The conventional wisdom among UK butterfly enthusiasts is that cold winters are generally good for butterflies. However, until recently there was little scientific evidence to support this perception.

Indeed, previous analyses correlating butterfly population size measured by the UK Butterfly Monitoring Scheme (UKBMS) with weather data had concluded that warm summer weather was the key positive factor associated with numbers of adult butterflies in most species1,2. 

In 2017, however, a team of scientists from the University of East Anglia, Butterfly Conservation and the Centre for Ecology & Hydrology (CEH) published new research into the effects of weather on the abundance of UK butterflies3. 

Peacock (Overwintering) - Iain Leach

The study, which examined the impact of extreme climatic events on UK butterfly populations over the period 1976-2012, and looked for effects across all life cycle stages, confirmed the importance of warm weather during the adult flight period but also uncovered strong effects during winter. The researchers found that extreme winter warmth exerted significant detrimental effects in just over half (51%) of the 41 species that were analysed. Extreme winter warmth was defined as days with a maximum temperature that was two standard deviations higher than the mean daily maximum temperature for the winter period at each UKBMS transect site. 

The 21 species negatively affected included both common ‘wider countryside’ butterflies and ‘habitat specialists’, and species that overwinter in all four different life cycle stages. Thus, 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 (5%) of the studied species (Wall and Holly Blue) was there a significant positive effect of very warm winters on population size. 

In addition, the abundance of 13 species (32%) 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. 

Thus while cold spells during winter tended to be neutral or beneficial in their impacts on butterfly populations, unusually warm spells in winter were generally detrimental. The mechanisms underlying these effects are not yet known and represent an obvious avenue for further research. It has been suggested that mortality caused by pathogens may increase in milder winters or that warm temperatures may disrupt dormancy resulting in harmful exposure to winter weather or depletion of stored energy reserves. 

Chalk Hill Blue

In general, extreme rainfall events during winter had less impact on butterflies than extreme temperatures. There were positive correlations for six species (15%) (including Small White and Small Heath), while populations of just three species (7%) (Speckled Wood, Ringlet and Meadow Brown) decreased in response to extreme winter rainfall. 

Another recent study also examined winter weather impacts on butterflies using the UKBMS dataset gathered by Butterfly Conservation and CEH. In this case, the researchers from Stockholm University studied the phenology of the spring flight period of five species that overwinter as pupae4. It is well known that higher spring temperatures can cause the earlier appearance of butterflies and this was again shown in the new study. However, 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 1 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 warm springs promote the earlier emergence of butterflies but warm winters have the opposite effect. 

Richard Fox

Associate Director of Recording & Monitoring, Butterfly Conservation 

 

  1. Pollard, E. (1988) Temperature, rainfall and butterfly numbers. Journal of Applied Ecology 25: 819–828.
     
  2. Roy, D.B., Rothery, P., Moss, D., Pollard, E. & Thomas, J.A. (2001) Butterfly numbers and weather: predicting historical trends in abundance and the future effects of climate change. Journal of Animal Ecology 70: 201–217. doi:10.1111/j.1365-2656.2001.00480.x
     
  3. McDermott Long, O., Warren, R., Price, J., Brereton, T.M., Botham, M.S. & Franco, A.M.A. (2017) Sensitivity of UK butterflies to local climatic extremes: which life stages are most at risk? Journal of Animal Ecology 86: 108–116. doi:10.1111/1365-2656.12594
     
  4. Stålhandske, S., Gotthard, K. & Leimar, O. (2017) Winter chilling speeds spring development of temperate butterflies. Journal of Animal Ecology 86: 718–729. doi:10.1111/1365-2656.12673