, there's a delightfully gregarious caterpillar known as Manduca sexta, or the tobacco hornworm. It's placed within the Sphingidae family, which is composed of the hawk moths. Take a look at the Wikipedia page for the hawk moths: they're all spectacular. Some of the adult moths resemble birds or bees, they're just amazing. To get back on topic, the tobacco hornworm feeds on tobacco (obviously), as well as other plants within the family Solanaceae, which includes tomatoes. This makes them pests to gardeners, which is pretty unfair. Tobacco hornworms develop into some beautiful moths and are quite large, so they're truly a sight to see once they reach maturity. Luckily for me, they also feed on Datura wrightii, a plant within the same family as tobacco and tomato, which grows near my house.

I had the fortune of finding a few of the caterpillars on the Datura plant, so I plucked them along with some leaves and placed them in a container in my room. They were still in their early instars (An instar is simply a stage in the life of a caterpillar. As it grows, it will molt and enter a different instar, gradually getting bigger as it enters each successive instar.) and so were relatively small, as you can see from the pictures. Of course, they did not stay small for long. I ended up plucking five or more leaves each day for each caterpillar in order to give them enough food, and I still wasn't sure if that amount was enough. They would eat through all the leaves in short order, and sometimes I found myself getting even more leaves for them throughout the day.




And so, they each progressed through their instars, growing larger and larger. I was surprised at just how big they were getting: these aren't small, inch-long creatures. If you're creeped out by large worms or snakes, then you won't like these guys (but you should anyway). Taking a look at the pictures now, you can start to notice their stripes and small eye-like spots on their sides. The stripes are one way to distinguish them from a similar species, the tomato hornworm, which also feeds on tomatoes. It has eight V-shaped markings, while the tobacco hornworm has the seven diagonal stripes*. The stripes serve another purpose as well: camouflage. The stripes break up the hornworm's pattern while it's feeding on leaves, helping it to blend in with the overall shape and color of the plant. If you encounter a plant with skeletonized leaves, it's a good indication that a caterpillar has been feeding on the plant, but even if the caterpillar is still on the plant while you're looking at it, it's no guarantee that you'll find it. They're remarkably hard to find, even when you know they're there (which is remarkably frustrating).

Now, the orange eye-like spots on the hornworm's side are its spiracles. Spiracles are how insects breathe, simply put. It probably doesn't hurt the insect's chances of survival that its spiracles look like eyes, either.

In the picture below, one of the caterpillars has undergone ecdysis (molting), and has entered its next instar. Tobacco hornworms usually have five instar stages, but can have more depending on growth conditions. Near the caterpillar's posterior end, you can see the remains of its previous molt. The caterpillar proceeded to eat these remains. When a kid is growing and needs nutrients, they're forced to eat vegetables. When you're a caterpillar, you eat what used to be your skin. Except for the head case. That part is too hard, so it just pops off and is left alone.

Pictured: a delicious gourmet meal

You can tell that the caterpillars have grown much larger than they used to be. The most mind-blowing fact about it is that these pictures were all taken over the course of about a week. Caterpillars can grow pretty quickly.

  Caterpillar excrement is known as "frass." It fertilizes plants and all that good stuff. Also, you can hear it falling down the leaves of trees and other plants like rain sometimes.

 The small reddish brown object is the pupa of another caterpillar, Ceratomia catalpae.

Still eating.

At a certain point during the hornworm's fifth instar, it starts undergoing physiological changes signaling its body that it's time to pupate. Different hormones start being produced, and the hornworm's behavior changes. Effectively, it's puberty, but with less pimples. One of the behavioral changes is quite pronounced, and results in the caterpillar entering a so-called "wandering stage." The caterpillar descends from its plant and starts walking around on the ground, looking for a good place to burrow into the dirt. Or, if the caterpillar is in an insect cage, it will walk around the enclosure and stomp on everything. (Various sources recommended that I remove excess frass to avoid a messy situation. It was a very helpful recommendation, and saved me a lot of cleanup time later on. If you decide to raise a hornworm, you would do well to heed the same advice.) After a little while of wandering and being much more active than it was during its feeding stages, the hornworm will start to dig into the soil. It's a fascinating process: the hornworm uses its body as a shovel. It doesn't so much as dig a burrow as pulverizes its way down into the dirt and decides "Yep, this'll do." I found I had new respect for my hornworm after watching it go underground.

