Every year at the end of summer our apple trees drop apples all around, leaving fermenting fruit to attract an impressive swarm of yellow jackets. For a few weeks yellow jackets seem to be everywhere, with nothing better to do than hang around like surly teenagers and get, literally, underfoot.
Then they disappear, not to be seen until next summer. Where are they hiding for all those long, cold winter months?
By the end of summer it is true that most wasps in the family Vespidae, which includes the infamous yellow jacket, have nothing to do. The summer’s work of building a paper nest and raising brood is over. The sterile female workers and male drones have fulfilled their duties to the nest and won’t live much longer, but for fertile female wasps, called gynes, a new challenge lies ahead: surviving the winter to found a new nest in the coming spring. They do it with the help of an adaptive trick that sounds like a super power: supercooling.
Supercooling is what happens when a fluid cools below its usual freezing point, but remains a fluid. There are a couple different ways to coax a liquid into supercooling, but many insects, including the gynes of vespid wasps, take advantage of this phenomenon to avoid freezing at winter temperatures by making “cryoprotectant” chemicals in their tissues. One common insect cryoprotectant is ethylene glycol, the same chemical we use as car antifreeze. Wasp gynes use sugars and glycerol for their cryoprotectants.
Cryoprotectants help the water in a wasp’s tissues to supercool by interfering with it’s ability to form the crystal structure of solid ice, which means damaging ice crystals won’t form inside a wasp’s cells, even as temperatures drop below 32ºF. Wasp gynes have been observed surviving temperatures of 1.5ºF. While it’s true gynes can survive the whole winter by supercooling when temperatures drop below freezing, it’s still a risky strategy. Any contact with external ice can cause the water in the wasp’s tissues to begin freezing too, because it acts like a seed crystal and jump starts a chain reaction of crystallization until the whole wasp is very frozen and very dead.
Since supercooling only works if a wasp doesn’t come in contact with ice, and ice is difficult to avoid in the middle of winter, choosing the right hibernation place is critical. When a wasp gyne is ready to overwinter she finds a “hibernaculum”: a cool, dry, and sheltered spot. Wasp gynes have been found using all kinds of places as hibernaculum: under loose bark, in attics, under rocks or logs, inside old machinery, down rodent burrows, and even between books on a shelf.
Considering the fierce reputation of many vespid wasps, a hibernating gyne is remarkably adorable. Hanging from the top of her hibernaculum spot, she tucks her wings beneath her body to protect them, holding them in place with her hind legs. She’s now ready to hibernate through the winter and, using supercooling, can survive hibernaculum temperatures well below the freezing point of water.
Yet even with supercooling on their side, wasp gynes have a tough time making it through to spring. Supercooling is an expensive way to overwinter; in one species of wasp it’s been estimated that gynes use up 70% of their lipid stores, 79% of their sugar stores, and over 80% of their stored glycogen. If a gyne hasn’t eaten enough over the summer, she doesn’t make it through the winter. Now you know why yellow jackets are so aggressive about stealing sips from your soda at the summer picnic!
In fact, there is evidence that a wasp’s lifetime access to good food affects their chances of becoming a gyne. Unlike bee queens, potential wasp queens look just like their worker sisters, but the wasps that become gynes have more stored fat. At the same time, they have a markedly greater tolerance to cold than worker wasps.
As temperatures drop through autumn, wasp gynes make their final preparations for winter. Most of their nestmates are long gone, they’ve mated, and before winter sets in for good, they hang out in temporary clusters getting in those last few energizing meals before they find their winterlong hibernaculum spot. If their hiding place keeps the ice away and if they have enough energy stored up as fat, they’ll supercool their way through winter, to found new nests in the spring.
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