Cold comfort

Peacock Butterfly

The Met Office is forecasting a cold month ahead and the conventional wisdom among butterfly enthusiasts is that cold winters are generally better for butterflies than warm ones. There was little hard evidence to back up this perception, until this year when a team of scientists from the University of East Anglia, Butterfly Conservation and the Centre for Ecology & Hydrology published new research into the effects of weather on the abundance of UK butterflies.

Previously, data collected through the UK Butterfly Monitoring Scheme (UKBMS) has been used to show a positive relationship between summertime temperatures and butterfly populations (in most species); most butterflies increase in numbers in warmer summers.

The new study, which examined the impact of extreme climatic events on UK butterfly populations over the period 1976-2012, found, for the first time, that extreme winter warmth had significant detrimental effects in just over half of the 41 species studied. 

Hibernating PeacockThe 21 species negatively affected include both common ‘wider countryside’ butterflies and ‘habitat specialists’,  and concluded that species could be impacted 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.

Chalk Hill BlueAdditionally, the population size of 13 species increased significantly in association with extremely cold winter days (including Large Skipper, Large White, Ringlet and Chalk Hill Blue), while only two species (Wall and Brown Argus) showed a negative relationship.

So, while cold spells during winter tended to be neutral or beneficial in their impacts on butterfly populations, warm spells in winter were generally detrimental.

Another interesting, recent study, this time from Stockholm University, also looked at winter weather impacts on butterflies using UKBMS information. In this case the researchers examined the timing (phenology) of the spring flight period of five species that overwinter as pupae. It is well known that warmer spring temperatures can cause the earlier appearance of butterflies and this was again shown in the new study. The scientists also showed a significant effect of winter temperature for three of the five species studied; Orange-tip, Green-veined White and Green Hairstreak flew earlier in the spring following cold winters. For example, for every 10 days of winter cold (defined as temperatures below 7.2°C) the Orange-tip flight period occurred one day earlier in spring on average. There was also an indication of the same effect in the other two species (Grizzled Skipper and Holly Blue) but the results were not statistically significant.

This study reveals a counterintuitive situation, whereby butterflies fly earlier in the year during a warm spring but not after a warm winter.

Winters are predicted to become milder in the UK as part of climate change, with negative and unexpected knock-on effects on our butterflies - a reason to hope for a white Christmas.

Richard Fox

Head of Recording, Butterfly Conservation