Field Notes—

A Halloween Wasp On A Katydid Hunt

By Curtiss Clark

They are the children of August nights, obeying an inverse convention of being heard but not seen. I wait through half a summer to hear the first of the katydids’ thrumming love songs, which seem such a vital part of the knot of sensations associated with sweating through a summer night. Lying awake in the chance heat and humidity of a near silent mid-July night is unnerving and almost unbearable without the zhuh-zhuh-zhuh, zhuh-zhuh-zhuh that in August shuffles even the most heat-sensitive souls into the stupor that passes for sleep on a too-hot night. The sound can be a great comfort to the discomforted.

I am not alone in my katydid vigil. Out there in the night is a black and orange creature, a winged minion of the jack-o-lantern, with more sinister intentions toward the katydids. It is the Great Golden Digger Wasp. At more than an inch long, it is one of the larger wasps you will find foraging in the gardens and borders. Sphex ichneumoneus is quite harmless despite its fearsome appearance, though it shows some nonaggressive curiosity about humans and their pets. I should say they are harmless unless you are a long-horned grasshopper of the tettigoniidae family, which includes katydids.

Great Golden Digger Wasps feed on nectar and pollen, but their survival as a species depends on their skills as katydid hunters. The female wasp, as her name suggests, digs her single-celled nest in the ground. (The males, as is often the case in the insect world, are useful for fertility and little else; they while away their lives among the flowers.)  The female will then stock the burrow with one to six live katydids. Once she finds her prey, whose leaflike body makes it nearly invisible, she stings and paralyzes it — but does not kill it. Ultimately, she will lay an egg in among the still-living katydids and then cover up the nest, satisfied that she has established an ample trust fund for the next generation. The emerging larva will feast on the fresh katydids until it is ready to leave the nest.

Sphex ichneumoneus seems to be the favorite insect of philosophers and social scientists, since they frequently use the digger wasp as an example of determinism, which is the doctrine that external forces and natural laws rather than free will determine events and actions. One experiment having to do with the female digger wasp’s mechanistic nest-stocking activities is often cited.

The female uniformly delivers a paralyzed katydid to the edge of the burrow, leaves it for a few moments to go into the burrow to prepare its place. She then returns to drag the prey into the burrow. (There is even video of this rote choreography on YouTube — search on “digger wasp.”) However, if while the digger wasp is in the burrow, the katydid is moved even an inch from where it was left, the female will emerge from the nest, return its prey to its original position at the edge of the burrow and restart the routine from the beginning, rather than just dragging the katydid the extra inch into the nest. One experimenter moved the katydid 40 times with the same monotonous result. The cognitive scientist and philosopher Douglas Hofstadter has even gone so far as to call free will “antisphexishness” in honor of the insect’s mechanical approach to its chores.

Behavioral researchers also point to the Great Golden Digger Wasp as an example of the Concorde fallacy, which is the tendency to continue investing in a project based on the past expenditure of time and resources rather than realistic prospects for future success. This “fallacy” is named after the ultra-expensive Concorde supersonic transport airplane funded for years by the British and French governments even long after it became apparent that no one would ever realize any profits from it.

Five to 15 percent of the time, a digger wasp nest will be stocked simultaneously by two females, according to a 1980 study. Researchers found that if the two females met at the nest, a fight would ensue, and the female that had the most invested in the nest (in the currency of katydids), would fight the longest and hardest to drive off the other female, even though the success of the nest was far from certain. The researchers also observed another scenario they called “winner take all,” which occurred when the females never met; one would decide to stop stocking the burrow, lay an egg, cover the nest, and reap the benefits of unwitting contributions of the other female.

Either way, the katydids come up short. It matters little to them whether a particular Sphex ichneumoneus is the grim reaper or the gravedigger, or both. This ghoul-of-a-wasp, all dressed up in black and orange for Halloween, is something for the grasshopper family to avoid when it is on the prowl during the day. The katydids go silent, lie low, and blend in. In the relative safety of the night, however, it is not hard to imagine that their zhuh-zhuh-zhuh, zhuh-zhuh-zhuh is something more than a love song — perhaps an urgent incantation against sudden discovery and a slow, hideous death.

So much for finding comfort in the sounds of the night.