Provisioning food for the next generation

By Jeff Mitton

My colleague Bill Bowman and I were walking up one of the canyons on the east side of the San Rafael Swell, looking for a petroglyph that Bill had seen years ago.

It was snowing when we left Boulder, so we were charmed by prickly pear cacti blooming in yellow and fuchsia. When we stopped to photograph them, Bill noticed a profusion of wasps, some of them attacking caterpillars.

Sphecid wasps are hunting wasps that capture prey to provision food for their offspring. Each sphecid wasp species is a specialist, hunting solely for grasshoppers, or katydids, or caterpillars, or spiders or Mormon crickets. This particular species,Podalonia sericea, hunts only for caterpillars.

I watched a wasp struggle to carry a caterpillar up a sandy hill. The wasp gripped the motionless caterpillar in its mouth, straddled it and labored to move it toward a burrow higher on the same hill. A few feet away, another wasp was excavating a burrow, tossing sand grains more than a foot.

When sphecid wasps encounter an individual of the appropriate species, they attack, stinging it repeatedly. The stings paralyze but do not kill the prey. Wasps drag live but motionless prey to a burrow and stack them inside. When the number of prey items reaches some critical threshold, the wasp lays one egg on top of the stacked food. It then fills the burrow entrance with soil and works the soil and adds sticks and leaf fragments to camouflage the sealed burrow, hiding all evidence of its recent activities and the stockpile of food.

Careful observations of sphecid wasps have revealed that some species, including those in the genusPodalonia, employ a tool when sealing the burrow. A wasp selects a pebble of the appropriate size and shape, grasps it with its mouth and tamps the dirt covering the burrow to make it resemble the soil in the surrounding area. At one time, we believed that tool use was one of the characteristics that made humans superior to all other species.

The primary threats to sphecid wasps are two groups of flies and two groups of wasps that either eat or parasitize the larvae in the burrows. To confuse and deter these insects, the sphecid wasps dig fake or accessory burrows near the camouflaged burrow. The predators and parasites explore the accessory tunnels intensively, searching for larvae.

In the foothills west of Boulder, the most common sphecid wasp is the blue mud wasp,Chalybion californicum. It is black with iridescent blue highlights and a chronic twitch that recurs every three or four seconds. The blue mud wasp is a specialist on spiders, and like other sphecid wasps, it digs a burrow, provisions it with paralyzed prey, lays a single egg, then closes and camouflages the burrow.

One group of sphecid wasps has evolved into cleptoparasites, relying on the industrious work of another sphecid species sharing the same environment. Some cleptoparasites detect a sealed burrow, open it, destroy the egg, lay their own egg and reseal the burrow. Other cleptoparasitic species simply open the burrow, lay an egg and reseal the burrow. The cleptoparasitic egg develops faster and hatches first; then the cleptoparasitic larva destroys the host's egg and feasts on the food stockpiled by the host species.

Jeff Mitton (mitton@colorado.edu) is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.

May 2011