Monday, January 24, 2011

Inventing the Wheel(bug)

Back in September, a good friend brought me two bugs he had found in Little Hocking, Ohio. He didn't know what they were, but figured that I would know something about them.

Eh, he was correct.

In fact, what he had found was a wheel bug, part of the assassin bug family, of the species Arilus cristatus. Two of them in fact, which was quite fortunate (one was a female, as I found out later when it laid an egg cluster). Wheel bugs are quite large: the wheel bug is one of the biggest true bugs in North America (1 inch to 1.25 inches in length), and, certainly in Ohio, is probably the biggest you're likely to find. If there are bigger true bugs (Hemiptera) in Ohio, I would love to find them. Female wheel bugs are bigger than males, as is the trend in the insect world. Wheel bugs range from Rhode Island to Nebraska, further west into California, and south into Florida and Texas. They've also been reported to be in Mexico and Guatemala, so if you find yourself down there, look around for them.

It actually has pretty good balance: it's keeping itself upright with only two of its legs. Thinking back to this moment, it could have been making an aggressive stance. If so, I dodged that bullet.

The most distinct feature of the wheel bug is also its namesake: the "cogwheel" on its back. Or chicken's comb, if you prefer. The wheelbug is the only insect in the United States with such a structure, and no one's quite sure what its use is. It's been speculated that it's useful for species recognition or to alert predators that it tastes bad, but from what I could gather from the published literature, no one has really bothered to study the wheelbug enough to figure out what function the cogwheel has. It's actually kind of embarrassing, not least of all because that's where the insect gets its common name from, and certainly something so unique and conspicuous deserves special attention. I don't buy either explanation, as they're both extremely lacking. If anything, it seems like it would serve to intimidate potential predators more than anything else. In my experience, it has had that effect on humans who have seen them. At any rate, it's a very interesting quirk that the bug has. The cog can have anywhere between 8 and 12 teeth on it, and the nymphs lack the crest. That must be an interesting molt.

The wheel bug has some interesting colors. The membrane on the wings reflects a bronze color, and its body is covered in fine yellow hairs (pubescence). Just in case you haven't noticed by now, this bug has a strange shape. All in all, the bug has its cogwheel, long legs, long antennae and its conspicuously wide abdomen, which juts out from under the wings. For some reason, the wheel bug has been described as "grotesque" before.

This is terrifying.

Obviously, this bug is a predator. It feeds on things like stink bugs, caterpillars and beetles. During its smaller nymphal stages, it will feed on smaller insects like aphids. The wheel bug eats some pest insects like defoliating caterpillars, and so is considered beneficial, but isn't the best at its job since it also eats some beneficial insects like honey bees. Its hunting strategy is akin to that of a mantis: that is, it's an ambush predator. It's usually slow-moving and waits in one spot for its prey. When it finds prey, it's kind of terrifying. The wheelbug's saliva contains some toxic compounds that paralyze and kill its prey within 15 to 30 seconds, and it then proceeds to suck out the (delicious?) fluids. I found a wonderful description of a wheel bug feeding on a beetle in a paper by Eisner & Aneshansley, which deserves a direct quotation. The full paper is cited at the end of this post, and also in the picture above this paragraph:
"It had impaled the beetle on its proboscis and was in the process of imbibing its contents. We collected eight such bugs at the site, and fed each an H. cyanea, with consistent results. The bug approached the beetle and straddled it, to which the beetle responded by clamping down. The bug then proceeded to probe the beetle until it found a membranous site for beak insertion. Within seconds after being pierced, the beetle went limp, and as it did, the bug simply lifted it up and pulled it off its hold (Fig. 3E).With its legs gone flaccid, the beetle seemed to detach readily. The beetles were thoroughly sucked out (mass of carcass=5.5 +-0.3 mg; n=8)."
 A paper by McCauley and Lawson corroborates the carnage:
"Victims were found either impaled on the proboscis of a wheelbug or lying on or under the plant on which the predator was located. Individuals killed by wheelbugs display a characteristic discoloration at the point of puncture but are otherwise intact and can be sexed easily"
This is not a bug to mess around with. The pain from its bite (more accurately described as a piercing) is ten times worse than a bee, wasp, or hornet sting, and numbness can last for days afterward. The full healing time takes about two weeks, but can take months or longer if you're acutely sensitive to it. (That's when you lose that particular genetic lottery.) Make sure you take care of the wound and it doesn't get infected. For treatment, ammonia water and magnesium sulfate (epsom salt) soaks are recommended, though the sources for any information about wheelbug bites are from 1919 (Barber), 1924 (Hall), and 1958 (Smith et al.). So, the recommendations are a bit dated now. No one wants to get bitten by a wheelbug for science anymore. On the bright side, while vicious in the wild (though that sounds like an exaggeration to me), they're less so after being in cages and become used to being handled quickly, which is sort of neat. Less neat is that cannibalism of the male by the female has been reported after mating when in cages, so maybe the viciousness is just sort of suppressed for a while.