 Lateral view, the hornworm at work. 

The problem with not having quite enough dirt in the insect cage.

A fascinating visual cue that the hornworm has entered its wandering stage is the presence of its aorta (its heart). On the "back" of the hornworm (its dorsal side, for those of you anatomically inclined), the hornworm's aorta pumps its blood (technically hemolymph, I do believe), which is quite conspicuous. 

video
The hornworm's aorta, hard at work.

After the wandering and digging, the hornworm settles in for some huge changes to its anatomy and physiology. 
Also notice how the posterior end of the hornworm looks a bit like a face. Some mimicry, perhaps.

While that hornworm is busy pupating, let us catch up with the other hornworm.

.....or not.

And now it is time to learn about one of the most interesting natural phenomena in the animal kingdom: Parasitoidism. You may know about parasites: nematodes, flatworms, and according to some, politicians (we won't get into that argument here). Parasitoids are similar to parasites in that they also live inside of a host and feed from it. However, parasitoids take parasitism one step further. Parasites live and feed off of a host, yes, but they do it in a way that doesn't kill the host. If the host dies, the parasite also dies since it loses its food supply. Parasitoids, on the other hand, have little concern for the well-being of the host. In a parasitoid-host relationship, the parasitoid feeds off the host in such a way that the host dies. The parasitoid has special adaptations that allow it to survive after the death of the host, so to the parasitoid, the final result for the host doesn't matter.

So what does that mean in the context of Manduca sexta
Manduca sexta has the misfortune of being prey for some species of wasps in the family Braconidae, which contains a variety of parasitoid wasps. These wasps seek out various caterpillars, and when they find them, they lay their eggs inside of the caterpillar. This is bad news for the caterpillar, as its defensive options are limited. The plants it feeds on contain toxic chemicals, and there is evidence that some caterpillars can ingest more leaves containing these chemicals to self-medicate against parasitoid infection**, though such a study has not been researched specifically for Manduca sexta. However, it is not too much of a leap to hypothesize that the hornworms could make use of toxic chemicals in an attempt to kill the parasitoids. 

As a side note, such parasitoidism is obviously beneficial for the host plant of the caterpillar, as it can suffer severe skeletonization of its leaves due to the caterpillar's voracious appetite. The plant is not entirely defenseless, however, and is able to release chemicals to attract such parasitoids to rid itself of the caterpillars. To learn more, check out this wonderful article from Scientific American

So, the hornworm has had the misfortune of being parasitized. What can it look forward to? Aside from the hope that the eggs won't hatch, not much. Its future is pretty bleak. First, the eggs will hatch. The wasp larvae will start eating the insides of the caterpillar. Since they are parasitoids, not parasites, they won't have much respect for the normal, healthy functioning of the various organs and systems of the caterpillar. The little respect for the caterpillar's health that the wasp larvae will have will simply be not eating the vital organs first, allowing the caterpillar to live so that the larvae will have a safe place to develop. Once they have grown sufficiently, the larvae move on to phase two. 

The larvae eat their way through the caterpillar's skin. If you go back to the picture of the larvae coming out of the hornworm, you will notice a few dark circles on the caterpillar. Those are wasp larvae eating their way out, almost breaking through the skin. Once the larvae have made it out of the caterpillar, they will immediately start spinning cocoons. They will stay in the cocoons for about a week or two, and then emerge as adults. During all this time, the caterpillar is still alive. Depending on nature's mercy, the caterpillar may die soon after the wasp larvae have burst from its body, or it may stay alive until the adult wasps emerge from their cocoons, and even for a few days after. I imagine the caterpillar curses its existence if it is in that predicament.

 Adult braconid wasps emerging from their cocoons.
The cocoons look like small white egg sacks. There can be as many as 50 or more on one caterpillar, depending on its size and unluckiness. 

 These adult wasps will each seek out other caterpillar targets.

The resulting caterpillar. 

  
For an even more gruesome lesson on parasitoidism, National Geographic has you covered.

Now, time to move on to a healthy hornworm. 

Pupal stage.

The caterpillar has undergone quite a change while underground, resulting in its pupal stage (also known as Metapod, to Pokémon fans). The most outstanding feature is the curved tube at its anterior end, which its tongue grows in. The pupa is not entirely defenseless. When touched, it will jerk its posterior end violently, which is quite startling if you do not expect it (I speak here from personal experience). The pupal stage also takes place while buried a few inches underground, which isolates it from many predators.

