Wild Side: Cicada killer wasps

These striking insects are typically not a threat to humans.

A cicada killer wasp. Only the female has a venom-producing stinger. — Matt Pelikan

The most common type of question I get in early August has to do with wasps: Big ones, sometimes described as frighteningly large, black and orange with white banding, often seen as “aggressive” because they dart and hover around humans observing them.

Calm down, folks. These wasps, if they stung you, would no doubt cause a good deal of pain. But they have no more interest in stinging you than you have in stinging them. They’re called cicada killers, Sphecius speciosus, and unless you happen to be a cicada, they’re simply not a threat unless you molest them. Cicada killers are the most prominent exponents of a very common life history among wasps, and they’re fascinating creatures.

It works like this. A female cicada killer digs a burrow in sandy ground. It’s a big burrow, roughly an inch in diameter and several inches deep, and the industrious wasp, scrabbling away like a motivated terrier, builds up a significant mound of sand around the mouth of the tunnel as she excavates. Sometimes more than one female wasp will collaborate on a burrow, each building her own underground chamber in a sort of condominium arrangement. The large size of the burrow is a necessity because of what happens next.

The wasp locates a cicada, usually as it sings up in a tree canopy. Just how they do this seems to be an open question. Perhaps they zero in on the droning call of the cicada. But in any event, once a female wasp has found a cicada, she stings it, injecting a venom that rapidly paralyzes the prey.

The female cicada killer then returns to her burrow with her victim. Though these wasps are large, well over an inch long, cicadas are typically even larger. The laws of aerodynamics make transport a challenge. But the wasp does her best, revving her wings up to full throttle and launching on a lazily descending flight path toward her burrow. Most times, she falls short, but, undeterred by the resulting controlled crash, she simply completes the journey by dragging her prey along the ground. If a cicada killer finds a cicada on the ground, she’ll often drag it up a tree and launch for home from an elevated perch.

Back at her burrow, the wasp drags the paralyzed cicada down into the waiting underground cell, lays an egg on it, and plugs the burrow. The egg hatches in a couple of days, and the larva happily feeds on the cicada. When mature, the larva spins a cocoon and prepares to hibernate until the following summer, when it tunnels to the surface and prepares to repeat the cycle. I always see my first cicada killers of the year within a couple days either way of when I hear my first cicada.

What about male cicada killers? While they’re equipped with a sort of pseudo-stinger at the ends of their abdomens, with which they can defend themselves by jabbing if necessary, they produce no venom and can’t truly sting. So they can be of no help in provisioning the nest.

But when they emerge as adults, they establish and defend little territories, patches of a couple of square meters of bare, sandy ground, and wait for a female to come by. They spend much of their time on the ground, sallying upward to chase away rival males or to chase females. Cicada killers that get up in your face are typically males, just letting you know that you’re encroaching on their air space. Since they can’t sting, they pose no actual threat to humans.

For all their fearsome activity on behalf of their offspring-to-be, adult cicada killers are surprisingly benign insects, interested in little besides cicadas and each other. Adults of both sexes feed only on nectar and pollen from flowers. If you move slowly and don’t obstruct their burrows, cicada killers are easy to observe, even quite closely.

This basic predatory life history, far from an aberration, is very common among wasps. Just offhand, in the same family as the cicada killer I can think of common Vineyard species that prey on beetles, stink bugs, and bees. In the closely related family Sphecidae – the “thread-waisted wasps” – some of our common representative ones prey on katydids, grasshoppers, and caterpillars. Members of one entire family, Pompilidae, stock their burrows with spiders. And the so-called “potter wasps,” close cousins of paper wasps and yellowjackets, build pot-like nests out of mud instead of digging tunnels, and they stock the pots with moth larvae.

That this habit shows up in multiple wasp lineages suggests that it evolved multiple times, in turn suggesting that it’s a highly effective strategy. And the diversity of prey species taken by predatory wasps means that these wasps play a huge role in regulating insect populations generally. Predatory wasps are a prominent feature in the complex web of ecological relationships, and their importance to a healthy ecosystem cannot be overstated. These are insects to appreciate, not fear.