While we humans have hot chocolate and Netflix to get us through winter, some species in our backyard have to literally freeze solid over the cold months - and survive.

Kiwi scientists have been studying how some of our species, including an alpine tree weta, have the bizarre ability to stay alive, even when ice has formed within their tissue.

Around the world, when winter arrives insects are forced to either migrate - Monarch butterflies in North America are one extreme example and have been recorded to fly more than 4000km to warmer climates - or put up with the cold.

Anna Probert, a PhD student at the University of Auckland's Centre for Biodiversity and Biosecurity, said insects had different options to deal with it.


Many spent the winters inside egg, larval, pupal, nymphal or adult stages, while populations of pesky summer species like ants, mosquitoes and wasps hibernated in nests, with little activity.

Yet insects, regardless of which life stage they spend winter, may be subjected to freezing temperatures, requiring them to adopt a strategy to avoid freezing to death.

These strategies tended to be one of either "freeze avoidance" or "freeze tolerance".

"Freeze avoidance, the basal trait for cold tolerance in insects, is essentially the ability to maintain body fluids at a liquid state at temperatures below zero, existing in what is called a super-cooled state," she said.

Insects achieved this by producing "anti-freeze" chemicals that prevented ice formation.

"For insects that adopt this strategy of cold tolerance, if the temperature slips below what they are able to maintain in their supercooled state, they usually freeze to death."

While this threshold varied largely between species, at the extreme end there is a species of parasitoid wasp that can supercool to -47C.

Hardy alpine stick insects can survive all that a New Zealand winter throws at them - even ice forming in their tissue. Photo / Luke Dunning, Landcare Research
Hardy alpine stick insects can survive all that a New Zealand winter throws at them - even ice forming in their tissue. Photo / Luke Dunning, Landcare Research

Freeze-tolerant insects, meanwhile, had the ability to survive the formation of ice within their tissues, through the production of ice-nucleating agents in their extracellular body fluid.


Interestingly, this phenomenon is something much more common in the Southern Hemisphere than on the other half of the planet.

"I think that has more to do with the fact that the weather fluctuates a lot in the Southern Hemisphere, whereas in the north, you have long periods of really cold weather in the winter."

In New Zealand, scientists had observed freeze tolerance in the alpine tree weta and the Niveaphasma genus of stick insect, both which can crawl away in spring after thawing out.

Genetic work by Landcare Research scientists also found that different populations of the alpine stick insects, which can survive at 1000m above sea level and have never been found north of Arthurs Pass, were restricted to a number of areas in the lower South Island during recent glacial advances.

"If your body tissue freezes, you'd think you'd just die," Probert said.

"So trying to figure out what genes are responsible for allowing this behaviour is important, because there are so many different applications you could use this for."


When the warm weather returned, hibernating insect species were reactivated by environmental triggers.

"So insects are still around during the winter, they may just not be as active, or in a stage of their life cycle where they are not so conspicuous," she said.

"Although I don't necessarily welcome the return of mosquitoes and social wasps into my life, it's neat to know a little bit more about the amazing adaptations insects have evolved to cope with the cold."