Last week, we kicked off our series on insect winter ecology with a look into a well-known overwintering strategy: Migration. But not all insects have the luxury of simply fleeing colder temperatures. Many species employ unique strategies in order to survive colder winter months, and these cold-hardiness strategies are divided into two categories: Freeze avoidance, and freeze tolerance.
The difference between these two strategies seems somewhat self-explanatory; you either freeze and bear it, or avoid freezing altogether. Remember, insects are ectothermic, meaning they cannot generate their own body heat; so for those that can’t migrate to warmer climates, winter presents some unique challenges.
Whether an insect uses freeze avoidance or freeze tolerance is largely dependent on the hemisphere it’s located in. Insects in the northern hemisphere have mostly adapted freeze avoidance strategies because the winter months are longer, colder, and more extreme. Insects that have adapted freeze-tolerant strategies tend to dominate the southern hemisphere, where cold temperatures are short-lived and less extreme. We’ll cover freeze tolerant insects in a separate issue.
But we’re talking about insects here, so you know it gets weirder than that.
In order for ice formation to occur, ice crystals need a nucleation site. Essentially, ice crystals won’t form if they don’t have something to “grab” onto. Water and other liquids can be cooled far beyond their freezing point without a nucleation source; water, for example, can reach -42°C (-43.6°F) and remain liquid if no nucleation source is introduced. This process is adequately named “supercooling,” and insects use it to depress the temperature at which their body fluids will freeze.
As the weather turns colder, insects begin cold-hardening, the process by which they prepare for colder temperatures. In freeze-avoidant insects, this process involves removing all potential ice nucleation sources from their body, including food particles and bacteria. Some simply stop eating, others will shed the lining of their midgut entirely during the molting process.
Some insects completely alter their metabolism in order to supercool their body fluids through the use of cryoprotectants. “Cryoprotectant” is a catch-all term referring to the various substances produced by ectothermic animals to protect their tissues from freezing. The most common cryoprotectant produced by insects is glycerol, which essentially renders their body fluids too viscous to form ice crystals at freezing temperatures.
Finally, one of the most fascinating freeze-avoidant strategies is the production of antifreeze proteins (AFP’s – or, if you really want to impress your friends, thermal hysteresis factors (THFs)). Antifreeze proteins are produced by all freeze-avoidant insects, and are quite literally a family of proteins that prevent freezing from occurring by inhibiting the growth of ice crystals. The production of antifreeze proteins is triggered by day length; as daylight decreases and insects experience a shorter photoperiod, the production of antifreeze proteins ramps up, and reaches its peak during the darkest months of winter.
In the coming weeks, we will explore the overwintering strategies of freeze-tolerant insects.