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!

References:

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:

Beneficial insects in the garden: #09 Wheel Bug (Arilus cristatus)

Wheel bug, Arilus cristatus from University of Florida