Where do butterflies go in Winter and what can we do to help their survival?

Despite their delicate nature, butterflies and moths are surprisingly resilient to the winter freeze.

Different species of these insects have adopted different strategies to survive the coldest months:  Some overwinter as dormant adults, sheltering in caves, buildings and vegetation; some as eggs; but the majority spend the winter as caterpillars or pupae, hidden underground or in cosy nooks of plants.  A few species even remain active either as adults or as caterpillars during the winter.

Providing habitats to allow overwintering for moths and butterflies in the form of leaf litter and log piles, or postponing pruning and leaving shrubberies more overgrown can only be good for protecting eggs, caterpillars and chrysalises.  (But please don’t ever be tempted to disturb these dormant creatures!)

If you have already seen a butterfly flying in the UK this year, it is likely to have been one of the species that overwinters as an adult – Red Admirals, Peacocks, Brimstones, Tortoiseshells and Commas – which all fall into this category. These butterflies will emerge to search for nectar on warmer days.

But how do the cold winters affect the overall populations of our butterfly and moths species?

Previous evidence has shown a positive correlation between temperatures in summer and butterfly populations – most butterfly species increase in numbers in warmer summers.  But new research at East Anglia University has revealed that many UK butterflies actually do better after a winter of colder overall temperatures.  Not only was cold weather found to improve life chances, over half of the 41 species studied were negatively affected by warm winter weather regardless of the life cycle stage at which they overwintered.

All four of the adult hibernators (Brimstone, Small Tortoiseshell, Peacock and Comma) showed negative relationships with extreme winter warmth, as did some species that spend the winter as eggs (e.g. Purple Hairstreak), as larvae (e.g. Dingy Skipper, Dark Green Fritillary, White Admiral, Common Blue) and as pupae (e.g. Green-veined White, Orange-tip, Green Hairstreak). Only in two of the studied species (Wall and Holly Blue) was there a significant positive effect of very warm winters.

So, the conclusion is that while cold spells during winter tend to be neutral or beneficial in impact on butterfly populations, warm spells in winter are generally detrimental.  And if winters are predicted to become milder in the UK as part of the bigger issue of climate change, the negative knock-on effects on our butterflies would be disastrous.