If you ever get the urge to go looking for some wheelbugs, they're diurnal, and found at lights at night (attracted to their prey rather than the light itself). They're also attracted to turpentine oil, oddly enough. Unfortunately, I have no idea why or any other information about that particular fun fact, since the publication is from 1928 (Metzger) and a bit difficult to find. Hagerty and McPherson published their findings of wheel bugs in southern Illinois from their study during 1997-1998, and they found nymphs starting in May, with adults being found starting in June. Plan your hunts accordingly.

 Figure 2, Hagerty & McPherson, 2000. (Click to enlarge)

If you capture one, it might extrude some orange-red scent sacs from its anus. So yeah, be prepared for that.

I've yet to hear the wheelbug produce any noises, but it's able to produce a chirping sound, and makes a loud droning sound when it flies. I would love to see one fly, it seems like it would look more than a little ridiculous. The reason for its chirping sounds isn't known.

This seems like a good place to interject: there is a lot about wheelbugs that we don't know. It's a pretty egregious oversight, considering how unique it is, and how large it is as well. At least with smaller bugs like leafhoppers there's the excuse that they're quite small, making them harder to study, but wheelbugs are pretty conspicuous. There's so much more to research, and we would probably get some pretty interesting answers (and even more questions) if more research was being done. Hagerty and McPherson hit on this well, stating that "Although a common species, most published information on its biology consists of scattered notes." I have a feeling we're missing out on a lot by ignoring the wheelbug.

Even the eggs of the wheel bug are interesting and unique. If you like to wax poetic with your descriptions, you might describe the eggs to "resemble miniature brown bottles with fancy white stoppers" (Mead). If you want to actually understand what they look like, a picture is below. They're arranged in a hexagonal cluster of around 40-180 eggs (I counted 139 in my picture), and are glued together with a gummy cement for protection from the elements and from predators. There's one generation per year (wheelbugs are univoltene), and the eggs overwinter and hatch in the Spring. After hatching, it takes about 3 months for the nymphs to mature to adults.

Fancy indeed

Hagerty & McPherson's study on wheelbugs is a particularly good paper about wheelbugs, so I'd like to take a look at some of their results. The full paper is cited at the end of this post, and a link to it is provided if you'd like to read it yourself.

The study listed some of the wheel bug's prey, which included:
  • Fall webworm, Hyphantria cunea, a pest species
  • Imported cabbageworm (also known as Small White), Pieris rapae, an imported pest species
  • Orange Dog (Giant swallowtail), Papilio cresphontes, a pest in citrus orchards
  • Tent caterpillar, Malacosoma
  • Bollworm, Helicoverpa zea, a pest of many crops
  • Mexican bean beetle, Epilachna varivestis, an agricultural pest
Those are some of the reasons why it's considered a beneficial insect, though its usefulness is tempered by its numbers (not always quite large enough to be an effective control) and the fact that it also feeds on some beneficial insects.

The study also explored the life history of Arilus cristatus, and measured the average incubation time for the eggs to be 60 days in lab. It was also found that a cold period is not necessary for the eggs to develop normally. While collecting eggs from the field, a total of 12 egg clusters were collected, and 10 of them were heavily parasitized (which speaks to why the wheel bug and other insects lay so many eggs at a time). The other two egg clusters had 315 eggs between them (123 and 192, respectively), and 252 of those hatched.