As the moth gets closer to eclosion (emergence from the pupal stage), the pupa will get darker and more translucent.
It will turn almost purple in some areas.

Finally, the moth will emerge from the pupa. I assume the orange-brown liquid to be hemolymph, though I haven't found a source identifying it for sure.

The pupal stage lasted a little over two weeks, and the final result was...


It's quite a beautiful moth, very "furry." It's also quite large, to the degree of a small bird. Knowing the size of its caterpillar and pupa, it's no surprise.

 Check out those eyes! Also notice the tongue curled up into a ball. The adult feeds on nectar.


 A full view of the body and wings. Notice the series of orange spots on its abdomen, absolutely stunning. There are six on each side, twelve in all. I assume these are what the sexta (Latin for six) in the species name refers to.

View of the closed wings. The newly emerged moth cannot fly, but must first pump hemolymph into its wings.

 Ventral view, with ruler for scale.

 It has quite the wingspan.

 Truly, it's a beautiful moth. It's a shame they're seen as pests.

This particular specimen is now located in the collections at Marietta College.

For more information about Manduca sexta, check out the following websites:


References:

39 comments:

  1. hey thanks for this really informative look at hornworms. i stumbled across your blog after finding out on bugguide that i've got treehoppers entilya carinata. i was reading your post on them, when i scrolled down and saw your hornworm post. i am thoroughly grossed out by tomato hornworms, which did quite a job on my plants last year, but i appreciate them in their moth form. i like moths and won't kill them, though i suspect their offspring may give me trouble.

    parasitoids though? nothing should have to suffer from that fate. ick.

    anyways, thanks for the post and great pics.

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  2. What a fascinating life. Thanks for the awesome write-up and pictures!

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  3. Please help, I have a horn worm that I have saved and he came out of cocoon during the night. His wings are not yet straight? How can I help them to be straighted? It is very dry here. Also, can I feed him, He is very strong and wants to fly, but can't yet How long does it take?

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    1. You should put the moth on a vertical surface, or give it something to climb up, such as a wad of paper towels or a stick. It will hang from those and pump its wings up with hemolymph, which will straighten out and not be damaged. Sphinx moths feed on flower nectar, so you could try sugar water, but the best thing to do is to let it go. Once its wings are ready (probably within an hour or two), it can fly, but may stay around for a few hours or until the night.

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  4. Thank you for the wonderful information. My kids and I enjoy these beauties so much :)

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  5. Thank you for your wonderful post! My daughter and I found a tobacco hornworm and we're interested in keeping it inside for a bit for better observation. We have a secure cage, dirt, and tomato leaves. Should we do anything else?

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    1. Mine also eats little green tomatoes (it is large and ready to pupate).

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    2. That's all we needed for ours. LOTS of tomato leaves. We didn't give it dirt until we saw signs it needed it, the easier to clean all the caterpillar poo up. Ours just emerged today! 3 weeks after burying itself.

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  6. Nice write-up and great photos. My guy is busy tunneling in the soil and pushing things up out of it like pieces of leaves and roots. He comes out every once and awhile then goes back into the dirt. Are they all this particular about their soil? Also curious why you called the hornworm gregarious. The one I have seems to be aware of me but think I might be imagining it.

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  7. It has been over a week since the wasps have hatched and the caterpillar is still alive. How much longer can it actually live?

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  8. My friends and I found a hornworm in St. George, UT a few weeks ago. We were all very fascinated by it. We call it Uniworn (unicorn worm... creative, right?) Anyways, I haven't been able to find anywhere explaining what the horn is for. Do you know? I really enjoyed reading this article. You have a great way of explaining everything! Thank you!

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    1. Most likely, the horn is used for sensing its environment, and possibly for looking a bit scarier than a normal caterpillar. Interestingly enough, some hornworms have lost their horn and it's replaced by a different structure. In the Abbot Sphinx, for example, the horn is instead a glossy button that looks like an eye. It's very cool!

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  9. Thanks for the great write-up and photos. I found a pupa while digging in my garden a couple days ago (November in Illinois) and didn't know if it was "friend or foe". Your post identified it as "foe" since tobacco hornworms are one of the most odious garden pests, in my opinion. Much as I would like to see the beautiful moth in the spring, I think I'm going to dispose of this guy!

