As summer draws to a close, there is still an abundance of butterflies here at the Botanic Garden, making the most of the late-flowering plants to gain enough energy to survive the winter. But have you ever wondered what butterflies actually do in winter – where do they all go?
Butterflies are ectothermic, meaning that they mainly rely on external sources of heat to warm their bodies to be active. This is why butterflies are often seen basking in sunshine or shivering by vibrating their flight muscles to raise their body temperature to prepare for flight. On the other hand, butterflies will seek shade under leaves and on tree trunks to cool down if they get too hot.
In temperate climates, such as here in Wales, the winter poses a problem for butterflies as they are unable to gain heat from the environment to be active and there is little food available. Some species, like the Painted Lady (Vanessa cardui) avoid winter conditions completely by migrating back to warmer climates each autumn. However, most UK species remain here and survive the harsh Welsh winters by going into hibernation. Technically, insects don’t hibernate, but rather enter a state of dormancy without any sign of life, with their normal physiological functions greatly altered. This allows them to live for many months without food or drink and to survive freezing temperatures.
Some butterflies hibernate as eggs, but the majority of butterflies overwinter in their larval stages (as caterpillars), including some of the Browns, Whites and Skippers. The Small Skipper (Thymelicus sylvestris) has only one generation a year and lays its eggs in sheaths of grasses. When the larvae emerge, they construct a silk cocoon within the sheath of grass, and hibernate in this chamber, before emerging again the following spring to feed and undergo metamorphosis. Similarly, Gatekeepers (Pyronia tithonus) also hibernate through winter as caterpillars, before continuing development the following spring.
The Large and Small Whites (Pieris brassicae and Pieris rapae) overwinter as pupa (chrysalis), rather than as larvae, slowing down their growth and emerging the following spring. For pupae that overwinter, this stage in their life cycle can last for many months, whereas the summer brood only takes a few weeks to transform into adult butterflies. The Speckled Wood (Pararge aegeria) can hibernate as either caterpillars or chrysalises, therefore some individuals hibernate as larvae and others as pupa, with different emergence times in spring.
Only a small number of our resident butterflies overwinter as adults, including the Brimstone (Gonepteryx rhamni), Small Tortoiseshell (Aglais urticae), Peacock (Aglais io) and Comma (Polygonia c-album). The Comma hibernates in woodland, often in hollow trees or log piles with their wings folded to show the dark undersides allowing them to remain camouflaged. Small Tortoiseshells and Peacock butterflies are often found overwintering inside houses and sheds, as they provide a suitably cool and dry shelter during the late summer and early autumn. However, this causes problems later in the year when the central heating inside our homes causes the sleeping butterflies to be awoken prematurely, and the weather outside is still cold with few sources of nectar available. Climate change also presents a problem for hibernating butterflies, because unusually high winter and spring temperatures can also wake them up too soon before there are enough food supplies to sustain them.
The Painted Lady has a completely different strategy, preferring to migrate rather than hibernate. This remarkable butterfly is a long-distance migrant, arriving in the UK every summer from Europe and Africa. Scientists have only recently discovered that they also undertake a return migration southward each autumn, making a 14,500 km round trip from tropical Africa to the Arctic Circle and back! The whole journey is not undertaken by individual butterflies, but rather by a series of up to six successive generations due to its unique system of mating throughout all seasons.
Butterflies have a wide variety of strategies to survive winter, but it is unclear why some butterflies favour different methods of hibernation or migration over others. As the weather begins to get colder and the days get shorter, many of the butterflies will be getting ready to go into hibernation, hopefully surviving the winter months ready to wake up and return to the Garden on the first warm days of spring!
References
Richard Fox, Butterfly Conservation, Available: https://butterfly-conservation.org/news-and-blog/where-do-butterflies-and-moths-go-in-winter
Butterfly Conservation, Available: https://butterfly-conservation.org/news-and-blog/painted-lady-migration-secrets-revealed