Not all of the hatchlings survived, and the study recorded how many of the nymphs successfully made it through each of their stadia (stages in their life history, i.e. their molts) to reach adulthood. The data were listed in a table, but I went ahead and graphed it to make it a little easier to understand.

The x-axis is the cumulative mean age (the amount of days it took for the average wheelbug to complete each stage) of the wheel bug nymphs, and the y-axis is the number of nymphs that completed each stage. As you can see, a little more than half (about 53%) of the nymphs reached adulthood. This graph doesn't include the number of nymphs that started the first stadium (210 healthy ones were selected from the original batch), nor does it include the number that hatched from their eggs (252, from which the 210 were selected), nor the original number of eggs (315). If you start from the number of eggs laid (315) and end at the number of nymphs that reached adulthood (93), you have a survival rate of 30%. So, after hatching, avoiding various ways to die during their youth, and successfully molting five times, about a third of the wheel bugs make it to adulthood.

It's an unforgiving world we live in.

Pictured: Success!


Eisner, T. & Aneshansley, D. J. 2000. Defence by foot adhesion in a beetle (Hemisphaerota cyanea). Proc. Natl Acad. Sci. USA 97, 6568–6573. (doi:10.1073/pnas.97.12.6568) Link.

Hagerty AM & McPherson JE. 2000. Life history and laboratory rearing of Arilus cristatus (Heteroptera: Reduviidae) in southern Illinois. Florida Entomologist. 83(1): 58-63. Link.

McCauley DE and Lawson EC. 1986. Mating Reduces Predation on Male Milkweed Beetles.
The American Naturalist 127(1): 112-117.

Mead FW. 1974. The Wheel Bug, Arilus cristatus (Linnaeus) (Hemiptera: Reduviidae). Entomology Circular No. 143. Florida Department of Agriculture and Consumer Services, Division of Plant Industry, Gainesville, FL. 

Stehr WC, and Farrell W. 1936. Two hemipterous enemies of the Mexican bean beetle in Ohio. Ohio Journal of Science 36: 332-333. Link.

Further Reading:


  1. You should totally post this to Noah!

    1. I found one of these in central Florida last week - May 29 2013. Never saw one before so I took a few picts. I can send attached photos if interested. They did let out an orange 'scent sac' as the photos show. But this bug must have been male because the cogs on the wheel was not as distinct in shape.

    2. I live in North Jersey....have seen 2 adults around my yard the last week....scary looking things! Got bit by one in North Carolina on my lower back over the summer. Absolutely horrible experience on vacation. In pain for 2 weeks! If you ever got bit you definitely want to avoid them.

    3. I live in North Carolina. I have two daughters, and I decided to do some research on this bug when my sister confused it with the "kissing bug," which is only a relative. After finding out that it has a powerful bite, I killed the specimen looming outside my door. My youngest child has Sickle Cell Disease and faces enough complications from her condition already. The next day there was another bug on the roof of my porch, which I had my brother dispose of. I'm not sure if they release some sort of attractive pheremone when killed. My apartment is near trees, and already this year I've had to battle stinging hair caterpillars, one of which was a pus caterpillar, as well as numerous stink bugs. I'm really having second thoughts about this place. I like most bugs to some degree. (I'm more into spiders) but I have to protect my kids. I just hope I'm not attracting more of them by killing the ones that keep showing up.

  2. I'm planning to! I have a backlog of pictures from last summer to post first, but I definitely won't skip this one.

  3. Derek,
    Thanks for a well-written and informative post on wheel bugs. I just found one this evening (central Illinois), was finally able to identify it (first one I think I've ever seen, 50+ years), and then happened on your blog in searching for more information. You certainly provided it! Good job!
    - Nancy

  4. Thanks for a very informative article. I found one of these yesterday and everyone was telling me I should step on it, but I didn't. I've never seen one of these before and it scared the heck out of me at first. I threw a stink bug at her to see what would happen, not too much later and the stink bug was belly up and lost some weight.

    I live in Pittsburgh and the past 2 or more Summers have been horrible with the stink bug around here, maybe they are following the food?

