How Butterflies Beat the Freeze by Martin Robinson

• In a recent blog we discussed how butterflies spend the winter, either as eggs, caterpillars, pupae or adult insects. One of the biggest challenges of making it through the coldest part of the year is to prevent being frozen.

  When water turns to ice it forms rigid crystal structures. We are all familiar with those wonderful pictures of individual snowflakes and their six-point starlike shapes. When water freezes inside the delicate tissues of creatures such as butterflies, the crystalline structures of ice disrupt and destroy the internal organs resulting in death. We can see the damage that severe subzero temperatures can do to humans from the experience of some polar explorers who have lost fingers and toes due to extreme frostbite.

  Peacock butterflies spend the winter as adult butterflies in hibernation, (to be more precise in a dormant state). They survive by trying to find sheltered places such as outhouses, garden sheds, and lofts in which to roost for the winter. These spaces are free from frost, and provide an environment with fairly even temperatures, especially when the outside temperature dips below zero.

  Many species of butterfly spend the winter as a caterpillar. The White Admiral is one such example, and although not a particularly common butterfly, it can be found flying during the summer in local woodlands such as Maidenhead Thicket, Ashley Hill Forest and Homefield Wood. However, in winter it spends its time as a dormant caterpillar. Before going into hibernation, the larva wraps itself in a honeysuckle leaf.  This not only disguises it from predators, but the leaf also acts as insulation against the cold. Grassland species like Meadow Browns and Gatekeepers adopt a different strategy. Their larvae spend winter low down on grass stems where there is plenty of shelter and on milder winter days will even rouse themselves from their dormant state to briefly feed.

  Scientific research has been carried out on how some insects can survive subzero temperatures. Incredibly before going into hibernation, some insects increase levels of natural antifreeze chemicals such as glycerol and trehalose (a type of sugar) within their bodies. These chemicals prevent the formation of ice crystal structures at sub-zero temperatures and so protect the internal tissues of the insect.

  The toll on hibernating butterflies in whatever state they chose to spend the winter is enormous. Great numbers are predated by birds, such as blue tits, who need a source of protein to sustain themselves during the winter. Many butterfly species, despite their best efforts, do succumb on very cold days. Ironically mild winters are also very damaging for butterfly populations. On warmer days, when the temperature gets into double figures and there is a bit of sunshine around, the conditions fool butterflies into thinking spring has arrived. Those such as Peacocks, will take to the wing using up valuable reserves of energy trying to seek out scarce nectar sources in vain.  Some caterpillars will also stir, looking for food sources which, in the majority of cases, are sadly absent. This results in depletion of their energy reserves below the level at which they can survive the winter. Unfortunately, due to climate change, milder wetter winters are becoming more common in the UK with the resulting negative impact on our native butterfly species.

  It seems a miracle that such fragile creatures can survive the ravages of winter, but they do, returning to our gardens and countryside, welcoming in each spring and raising our spirits again!