Post by Anna Frances Probert @AFProbert
I often get asked where all the insects go in winter, which is a pretty good question considering how conspicuous mosquitoes, ants, wasps and cicadas are during the summer months. The presence of insects may be obvious in summer although they are seemingly absent during winter. We are all aware that there are some pretty amazing animals out there that spend the coldest months of the year in a torpid or hibernation state. But what about all our invertebrate friends that seem to disappear in the cooler months?
Like all animals, insects are faced with two options when it comes to the cold: move away, or somehow deal with it. The first option of moving away is something we might generally associate with larger animals – migration. Although insect migration differs slightly compared to that of birds and mammals in that the ‘round trip’ of the migratory path is usually made up of multiple generations, they can indeed travel vast distances to head to warmer climes to overwinter.
This is best exemplified with the North American migration of the Monarch butterfly (Danaus plexippus). The winter months in Canada and in many places in the United States proves to be too cold for the Monarch butterfly to survive. So to avoid certain death via freezing, come autumn, populations start moving south towards the equator, where the climate becomes more forgiving. Although a complete round-journey involves at least four generations of Monarch butterflies, individual butterflies have been recorded to fly more than 4000km.
But what if you don’t have the ability to move to avoid the cold? Overwintering and being “cold tolerant” becomes your only option. However, when you’re an insect you have different options of how to spend winter time. You can either overwinter in your egg, larval, pupal, nymphal or adult stages, depending on your life history. 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. Essentially, like all ectotherms, insects subjected to freezing temperatures need to adopt a freeze avoidance or freeze tolerance strategy.
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 “supercooled” state. Insects achieve this by producing “anti-freeze” chemicals that prevent 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. Whilst this threshold varies largely between species, at the extreme end there is a species of parasitoid wasp that can supercool to -47°C.
On the other hand, freeze tolerant insects have the ability to survive the formation of ice within their tissues, via the production of ice nucleating agents in the extracellular body fluid. Freeze tolerance is a strategy adopted by many Southern Hemisphere insects including a species of alpine tree weta here in New Zealand.
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. 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.
Anna Probert is a PhD student in the Centre for Biodiversity & Biosecurity, School of Biological Sciences, University of Auckland. She is using ants as a model to assess the risk posed by exotic invertebrates to native ecosystems. She is supervised by Margaret Stanley, Jacqueline Beggs, and Darren Ward.