Wild Side: Predatory beetles

When it comes to prey, it’s a battle to the finish.

Improbable partners: a paper wasp and a tiger beetle share a caterpillar lunch. — Matt Pelikan

While I have my favorite places to look for insects, I’m always alert for habitats that look like they might hold something interesting. This tactic recently took me onto a vacant lot in Edgartown, where a strange story unfolded.

The lot had been cleared and graded, presumably for sale or construction. While there was a smattering of low vegetation — Pennsylvania sedge, sweet fern, tangles of prickly dewberries, a few asters in bloom – the site was mostly exposed mineral soil. It was this coarse sand that caught my eye, since it looked like a substrate that would appeal to any of the many kinds of insects that spend part of their life cycles in burrows.

And right I was. From my first step onto the open area, I realized I had stumbled over a small but incredibly dense population of oblique-lined tiger beetles, Cicindela tranquebarica. This predatory beetle is a common one on the Vineyard, present in nearly any dry, open habitat. Like all of our tiger beetles, its immature or larval stage lives underground, hunting in ambush from the mouth of a burrow for any smaller passing insects or spiders.

When I say “dense population,” I mean it. Each step I took, it seemed, flushed a small handful of these voracious little predators off the ground. It’s hard to get a precise count of tiger beetles because they are highly mobile, sometimes (but not always) circling back to their original location after they’re disturbed. But I can’t believe there was less than a beetle per square meter on the site, and if we call it a half-acre, that adds up to 2,000 beetles. A rough estimate, but probably a conservative one.

And when I say “predatory beetle,” I mean that as well. Tiger beetles mostly take live prey — ants, other beetles, spiders, flies — that they capture on the ground with blindingly fast sprinting attacks. Which raised an interesting problem: What were all those beetles eating?

The answer, as far as I could tell, was not very much. Aside from the beetles, the site (as you might expect from its largely bare condition) seemed to hold little insect life. There must have been plenty of prey at one point, or all those beetles couldn’t have survived to maturity. But whatever it had (my guess is ants of some kind), it must have been seasonal in occurrence. And its season was done.

The result was that I had stumbled over a large population of ravenous tiger beetles. As I watched, the implications of that became clear. Never a very sociable insect, these beetles were downright irascible, chasing each other and wrestling in what were either territorial disputes or attempts at cannibalism.

As I watched the frenetic activity, I came across a group of three tiger beetles wrestling over a small caterpillar that they had been lucky enough to find. Caterpillars, which spend most of their time off the ground on vegetation, are not frequently the victims of tiger beetles. But in this case, the larva was a welcome resource, and had spawned a vigorous tug-of-war among the beetles.

As I watched, another predator joined the fray: a Northern paper wasp, Polistes fuscatus. Perhaps attracted by the activity of beetles’ dispute, it landed, got its own grip on the caterpillar, which was chewed beyond recognition by this point, and tried to steal the prey for itself.

This launched a brief flurry of violence. Beetles attacked beetles; beetles attacked the wasp; wasp fought back. At one point, one of the beetles had its jaws on the wasp’s head and was evidently trying to kill it, the beetle wriggling to avoid the stinger that the wasp attempted to bring into play. The action was too fast for me to follow clearly.

I finally remembered my camera, which I unslung, turned on, and brought to bear on the quartet of hungry predators. But I did so clumsily, closing in to macro range too fast, disturbing two of the beetles, which flew away.

Instantly the tenor of the situation changed. With just two competitors now vying for the caterpillar, the wasp and the remaining beetle evidently decided, through some vague insect logic, that there was plenty of caterpillar to go around, and hence no longer any point in mortal combat. Both insects abruptly stood down from their belligerence and, settling in on opposite sides of one end of the caterpillar, began contentedly munching on the prey they had won, amiable strangers sharing a lunch counter.

A strange partnership! But then, it was a strange situation. Perfect soil conditions plus plenty of some unknown prey species produced a bloom of adult tiger beetles. The prey disappeared, leaving a high population and a collapsed food supply, creating a Hobbesian dystopia of beetle against beetle. The result was an unusual prey choice, a struggle for possession, and then, at last, a moment of tranquility for two oddly matched, ad hoc, interspecific friends.