    Thanks again,

  5. Thank you both, I'm glad you found this post informative. I'm glad you didn't step on it, Curt, that would be unfortunate. And who knows? Maybe with all these stink bugs, we'll find more wheel bugs around too.

  6. We saw one today carrying a smaller wheel bug on it's back. Would this be it's mate? This my first encounter with a wheel bug. I had no idea what it was but was fascinated and had to look it up. What a cool bug! I'm also glad I didn't let my 4 yr old son touch it. Sting or stink would have been bad!

  7. Tamahawk, yes, that would be the wheel bug's mate. I actually just found a mating pair a few hours ago, it's quite exciting!

    The female wheel bug is larger than the male (the smaller one you saw on top), which follows the pattern of sexual dimorphism in most insects--the females are bigger, which is helpful for storing eggs. The males will ride on top of the females during mating, and they can stay in this position for hours.

  8. Are wheel bugs harmful to dogs.

  9. Conceivably, they could be, though it would be incredibly unlikely. The wheel bug would have to land on the dog to bite it with the dog standing still long enough for the wheel bug to stay on. Then the wheel bug would have to get through all the fur. Humans are probably more likely to be bitten than dogs, so I wouldn't worry.

  10. Barbara in Baltimore, MD, areaSeptember 6, 2012 at 6:16 PM

    I had one on my deck last year at this time but didn't try to catch it--first one I've ever seen in my life and I have always been bug-curious. Last week I saw two on a leaf on my Japanese Snowbell Tree (relatively small ornamental) It had the male on the back, I thought the bottom one was the male as it had a bright reddish orange thing protruding from it's tail, was that the thing they kill other bugs with or was that a penis? I was busy catching stink bugs in soapy water so I lost sight of the wheel bug(s). Saw two together again yesterday, might have been the same two but would they have stayed together that long? So I knocked them in the hot soapy water with the stink bugs. They died and sunk immediatly whereas the stink bugs float even when they die. I fished out the wheel bugs before flushing the stink bugs down the toilet and amazingly they were still together even though dead. I wish I would have looked with a magnifying glass before I pulled them apart to see just how they were attached. When first reading your blog and knowing they killed stink bugs I was very sad I had drowned them until I saw that they don't really get to destroy that many of them. I also have three very small dogs that are always on the deck with me and I don't want them stung even though you say it is not likely. This particular tree is covered with stink bugs, on the branches, leaves, trunk. I wondered if it was because I had two stink bug traps hanging from it and actually attracted them. The trap caught so many nymphs in June and July I never could have counted them and I thought there would be none left to grow to adults but no such luck - eventually the adults started going in. No doubt that the trap captured them but I worried that I was attracting them so I checked another Snowbell tree much farther from my house (with no trap) and discovered it was loaded with them also, although not as many as the one next to my deck. I've tried other methods to kill them, double-sided tape around the tree (like used to trap the catapillers from several years ago that were on all my oak trees--by the way they haven't been around for some time--what happened to them--can't remember their name at this time) Used a vacuum cleaner on them on the tree trunk and back of my house last summer and was so sickened by the smell in the vacuum I had to trash it. This year I'm using a vac with a plastic hose so their smell doesn't permeate it (or if so I can soak it in a laundry tub with a strong detergent) also throwing the bags out every day. Last year I wanted to be frugal and didn't throw the bag out until I had used it for many days. Maybe no one will read this comment as it is so long and boring to anyone who is not as obsessed with stink bugs as I am. I became this obsessed after one must have gotten on my pillow while I was sleeping and released it's odor on my hair. I had to wash my hair numerous times to get rid of the smell. The smell never came out of the pillow case even after washing it many times. It had gotten through to the pillow-case cover and the pillow itself causing me to throw it all out (was an expensive pillow) I also thought about spraying the tree with Talstar Pro (read it was the best to kill them) but I am afraid it could hurt my dogs (and me)so I haven't done it. I also purchased the Stroub light trap to put in my garage as soon as they start coming inside, I think it will be any day now. Maybe no one is looking at this blog anymore but at least I had a chance to vent as my friends and relatives are so sick of me talking about stink bugs!