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    1. I hope you don't. They may eat parts of some plants, but it's unlikely to be significant. Plus, that moth may end up as food for a bird or other predator before it could even mate and produce any caterpillars.

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  10. We have one we have raised since fall, went into cocoon stage all winter under dirt inside. He emerged from his cocoon this morning and is now on a stick but isn't moving much and doesn't appear to be eating anything. I put some sugar water on his branch, he moved his leg a little, but otherwise isn't moving. What should we do?

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  11. I have a question regarding the hornworm droppings. We had some hornworms dropped off at our place of business a couple of days ago and the droppings were like little peanuts. But then had run out of the food they were dropped off with and my Coworker brought in some baby potatoes to feed them. Now the dropping are like mush and all over the place. I am not too sure what this means or how I should correct it. Please help.

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  13. Thanks for your post and photos, it was very informative. I have recently acquired two rather large Hornworms from a friend and have placed them in a tank with some leaves from the plant it was found on and an inch and a half of peat soil. I plan to take it to my kids school to observe. At the moment we are toward the end of Autumn, coming into winter and I wonder if they will eclose much later than normal given we only have around 11 hrs of daylight this time of year?

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  14. what type of plant is that in your photos? I got some hornworms and tried feeding them a commercial diet recipe i found online and they wont touch the stuff

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    1. they like tomato plant leaves, tobacco leaves, pepper plant. Those are the ones I know for sure. I Can't stand killing them when I find them and I always just keep them and feed them, let the kids watch them go through their stages. I was wondering if their frass is worth keeping as fertilizer? any thoughts anyone

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  15. what type of plant is that in your photos? I got some hornworms and tried feeding them a commercial diet recipe i found online and they wont touch the stuff

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  16. Thank you for this! Ours just eclosed (?) today, I was afraid it was injured as its wings leaked drops of neon green liquid, but it looks normal now. Releasing it shortly, so it can find food. Sure wish we could keep it to observe, but it probably has a short life span? Want it to enjoy the time it has :)

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  17. Hi, my daughters and I have been raising Hornworms and one just went Chrysalis. We have been keeping them in a butterfly net cage. Just today, I found 2 more species of caterpillars in my garden. I was wondering, would it be safe to place them all together, or might they eat each other?

    Also, is it safe to pet the moths once they emerge? Thanks! Carrie

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    1. Hello Carrie, it's safe to put all your caterpillars together; they're herbivores and won't try to eat each other. You'll want to make sure they have at least an inch of dirt at the bottom of their container to pupate in though; the caterpillars will dig down when they're ready to pupate and form their pupa in the soil. (They can be gently dug up to see what the pupa looks like.)

      When the moths emerge, you'll want to make sure they have something to climb up (whether it's the side of a cloth cage or some added sticks that can hold their weight), as they rely on gravity to pump up their wings. If they try to pump up their wings while they're on the ground, they're at a high risk of having their wings be disformed. But, after they've pumped up their wings, they hang around for a while and can be (gently) petted as they're sitting there. Just make sure to take them outside at night so they can fly and find other moths to mate with!

      I hope your daughters enjoy raising the caterpillars!

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  18. Has anyone ever seen a tomato hornworm with all the white oval on it, but instead of being bright green, it's totally black.????

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  19. I just found one of these beauties outside today my husband knocked off all the wasp coocoons off of it and he seems to be moving more he is very large about 4 inches already I would like to know if he may live I'm setting up a encloser for him as we speak in hopes that he will live

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  20. Thanks for this! I found a hornworm on a tomato plant and would like to bring it inside to keep for my toddler son to watch as it grows. What kind of "house" do you recommend, and is there anything else I need to know? How long does it take to pupate? thanks for any info!

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  21. Our tobacco hornworm has gone to ground, for about 3 days, but popped out of the dirt this morning. Not sure why. Very dull in colour, but still moving a bit. Perhaps it's too dry? Any ideas?

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  22. I have some pupae and it has been about 2 weeks exactly. How can you tell if they've died during this stage? I picked one up and it didn't jerk its abdomen at all. Does this mean it's dead? I have them on Eco Earth (coconut fiber substrate) and they never burrowed. I got them from a friend who said they were too big to feed her gecko. from what i can tell they were at the wandering stage well before I got them, because they pupated almost overnight as soon as they got onto dirt. I have been occasionally misting the dirt with water. am I doing everything right?

