Insect winter ecology
Loss of enzymatic function and eventual freezing due to low temperatures daily threatens the livelihood of these organisms during winter.
Not surprisingly, insects have evolved a number of strategies to deal with the rigors of winter temperatures in places where they would otherwise not survive.
Two broad strategies for winter survival have evolved within Insecta as solutions to their inability to generate significant heat metabolically.
The longest one-way flight on record for monarchs is 3,009 km from Ontario, Canada to San Luis Potosí, Mexico.
Insects that do not migrate from regions with the onset of colder temperatures must devise strategies to either tolerate or avoid lethal freezing of intracellular and extracellular body fluids.
In temperate regions of the northern hemisphere where cold temperatures are expected seasonally and are usually for long periods of time, the main strategy is freeze avoidance.
In temperate regions of the southern hemisphere, where seasonal cold temperatures are not as extreme or long lasting, freeze tolerance is more common.
Alternatively, substances that facilitate the aggregation of water molecules can increase the probability that they will reach the critical size necessary for ice formation.
[10] Therefore, when an insect maintains its body fluids in a supercooled state, there is the risk that spontaneous ice nucleation will occur.
[14] Thus, some insects avoid freezing by selecting a dry hibernation site in which no ice nucleation from an external source can occur.
[4] The stage of development at which an insect over-winters varies across species, but can occur at any point of the life cycle (i.e., egg, pupa, larva, and adult).
Although polyols such as sorbitol, mannitol, and ethylene glycol can also be found, glycerol is by far the most common cryoprotectant and can be equivalent to ~20% of the total body mass.
[17] Glycerol is distributed uniformly throughout the head, the thorax, and the abdomen of insects, and is in equal concentration in intracellular and extracellular compartments.
At colder temperatures (below 0 °C), glycogen production is inhibited, and the breakdown of glycogen into glycerol is enhanced, resulting in the glycerol levels in freeze-avoidant insects reaching levels five times higher than those in freeze tolerant insects[19] which do not need to cope with extended periods of cold temperatures.
[20] A seasonal photoperiodic timing mechanism is responsible for increasing the antifreeze protein levels with concentrations reaching their highest in the winter.
Insects that have evolved freeze-tolerance strategies manage to avoid tissue damage by controlling where, when, and to what extent ice forms.
[25] The fat body is an insect tissue that is important for lipid, protein and carbohydrate metabolism (analogous to the mammalian liver).
[27] Although freeze-avoidance strategies predominate in the insects, freeze tolerance has evolved at least six times within this group (in the Lepidoptera, Blattodea, Diptera, Orthoptera, Coleoptera, and Hymenoptera).
It has been suggested that this may be due to the Southern Hemisphere's greater climate variability, where insects must be able to survive sudden cold snaps yet take advantage of unseasonably warm weather as well.
This is in contrast to the Northern Hemisphere, where predictable weather makes it more advantageous to overwinter after extensive seasonal cold hardening.
As a result, cryoprotectants like glycerol decrease the amount of ice that forms outside of cells and reduce cellular dehydration.
Ladybugs practice communal hibernation by stacking one on top of one another on stumps and under rocks to share heat and buffer themselves against winter temperatures.
[37][38] Other methods of hibernation include the inhabitance of bark, where insects nest more toward the southern side of the tree for heat provided by the sun.
[15]: 149 In addition to using freeze tolerance, many aquatic insects migrate deeper into the water body where the temperatures are higher than at the surface.
