For weeks, the blacklight in my yard in the Creosote/Saguaro flats of Picture Rocks, Pima Co, Arizona has been overrun by huge numbers big, black beetles. They densely cover over 30 square feet of wall and desert floor around my light. The longer my lighting session goes on, the more arrive.
This is a real population explosion of Calosoma probably Calosoma peregrinator. We have a number of very similar Calosoma species here, so I will just use the genus name from here on.
Fast on its strong legs and equipped with big prognath mandibles, Calosoma is a potent predator that grabs whatever other insect they can catch. Moths, desert roaches, ant lions, other beetles, carcasses – Calosoma is not selective at all. I am actually giving up black lighting until the curve of this population explosion is on the down-slope. That may take a while, because the individual beetles can live for several years.
Of course other people in Arizona also find the beetles around their porch lights and swimming pool illumination. Lighted storefronts attract them at night, store owners spray against ‘this pest’ and dead beetles litter the walkways in the morning.
So even though the beetles are mostly night-active or crepuscular (dawn and dusk-active) and during the day only single individuals are visible running through the cinch-weed, people are aware of them and are getting annoyed. Thanks to social media, speculations as to what they are and what to do about them abound.
This is what they are not:
Many Arizonans want to call them Stink Bugs. But Stink Bug is a folksy name for the Desert Stink-beetle or Pinacate Beetle. Similar in size and some behavior to Calosoma, Stink-beetles are darkling beetles in the genus Eleodes. They got their popular name for a good reason: Eleodes has 2 scent glands that open to the very tip of the beetle’s abdomen. When threatened, Eleodes releases the smelly, staining fluid (containing a potent mix of benzoquinones and caprylic acid) – either directing the chemical spray directly at its attacker, or just pouring it all over itself by standing on its head, ejecting the chemical and letting gravity do the rest, rendering the beetle distasteful to many attackers. This ability gave the Stink-beetle its name, and its headstand is a very well-recognized warning that many predators understand. It also gave the Stink-beetle the confidence to stay out in the open at dusk where people saw it and noticed it. So It got not only one, but several common names that are well-known all over the southwest. The Desert Stink-beetle is a peaceful, slow moving feeder on starchy seeds and fungal matter. I often see many coming together where Creosote bushes drop their fuzzy fruit. Eleodes have fused elytra that give them a hard shell and protection from evaporation. But they have lost the ability to fly.
There is another flightless beetle walking the desert, also black and of similar size, and even shape: the Cactus Longhorn Moneleima. When these longhorns move between cacti across a stretch of bare sand, you may sometimes see them stop and point their hind end upwards. They do this very much in the manner of Eleodes, though in fact, they lack stink glands. They are mimics of the Desert Stink-beetle. The existence of mimics that not only look like Eleodes, but even imitate its behavior, demonstrates how effective the protection is that Eleodes gains from its chemical warfare. Cactus Longhorns and Desert Stink-beetles are often confused even by human observers.
But back to Calosoma, the predatory Carabid whose population explosion bothers so many people. Even though it also has a distinctive smell (Hydroquinone) it is actually quite different from the Stink-beetle, being a fast runner and able to fly very well. And while the Stink-beetle larvae could be compared to big meal worms (they are in the same family) that feed on starchy stuff, Calosoma larvae are already potent predators, that go after all kinds of soft-bodied arthropods and also, opportunistically, carcasses.
I think it is this high protein diet that makes the larvae of Calosoma to grow up quickly (a few weeks) and prepares the species to take advantage of sudden abundance of prey species as we are seeing after this monsoon season. When hundreds of White-lined Sphinx caterpillars were appearing in the desert, feeding on herbs that were sprouting everywhere, Calosoma armies were ready to attack them, together with tarantulas, tortoises, lizards, kestrels, coyotes – you name it. Of all of these, only Calosoma has a generation sequence short enough to result in visible, sudden population increases. I am sure that other insect predators and parasites might have reached similar population peaks, but little braconid wasps and even tachinid flies would be much less obvious to humans.
Contributing is the fact that Calosoma is a night active, flying insect that uses light sources (the moon in nature) for orientation. Artificial lights, especially those with a high UV component, are irresistible to those big black beetles, so they can quickly congregate by the hundreds.
So at your porch lights running to invade your doors is Calosoma, the Caterpillar-hunter. It’s that predatory Carabid that everybody sees and gets upset about. Not the slow Desert Stink-beetle Eleodes, a Darkling Beetle, and also not the Cactus Longhorn Moneleima, though single individuals of both genera can be seen stalking (not running!) over the desert sand at dusk and dawn.
Ironically, the role of Calosoma as a predator that very quickly reacts to sudden abundances of ‘destructive’ caterpillars has been well known by many forestry experts for a long time. Calosoma inquisitor, a European Species, was actually introduced to the US for biocontrol of certain periodic caterpillars. How strange that avid gardeners here in Arizona are now the ones that spray and kill an endemic Calosoma because they are ‘grossed out’ by the usually mis-identified beetles.
There is a very close relative to our black Calosoma species, Calosoms scrutator. This one doesn't like the hot dry desert as much as its dark relatives. You can find this one in the medium elevation of the sky islands of the Southwest.