In the summer, when the air is sticky and hot, insects are everywhere: circling your fruit bowl, marching across your picnic, buzzing in your ear, and sometimes — the worst of times — flying directly into your mouth.
But when temperatures drop, icicles form, and parkas come out of storage, the bugs seem to vanish completely from the chilliest corners of the world — until months later, like some kind of magic trick... they reappear, as if they'd been there all along.
Where did they go?
To find out, we talked to someone who has spent much of his research career pondering this very question: Brent Sinclair, the director of the Insect Low Temperature Biology Lab at the University of Western Ontario in Canada, where he is also an associate professor.
One reason most people are mystified by the fate of insects in the winter is because there is not a simple answer. Some survive as eggs, larvae, or pupae, while others make it through the winter as fully-grown adults.
In general though, there are three distinct survival strategies that get insects — and related critters, like spiders — through the winter. One of them (the last on our list!) seems strange enough to be science fiction.
1. Avoid the cold
Sonia AltizerThe first strategy is perhaps the most straightforward. Many insects survive the cold winters by simply avoiding them, employing what Sinclair dubs "the snowbird approach."
Some, like butterflies and dragonflies, migrate much like songbirds do, heading south en masse as soon as the cold sets in. (Researchers have actually attached tiny radio transmitters to dragonflies to track these migration patterns.)
North American monarch butterflies, the most famous migrating insects, make a long and somewhat miraculous journey to central Mexico each winter. (Swallowtail butterflies do no such thing, sticking out the winter safely encased as a chrysalis instead.)
For other insects, avoiding sub-zero temperatures means a journey of inches, not miles. Many aquatic insects wait out the winter at the bottoms of ponds, where they can remain relatively comfortable even when the surface freezes, Sinclair explains. Others do the same in the soil, burrowing deep below the frost.
Different types of mosquitoes have different winter survival strategies, but some are able to survive cold temperatures by hiding out in sheltered places like "inside the envelope of a house or under a bridge," Sinclair says, where they lay in wait in a state called "quiescence." Their next meal won't come till springtime.
2. Carry on as usual
Robbie Sproule / FlickrWhile insects seem scarce when December rolls around, some — a rare and hardy few — continue doing their thing, just like those headstrong outdoorsmen who go camping in the dead of winter.
"If you were to put a little trap underneath the snow, you would find some small primitive insects," Sinclair tells us. Some crawl within warm pockets carved out by grass and leaves, while others survive on the surface.
Mites, springtails (called "snow fleas," though they are technically not insects), and certain spiders (also not technically insects, of course) can all appear as unassuming black dots on the snow. But don't be fooled: they're very much alive.
U.S. Department of Agriculture via FlickrThe most sci-fi winter survival strategy insects rely on is similar in spirit if not mechanics to the hibernation of many warm-blooded animals.
It's called diapause: a dormant, semi-frozen state some insects enter until they thaw out in the spring and crawl off as if nothing had happened. (Despite our fantasies of full-body cryogenics, humans definitely can't do that yet — though some mosquitoes pull off something a lot like it.)
The emerald ash borer, a tree-killing invasive species in North America, enters diapause in the winter, which (unfortunately for the northern regions it's infested) means it can survive freezing temperatures. In this state of suspended animation, "they don’t do anything," Sinclair says, "They don’t develop. They just sit under the bark of trees where they’ve been feeding all summer."
The environmentally damaging creatures are able to stay unfrozen and alive in the cold because a high concentration of their blood is made up of something called glycerol, which acts as an antifreeze.
Woolly bear caterpillars, meanwhile, actually freeze into tiny statues but still live through the winter. "Ice forms inside their bodies — you tap them and they’re solid," Sinclair explains. "It's an amazing trick." They can survive, Sinclair has found in his lab, at temperatures well below anything found on Earth. A lot of body processes shut down so they aren't injured in the meantime. (Goldenrod gall flies perform a similar stunt.)
The undead invasion
Many insects actually do die in the winter, leaving nothing but eggs behind. That means they are replaced by an entirely new generation in the spring.
"You know the crickets that you hear singing in the fall? Those adults that are singing are all going to die in the winter," Sinclair explains. "They lay eggs in the soil, and those hatch in the spring."
One problem with our warming winters — yes, parts of the US are experiencing a cold snap, but nationally it's one of the warmest winters ever recorded — is that insects that are supposed to die off don't, and those that normally can't survive in the coldest areas are moving in and setting up shop.
"If you have more and more warm winters, you can get invasive species moving further up and into cold areas," Sinclair says. And all that's stopping non-invasive species, like the mountain pine beetle, "from eating eastern North America," is the blistering cold of the Rockies — something we may not be able to rely on for long.
"Elevated temperatures at high elevations across western North America have allowed mountain pine beetle populations to develop in a single year in areas where two or more years were previously required," noted a report from the US Forest Service.
Still a bit of mystery
Scientists can detail many of the different ways insects make it through the winter, but there's a lot we still don't understand.
The creatures that freeze or enter diapause and then spring back to life are especially mysterious. They're also some of the most intriguing insects, since while humans have found ways to freeze eggs and sperm, we are still trying to figure out if there is a way to freeze and preserve whole tissues and organs without damaging them.
On that measure, the insects have us beat.
"For over 200 years, we’ve known that some insects can survive freezing," says Sinclair. "But we still don't know what the magic bullet is that lets them do that."