  11. Barbara in Baltimore areaSeptember 6, 2012 at 6:38 PM

    just found this on the website

    Which leads us to the Wheel Bug’s odor. Their smell isn't as potent as their cousins, the stink bug, but the scent is strong enough to make an impression on a potential predator, even humans. When disturbed, the Wheel Bug extrudes a pair of bright, orange-red scent sacs from its anus, giving off a pungent stench.

  12. What is the purpose of the wheel on the Wheel Bug? I agree with you, the reasons you have heard guessed do not sound like very likely reasons for the assassin bug have a toothed wheel on its back.

    I noticed that the bug can flex the joint between its 1st and 2nd thoracic segments. I think these features have co-evolved because this allows it to rotate its "wheel" up and down almost like a sawing motion. If a predator (maybe a bird) has grabbed the wheel bug and it flexes the 1st and 2nd thoracic segments, is that like activating a reciprocating circular saw in the bird's beak--maybe making the bird think that it is being bitten.

    Your pictures of the wheel bug show its 1st and 2nd thoracic segments in different positions. I also have a 3D video of the Wheel Bug that shows it flexing that joint(

    This is the only live Wheel Bug I have found. It may be difficult to locate enough for an experiment with a bird. Maybe one could experiment with an artificial bird beak to see if the bug flexes that joint while caught....

  13. My husband and I have found an infestation of wheel bug nymphs on our dogwood tree. The dogwood is not looking well; it is losing bark and the leaves are already turning brown and falling off and it is only June. Could the wheel bug's be parasitizing the dogwood tree?

  14. Since wheel bugs are predatory, they don't affect plants, so no, your dogwood must have something else wrong with it. I'm not a botanist, so I'm not sure what it could be.

  15. I have found wheelbugs on and around my bunny hutches. Are they harmful to small animals? Is there a way to kill them?

  16. I've seen these bugs often in mid- Maryland. I never harm or disturb them because they are my friends eating other bugs. I did not know they would bite and had venom. This week, a pair of them were on a corner of my patio screen door, mounted. I figured they were a mating pair and so did not disturb them. They were there for days. Now today, I see there is only the larger female. I don't know if she ate the male or if he moved on. I moved the screen door in hopes that she would move to the open side and eventually lay her eggs somewhere else. Her belly is to the inside glass side so for the first time, I saw the underside. She moved the orange sac in and out (thanks, did not know about that either). Then she moved something white in and out a few times before moving to the other side of the screen like I wanted her to. Is the anus also the location for the ovipositor? What do they do over winter? Anything that eats stink bugs is a friend of mine!

  17. I was a victim of the wheel bug's sting Friday. The sting was excrucatingly (spelling?) EXTREMELY painful that lasted for about 4 hours. I was riding a elevator and the wheel bug landed on me. I freaked out and shook my shirt and next thing I knew I was sting on the lower back. The pain was definately stronger than a wasp or bee which I have been stung by both. I did kill the instect and kept it in an envelope so I could figure out what it was as I have never seen on before. The pain radiated about a 6 inch arear aound the puncture area. The area became red, the puncture site there is a blood spot and raised like a mosquito bite and hot to the touch. I treated myself with ibprophen and benedryl then treated then area with anticeptic. I phoned my doctor who was more concerned with a reaction such as one would have from a bee such as hives, restricted breathing and swelling or infection at the puncture site to seek medical attention immediately. Its been 24 hours since the puncture from the insects beak and the site itself is still sore and the blood spot is the size of a pencil eraser and hardened. I am treating the area with neosporin. I have learned alot about this insect over the last 24 hours and found it to be an incredible insect and I am sadden that I killed the wheel bug. I learned this insect eats other insects but injecting a paralizing fluid which is a soft tissue dissolvant - Not good if you are the victim. I think if I had not freaked out that this large bug was in the elevator with me - landing on me and shook out my shirt - but rather if I would of remained calm and slowly brushed it off me then it might not of stuck me - but whose to say.