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  23. everybody especally jen just be happy that no parasitoids go for us hey right

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  24. I live in Canada and it's the middle of winter. What can I do with the moth? I have a bearded dragon, lunch maybe?

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    1. A little late for you, but yes if you have one please feel free to feed it to your dragon, or to a bird. Chickens love them.

      The adults might look nice, but releasing them to attack the neighbor's garden is not very neighborly.

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  25. Should you water the soil that the horn worms are pupating in?

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  26. I have tomato hornworms as part of rotational food for my veiled chameleon and leopard geckos. I was wondering what the translucent clear stuff that covers their "nose" is? It's hard and sometimes falls off? Should I be worried about them (the Hornworms) feeding?

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  27. I have one we found in the fall I'm in iowa and it over winter in the dirt provided. It's late may now and still hasn't come out yet it does jerk around if messed with though. Is it ok or do you think it's doomed?

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  28. Hi there!! So about 4 days ago, my toddler found a hornworm in our backyard, abt 30-40' from our garden, where the tomato plants r located. After reading abt them, I concluded that it was probably traveling/searching for a place to pupate. It was abt 4-5" long, and after reading abt them, we decided to keep it as a pet to watch the life cycle.
    On the first day, we made a home for it. We put approx 4-5" of dirt in a gallon size ice cream bucket with a few tomato leaves. We made adequate breathing holes in the ice cream container lid and left it alone for a few hours. We chkd on it before bed and it was not moving much, or eating (at all), and had started to change colors (darker) on its dorsal side. The next morning (day 2), we chkd on it in the am and it was already underground (we could c the entrance to the hole it made to burrow in). It is now the 3rd day that we have had it in the container (2 whole days of being underground) and I'm wondering:
    1) How long will it stay underground? We live in southern Nevada, and it's abt 100°F during the day, give or take a few degrees.
    2) How long does it take to form it's cocoon? In other words, when could (or, should) we (gently) dig it up to inspect/observe the cocoon? And IF we do dig it up, will we interfere with it's natural process?
    3) Should we mist the top of the dirt with water? (I've read conflicting info on this.)
    4) If we do NOT dig it up, will it immediately fly away when pupation is complete and it emerges as a hawk moth? (I have the lid on it bcuz I did not want it to fly away in the middle of the night or early morning, before my little ones get to c it, as they r SO super excited to observe this process of life, however, I also do not want to interfere with it's natural process and cause it to die bcuz it couldn't manage to get out when it needed to, and do what it would do in nature - ie. Fly away).
    I'm sure I have more questions, but these r the most important ones I could think of.
    Someone plz help! This is our FIRST time finding, and subsequently, trying to raise, a hornworm, so I need detailed info (lol).
    Any and all I do will b much appreciated.
    Thanks, in advance, from my little ones and I (as well as our new "pet"). =)

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    1. Mine pupated in about one or two days of being under the dirt, however, I would wait a week because sometimes they take longer and are very fragile at that point. The moths could take about a month to emerge, and will take longer in lower temperatures. Also, if you are keeping the container outside, direct sunlight could heat the container enough to cook the insect so make sure you keep it in a temperature controlled area.

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  29. Also in southern Nevada like the previous poster! First time caring for a hornworm. Ours is an Achemon Sphinx actually. Which will be so cool as long as it survives pupation! Right now it's half in the dirt and half out, by his own doing. He is head down, and only the half out of the dirt has cocooned. I gently dug him up and his head-half is still a caterpillar. Should I bring him to the surface completely? Is the air helping him pupate? Also, can someone please answer the question of misting the dirt with water. It is VERY dry here, however our indoor humidity (where the cocoon is) is around 35%.

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  30. I found a horn worm on our pepper plants so I clipped off the branch with the worm and put it into a butterfly enclosure in the house. Then I bought a small tomato plant and put that inside. The worm ate through the biodegradable container and disappeared in the dirt! After I Week I went to replant the tomato plant and keep the cocoon...but it was nowhere to be found. What happened to my tobacco horn worm??? I sifted through the dirt and found nothing. I think he disintegrated ☹️

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