Even without the heat, you can tell it's summer in the desert by paying attention to the animals. The baby quail are in their gawky awkward phase. Scotts' Orioles have arrived from their wintering spots in Central America, and they lend a flash of bright yellow and cheerful song to the desert dusk. And all around the desert, emerging from their holes beneath the trees, come the "Mother of God, what for the sake of all that's holy is that?"
"That," folks, is the palo verde root borer, Derobrachus hovorei, which at up to four inches in length may well be the largest beetle in North America. It's certainly the largest insect in the desert.
I first learned of the palo verde root borer the way most people do: I saw one in the desert and had no idea what on Earth it was. It was emerging from a hole in the ground next to the house I'd just moved into. I happened to have my camera in hand, and I took a frustratingly blurry photo.
I'm not by any means phobic about insects: as a rule, I like them. But this guy was pushing it by his very existence. He looked like he was built of cast iron and axle grease, a ridiculous steampunk insect just wandering out into the sunlight. I half expected to hear steam-driven pistons as he walked. This sculpture gives you an idea how it felt to watch the thing, though I will admit the sculptor has mildly exaggerated the insect's size.
That was in Nipton, on the north edge of the Mojave National Preserve. I lived there for much of 2008. Over those few weeks of summer I grew accustomed to the root borers, which had emerged en masse. The borers turned out to be inoffensive. My non-airconditioned house there had a screen porch where I spent much of my time. One day I was sitting out there barefoot and a tickling feeling distracted me from my reading. A huge Derobrachus had climbed up atop my foot, and was waving its heavily segmented antennae more or less at me. I suppressed a flinch and steeled myself for a bite from its powerful-looking jaw. It was just getting its bearings, though, and it clambered down the other side of Mount Tarsus and wandered off under the exercise bicycle.
I took a photo of it an hour later on his way back out from under the bike and put it on flickr with the title Palo verde root borer finished with his workout. "When it can curl five pounds," I noted, "I'm leaving."
Beetles are famously diverse critters, accounting for at least a quarter of all known animal species. Derobrachus is no exception: there are more than 20 species now known in the genus, and that number is almost certain to increase whenever some graduate student takes an interest. Derobrachus hovorei didn't even exist as a species until 2007, as far as coleopterists knew: they were considered part of the species Derobrachus geminatus until Antonio Santos-Silva determined that that earlier species needed to be broken up. Derobrachus geminatus do exist in the California desert, but they're rare. You can tell them apart from Derobrachus hovorei easily: their wing covers -- called "elytra" -- are shiny, where D. hovorei's have more of a matte finish. The more common D. hovorei also tends to be darker, like walnut heartwood, as opposed to D. geminatus' mahogany. This is not a hard and fast separation between the species, and the fact that the related Derobrachus leechi also lives in the California desert makes positive identification even harder. When in doubt, consult your coleopterist.
Derobrachus as a whole are found through the Americas from the southern tier of the United States southward to the Andes. They're in the longhorned beetle family. California's Derobrachus pretty much make their living the same way. Adult females lay their eggs at the bases of appropriate trees like so:
Those eggs hatch out. The larvae within immediately eat their eggshells, which, as folklore has it, contain microorganisms that help the beetles digest cellulose. They then seek out the tree's roots, eating them as they grow. Palo verde root borers get their name from this larval culinary preference, though they will also eat cottonwoods, citrus, and -- in the specific example of the beetles in Nipton -- eucalyptus, as long as it's irrigated.
If you think the adult beetles are unsettlingly large, consider this fact: the larvae can get even bigger. Palo verde root borer larvae have been measured at five inches long. As you sit in the welcome shade of that desert palo verde, consider the fact that dozens of five-inch grubs may be just inches beneath the part of you you're sitting on. You're welcome.
Despite their relatively gargantuan size, the larvae rarely harm a healthy tree, though they can cause damage to stressed trees. After about three years as a grub they metamorphose into pupae, which are essentially the beetles' equivalent of a butterfly's cocoon. After a month or so the adult emerges from the shell of the pupa, tunnels its way up into the sunlight, and goes about the business of making a new generation.
And that's when most people see them, on summer evenings, flittering clumsily around porch lights and other illuminated spaces. Adults feed on nectar and fruit and, in my experience, on the leavings of soft drinks from cans desert travelers toss out their window into my front yard in Nipton. The adults' emergence offers a bounty for other hungry desert dwellers such as coyotes. In Nipton in 2008, the local litter of four-month-old feral kittens grew fat on palo verde root borer adults. It was a boon for them, though it was hard to get to sleep at nights what with all the crunching. You're welcome.