  18. To the woman from Baltimore with the stink bug problems, i believe i would remove the tree they are attracted to. I also hail from Charm City and have had little problems with them.
    Last year in my small urban back yard i became interested in an insect i believed to be a hummingbird. After researching it was a hummingbird moth, a very beautiful creature as well.
    It seems strange, some insects like the caterpillars from my youth that would be on all the catawba (cigar) trees have disappeared and lots of new bugs ive never seen before are around. When i was a child i would spend countless ours looking at and learning about bugs. They are like aliens right here on earth. Some are quite other-worldly.

  19. I found a wheel bug yesterday, didn't know what it was, thought it was in the stink bug family, but reading this it must not be. I live in Louisa, Virginia. I just found it interesting. I didn't want to smash it so I put vinegar in a cup and of course it died....I wanted to see it close up without getting bit. Very strange indeed.

    1. I found a nymph on my deck in Louisa, VA last July. I only recently learned what it was. I am trying to find more! I haven't seen any eggs, nymphs or adults since.
      I have found a very large number 12-24 dead beetles around the outside of my house. Is it possible the wheel bugs could have killed that many?

  20. Just had wheel bug nymphs hatch today. They are contained and I would like to know how to keep them until spring. We live in Pittsburgh.

  21. Olney, MD
    I have a pair of wheel bugs outside and left of my front door along where the pavement meets the wall. Yes, there is a small group of eggs there as well, and the female (I assume) has not moved more than an inch away from them. The male (again I'm assuming) ventures out a bit, but does come back near the egg clutch. They have been there for about a week now, and it's pretty much a protected area from the elements. This is the 1st time I've encountered these guys, and it's been interesting. I am curious about something....the "female" is always guarding her eggs, I haven't seen her in search for food. Will she just continue guarding and not eating? How does that work?
    Thank you again for this site, very informative!

  22. I found a batch of wheelbug eggs on a hoe handle and photographed it.
    In moving the tool I accidentally crushed a couple of the eggs and was surprised to
    detect the odor of pine or turpentine. Various reports state that the bugs are attracted to turpentine, but I wondered if they produced a turpentine mimic odor to help protect the eggs.
    Has anyone else detected this odor?
    As to the general presence of these bugs, one usually hangs around on my porch for a couple of weeks until frost. They are not aggressive, and move very slowly.

    1. Excited to find your post! Found three egg clusters on my apple tree here near Elkton, VA and when scraping them off with my fingers, detected a strong pine resin smell. Did not at the time know they were wheelbugs till spent time researching. I too wondered about a previous post that they are attracted to turpentine. Plenty of pines close by, wonder if they "gather" it to glue the eggs in place!

  23. Last summer I had an assassin bug that's very similar to the wheel bug living in my yard. I placed him on a hanging pepper plant and fed him regularly. His diet consisted mostly of injured half dead flies and other small bugs. He lived for several months then began turning grey. He claimed down from the plant and made his way to my she'd where I later found him dead. A very interesting summer "pet."

  24. Last summer I had an assassin bug that's very similar to the wheel bug living in my yard. I placed him on a hanging pepper plant and fed him regularly. His diet consisted mostly of injured half dead flies and other small bugs. He lived for several months then began turning grey. He claimed down from the plant and made his way to my she'd where I later found him dead. A very interesting summer "pet."

  25. Where in pa would one go hunting for one of these.I've spent most of my life in the forest and can't say I've seen one.I would like to show a live bite of one on YouTube. Yes, I'm willing to take a whack from one for entertainment of others! I just need to find one first!

  26. Where in pa would one go hunting for one of these.I've spent most of my life in the forest and can't say I've seen one.I would like to show a live bite of one on YouTube. Yes, I'm willing to take a whack from one for entertainment of others! I just need to find one first!

    1. I live in Airville PA and keep finding them on my purple coneflowers where they ambush moths and butterflies. They seem to prefer the highest flowers on the plant.

  27. i grow plants in greenhouses in leola , pa. we have gone biologically friendly and this wheel bug has ventured in on its own, we also spotted edd sacks out side on a rusty barrel.

  28. I just found one of these on the back of my house, in Columbus ohio. Had no idea what it was, took pics, and after reading about it, realize I was far too close! Amazing creature of God's doing.

  29. Well done! Your writing style makes me smile, as well as being informative and witty, it is grammatically clean and shiny!!

    -The Grammar-Gramma


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