#wheel bugs

[ID: 1: Two grey and black wheel bugs on leaves.

2: A grey and black wheel bug on a white-painted wall, holding a ladybug. /End ID]

NORTH AMERICAN WHEEL BUGS! I was graced with the presence of three separate specimens while I was walking around a nature area outside of my local natural history museum, and extra graced by finding one mid-meal! You can tell that these aren't adults because they don't have their "wheel" yet!

Wheel Bugs gettin' dirty on the sidewalk outside Wescoe Hall after class today

As March was approaching, I knew I was ‘on deck’ to produce a blog entry originating from our Section of Invertebrate Zoology. Looking at the calendar, I saw that the deadline would land around the 15th of the month, and something nagged at me about that date – what was the significance? Why, yes – the Ides of March was looming! Shakespeare’s Caesar failed to heed the warnings, and in the end… met his assassin…

So, let’s take a look at an insect that carries the moniker of “assassin bug” – a species that possesses a bite of which everyone should truly beware. The insect is Arilus cristatus (Linnaeus), a species of ‘true bug’ in the order Heteroptera, in the family Reduviidae, collectively known as the assassin bugs. Arilus cristatus also carries the common name of ‘wheel bug’ due to the distinctive, serrated crest on its pronotum that in profile resembles a portion of a wheel or gear. No other insect in the United States possesses such a structure and the ‘wheel’ allows this species to be readily identified. In addition to the odd cog-like crest, the bug is large – nearly 1 ½ inches in length in mature adults and is dull gray in coloration. The immatures, or nymphs, look entirely different – they are small, bright red and lack the ‘wheel.’

Wheel bug, Arilus cristatus (Linnaeus) (Heteroptera: Reduviidae) (Image ©Rich Kelly, New Hyde Park, NY. Used with permission)

The wheel bug occurs throughout the southern half of the United States, ranging northward to the upper Midwest and southern New England. While Southwest Pennsylvania is within its natural distributional range, it appears to have become more common in our area over the last decade. People began bringing specimens into the museum for identification at an increasing rate starting around 10 or so years ago. While the evidence is anecdotal, the apparent increase in their abundance in our area could be a result of our changing climate – as our region becomes warmer on average, the environment becomes more suitable for the wheel bug, allowing it to thrive. Another potential factor for its increased numbers in our area is the introduction of an invasive species, the brown marmorated stinkbug, Halyomorpha halys Stål — a true bug species native to eastern Asia. With an increase in easily captured prey, the wheel bug may be exploiting this new food source. On a number of occasions, I have witnessed wheel bugs feeding on the introduced stinkbugs in the field.

Immature of the wheel bug, Arilus cristatus (Image ©Seth Ausubel, Washington Crossing, PA. Used with permission)

All of the reduviids are predatory on other invertebrates, using their beak-like mouthparts to pierce their prey and inject a powerful mix of enzymes that kills and begins digesting their prey from the inside, similar to the feeding habits of spiders. I can attest, from personal experience, to the extreme pain this insect can inflict by its bite. Wheel bugs, like many Insects, can be attracted to lights at night, and while collecting around some bright gas station lights some years ago, I foolishly decided to pick one up by the wheel with my bare fingers, assuming it could not reach me with its relatively short beak – and oh, how wrong I was! The initial bite was not terribly bad, but unusual, feeling like a tiny electrical shock. In less than a minute, however, a sharp, burning sensation began spreading the length of my thumb. The pain reached a crescendo in about 5 minutes and stayed at that level for several hours. The next day, the burning had subsided, but was replaced by a dull, throbbing ache that felt as if I’d smashed my thumb with a hammer. That discomfort persisted for a couple more days, yet oddly, there was no swelling and no obvious redness or sore at the site of the bite.

Arilus cristatus feeding on a hymenopteran (Image ©Seth Ausubel, Washington Crossing, PA. Used with permission)

While the bite of the wheel bug can certainly be a painful experience, and potentially worse in individuals that have a sensitivity or allergic reaction to the bite, they are rightfully considered a beneficial insect. Their predatory behavior helps rid gardens and forests of a wide variety of pest insects, from leaf-feeding beetles to caterpillars — a process of natural pest elimination known as biocontrol. So, if you should encounter a wheel bug — mid- to late summer is their peak time of activity — enjoy observing this odd insect and appreciate it for the role it plays in the environment. But heed my warning — resist any temptation to pick it up for a closer look!

Wheel Out the Wheel Bug

The wheel bug, also known as Arilus cristatus, is a member of the assassin bug family found throughout eastern North and Central America. They thrive in a variety of environments including grasslands, deciduous and tropical forests, and gardens; anywhere where flowers are abundant.

The wheel bug is so named for the distinctive, spiny wheel-shaped hump along its thorax and abdomen. The body is otherwise elongated and dark grey with white spotting. As an assassin bug, the species also sports an enlarged proboscis for deeding. This species is also noted for being one of the largest true bugs endemic to North America, at a whopping 3.8 cm (1.5 in) long.

As an assassin bug, A. cristatus is a voracious predator within its natural habitat. Individuals catch caterpillars, beetles, sawfly larvae, aphids, and stink bugs with their long forelimbs and then inject their target with a potent paralytic, dissolving its insides and slurping up the resulting soup. Due to their spiky bodies and painful bite, the wheel bug has few natural predators. However, eggs are often prey to parasitic wasps.

Wheel bugs begin mating in September and continues throughout the fall. After mating, the female lays anywhere from 40 to 200 eggs on a leaf, in a cluster resembling a cylindrical honeycomb. The eggs incubate over the winter and juveniles emerge the following spring. They mature throughout the summer and become full adults the following fall, where they mate and then die.

Conservation status: The wheel bug has not been evaluated by the IUCN, but populations are widely dispersed throughout its range and the species is generally ecologically stable. In many areas, it is one of the only predators of other invasive or pest insect species.

Photos

Rick Kelly

Seth Ausubel