Home & Garden Columns

Wild Neighbors: Bug Bombs: The Stink Beetle Meets the Killer Mouse

By Joe Eaton, Special to the Planet
Tuesday January 30, 2007

First, my apologies for the last column’s headline, which I suspect was a spell check-inflicted error. “Scooter” is one of the surf scoter’s many vernacular names, along with “skunkhead coot,” “blossom bill,” “tar-bucket,” and several that involve distasteful ethnic references. But officially, it’s “scoter.” 


I’ve always been fascinated by evolutionary arms races. In his The Ancestor’s Tale Richard Dawkins makes the point that if you can see progress anywhere in evolution, it’s in these ongoing duels between predator and prey. Each, over time, gets better at attack or defense, or dies out. At a minimum, as Geerat Vermeij has argued, arms races have made the natural world a more complex place.  

Take just one example: the skin of the California newt is laced with tarichatoxin, nearly identical to the active ingredient in the notorious fugu fish—enough to kill a human-sized predator if swallowed (yes, it’s happened.) But some West Coast garter snakes have evolved a resistance to the toxin. The snakes aren’t totally immune—they do get sick; but they survive. This puts pressure on the newts to ratchet up their toxicity. And so on. 

Insects and other arthropods, though, are the true masters of chemical defense. I refer you to the work of pioneering chemical ecologist Thomas Eisner, especially his For the Love of Insects. Here you’ll meet millipedes that secrete tranquilizers, stick insects that protect themselves with the natural equivalent of mothballs, butterfly and moth larvae that sequester toxins from the plants they eat and render themselves unpalatable. 

For really baroque defensive systems, you can’t beat the beetles. Best known is the bombardier beetle, a staple of creationist literature. This insect responds to attackers by squirting scalding-hot fluid from its rear end. It stores hydroquinones and hydrogen peroxide in large abdominal glands. Adding an enzyme triggers a chemical reaction that turns the hydroquinones into benzoquinones and frees oxygen, forcing the mixture out of the bombardier’s body. Creationists claim this is an irreducibly complex system which could not have arisen through evolutionary tinkering.  

But another chemically-armed beetle belies that claim. Stink beetles of the genus Eleodes, a variety of darkling beetle, are common in California and the Southwest. They’re shiny black creatures, flightless foragers for plant matter. When alarmed, they point their rear ends toward the sky. There’s a passage in Steinbeck’s Cannery Row in which Hazel, one of the Row’s derelicts, speculates to Doc, the Ed Ricketts character, about this behavior: “’I wonder why they got their asses up in the air for? … I think they’re praying,’ said Doc.” But no; the posture is preliminary to firing. 

The stink beetle’s weapons system is a simpler version of the bombardier’s. Instead of mixing chemicals in a reaction chamber to form benzoquinones, it stores them ready-made. Caprylic acid provides the stink. Eleodes is well-enough defended that unrelated (and unarmed) beetles have come to mimic its distinctive posture.  

At least one predator, though, has figured out how to circumvent the defense. When working in Arizona, Eisner kept finding stink beetle remains strewn about the desert. The wing covers appeared to have been chewed off, apparently by a rodent. Trapping identified the predator as the southern grasshopper mouse (Onychomys torridus.) That may seem like an incongruous notion, but this is no ordinary mouse. Grasshopper mice also prey on scorpions, and even other mice. And territorial males throw back their heads and howl, like miniature wolves. 

Eisner presented a captive grasshopper mouse with a stink beetle.  

Before the beetle could even assume its headstand, the rodent grabbed it with its front paws and jammed it butt-down into the soil of its enclosure. Holding the insect in place, it proceeded to eat it starting with the head. The tip of the abdomen, containing the chemical storage glands, was discarded. 

The next move, in an evolutionary sense, would appear to be up to the beetle. You have to wonder what the stink beetle and grasshopper mouse will come up with, if they have another few million years to work things out.  



Photograph: N. Ludman 

A stink beetle assumes its defensive posture.  



Joe Eaton is a former professional gardener and arborist. His “Wild Neighbors” column appears every other Tuesday in the Berkeley Daily Planet, alternating with Ron Sullivan’s “Green Neighbors” column.