Butterfly numbers bring some good news

In recent years those of us monitoring Yorkshire's butterflies and moths have been getting used to receiving an ever-increasing number of mid-winter records. These are invariably individuals which have gone into hibernation and can readily awake if there is strong sunshine and an air temperature rising to 12-13C or above. Not this year.

So far, I have only received a handful of winter butterfly reports, including a small tortoiseshell at Spurn in January and a peacock in Wath-on-Dearne in February. Clear skies and strong sunshine in early March tempted out a few more peacocks, one of which was rescued from the sea in Scarborough. Soon there should be more.

It usually takes until at least mid-March for the butterfly flight season proper to get underway and it may be April before there is a widespread appearance. Cold winters are thought to be good for butterfly hibernators. They depend on finding nectar when they awake and are unlikely to survive without flowers. The later they wake, the more likely they are to find a food source.

Butterflies spend winter in many different ways. Besides the hibernators like small tortoiseshell, brimstone and peacock, some, like the small white and large white are warmly wrapped up as a chrysalis or pupa. Inside the casing is a fully-formed butterfly ready to unfurl and fly away.

Yet other species spend winter as caterpillars, able to feed when it is mild enough and curl up to sleep in deep vegetation when it is too cold. A further group are still in the form of tiny eggs, stuck on a twig or a plant and almost invisible. As it gets warmer, a very small caterpillar will emerge and start eating. Its turn to be a butterfly will be later in spring or even well into summer.

I spend winters, not in hibernation unfortunately, but stuck in front of my computer amassing statistics from the records sent in by members of our recording team. If you followed my monthly series of articles last year, you will be aware of our growing concerns about the effects of the heavy summer rains and floods in 2007 and 2008. Butterfly numbers were certainly shrinking. Many species seem particularly vulnerable to heavy rains when the caterpillars are just emerging and can get washed off their food plants, and more readily become prey for other creatures.

The good news is that 2009 saw a marked recovery. Our rarest species are still under threat, but, with a couple of exceptions our regular 30 species did much better than expected. Benefiting from the same number of regular recorders in each year, 2008 produced a rounded total of 31,000 reports, and 2009 an all-time record of 47,000 reports.

Some of the increase might be attributed to extra recorder effort in better weather. But there is no doubt that on many sites there were simply more butterflies to count, and that wasn't due to the big influx of painted ladies alone. In 2008 the total count across all species was 172,000 butterflies; in 2009, 322,000.

This doesn't reflect the situation in every part of the county. Much of the increase was limited to the eastern half. The west suffered considerable rainfall in July and August while the east remained much drier and sunnier. At a recent national conference of butterfly recorders, I learned that this was also the case throughout much of Britain. However, in Yorkshire, two species which were showing alarming drops in population, the small tortoiseshell and the wall, saw re-assuring increases. Let's hope it continues.

Howard M Frost is Butterfly Conservation's voluntary organiser of butterfly recording in Yorkshire. He can be contacted via www.yorkshirebutterflies.org.uk, where you can send in reports and find daily updates of the butterflies and moths being recorded around the county in spring.