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Butterflies after the big freeze

Jeremy Torrance web producer Jeremy Torrance web producer | 14:28 UK time, Wednesday, 28 July 2010

Guest blogger: Ray Barnett is a naturalist and entomologist who has worked in museums for over 20 years. Publications include the main author for the 'Butterflies of the Bristol Region' and the 'Moths of the Bristol Region', with a volume on dragonflies in the pipeline.

It may seem a distant memory now but cast your minds back to the snow and ice of last winter. We shivered and slithered in conditions which have become less familiar to a younger generation. What impact did the snow cover and low temperatures have on our native wildlife?

The smaller bird species are vulnerable to freezing to death, losing body heat thanks to their relative high surface area to volume ratio. Many other species of bird and mammal may struggle to find food, but what about some of those food sources themselves? In particular, what impact does a hard winter have on our insect fauna?

Perhaps surprisingly a hard winter may actually assist many species which we usually consider to be common native species. Butterflies that hibernate as the adults - especially small tortoiseshell, peacock and brimstone - may actually benefit from a 'proper' winter.

In mild winters relatively high temperatures may result in early emergence from hibernation at a time when nectar sources from which to feed are hard to find. What's more, if the warm period is followed by a cold snap, these insects are highly unlikely to be able to re-enter hibernation and most likely to die.

So is there any evidence to show that last winter did have a positive affect on butterfly populations? Finding hard evidence and being able to correlate that directly to one cause is very difficult. However, there is some anecdotal evidence that some species have done better this year than in some recent ones. Small tortoiseshell may be a case in point, although any resurgence may be due to changing population levels of the parasitoid fly Sturmia bella (a parasitoid is a parasite which ends up killing its host).

But here's the twist, S. bella is one example of numerous species of insects which have invaded this country from continental Europe in the last 20 years or so, possibly because they have been able to survive as our winters have become milder. So did a harder winter actually reduce the number of these parasitoids thereby assisting the small tortoiseshell numbers? Not at least in the spring as the fly does not affect the hibernating adult butterfly only the developing caterpillars but reduced numbers of the parasitoid may have helped greater numbers of the spring brood of larvae to survive to adulthood.

Have other recent arrivals faired badly? One other example is that of the red admiral butterfly. Always a common species as an annual immigrant from the continent but only in recent years able to survive as an over-wintering adult in the UK. Initially it looks like the winter has not adversely affected these new colonists but a succession of hard winters might prove a different kettle of fish.


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