Warm Spring Helps Yellow Jackets Crash the Party

Jamie Barger raked his fingers through a wet mat of birch leaves and exposed a frosted floor of decomposing forest litter. His trained eye caught a familiar image---the vivid yellow and black striped abdomen of a queen yellow jacket wasp, clinging like a frozen water droplet to the underside of a leaf she'd attached herself to last fall.

Barger, a graduate student in biology at the University of Alaska Fairbanks, dropped the queen in a jar and resumed his search. He, along with Rutgers transfer student Jessie Seares, graduate student Gerry Zuercher and UAF Associate Professor of Zoophysiology Brian Barnes were taking advantage of a warm spring day in an attempt to understand more about Alaska's most cussed-at wasp, the yellow jacket also known as Vespula vulgaris.

A scavenger, Vespula vulgaris has a taste for garbage and good barbecues, where it often inspires screams by crawling between hamburger buns. To punish vulgaris for crashing the party, people often make the mistake of destroying the gray, paper-mache-looking, tree-hanging nests of vulgaris' cousin, Vespula arenaria, who prefer insect larvae to unattended steak.

Vespula vulgaris nest in ground cavities. Egg-laden yellow jacket queens have the lonely responsibility of keeping their species alive during the winter, because all female workers and the male drones die in the fall.

Last fall, Barnes and his students gently probed around the leaves in a birch forest until they found a few queen yellow jackets clinging upside-down to leaves, perhaps in an effort to keep dry while hibernating. They sunk temperature probes next to the wasp's bodies, and recorded the temperature every four hours from October until late April.

The temperature in the wasp's leaf litter bed never got lower than 25 degrees, even when the air in the Interior reached 40 below zero. Because 25 degrees seems more than cold enough to freeze a wasp's vital fluids, the researchers sought to find out why queen yellow jackets don't die from ice crystals bursting their cell walls.

As the captured queens twitched out of dormancy in the warmth of a lab, Barger and Seares strapped the now-wiggling, stinger-wielding wasps to a temperature probe with dental floss and dropped them in test tubes that were lowered into a cold bath of ethanol.

The wasps' body temperatures dropped to about seven degrees before the probe showed a rapid rise in temperature, a sign queens had given up all their body heat and died. Their death showed Barnes that the method queen yellow jackets used to survive the winter was supercooling, a process by which an organism has the ability to rid fluids in its body of impurities that trigger the formation of ice. Supercooling allows the body of a queen wasp to cool well below 32 degrees without damage to its cell walls.

The supercooling mechanism may make springtime weather the major factor in the number of yellow jackets that show up at the picnic table this summer, Barnes said. A consistently warm, dry spring could allow huge numbers of dormant queens to wake up comfortably, set up nests and succeed at their life's mission of producing hundreds of workers and drones.

A wet spring would hamper the yellow jackets' efforts to forage and set up a nest, which could prevent a queen from laying her eggs. A cold snap could also knock down yellow jacket numbers. Many queens would die if the temperature dropped below the yellow jacket's apparent supercooling minimum of seven degrees after previous warm weather had melted the insulating snowpack.

Barnes said early May is the time for homeowners to act if they want to go on an anti-yellow jacket campaign. He baits commercial "bee- traps" with bologna and apple juice concentrate. Getting the queen in early spring is infinitely more effective than trying to eliminate the yellow jackets later on, Barnes said.

He figured that each egg-carrying queen that succeeds in building a nest will produce several thousand worker wasps.

"Every queen people kill now will keep up to 3-to-4,000 yellow jackets from pestering them at their barbecues," Barnes said.