The results of this year’s Big Butterfly Count have just been announced by Butterfly Conservation. This is the charity dedicated to butterflies and moths which is behind this annual event in citizen science. The results make stark reading. The overall number of butterflies recorded per count was the lowest ever since the Big Butterfly Count began 12 years ago. Butterfly numbers fluctuate naturally from year to year, so it is tempting to view the 2021 results as a ‘one off’. Undoubtedly this year’s weather had a significant impact on butterfly numbers. Nighttime temperatures in April were very low, and May was a particularly poor month weatherwise, being very wet and cool. These factors had an impact on butterflies’ ability to feed and breed. The biggest effect was on spring butterflies and particularly those species which have two broods a year such as Small Tortoiseshells, Peacocks and Common Blues The latter two species were particularly hardest hit nationally, being down 63% and 59% respectively on last year’s figures.
It would be easier to dismiss 2021 as an aberration if all was well with the environment. Longer term trends in butterfly numbers show a worrying decline. “Since 1976, 76% of butterflies have declined in abundance and distribution and the downward trend continues”, stated Julie Williams, CEO of Butterfly Conservation, on the organisation's website. The decline is probably due to a combination of factors including loss of habitat by intensive farming practices, lack of woodland management and the loss of unspoilt countryside to irresponsible industrial and commercial development. Climate change has also played its part resulting in an increase in extreme weather events such as violent summer storms, flooding and mild winters.
How did butterfly numbers fare in the Marlow area compared to previous years? In the surveys I carry out, local factors can play a big part in the number of butterflies seen from year to year. For instance, whether a field is laid to crop (and which crop) or left fallow may have a significant impact on particular species of butterfly seen from year to year in a specific area. Some of the trends I have seen locally do not match national results. According to my observations, Peacocks have generally had a good year in the Marlow area, but they were largely the exception. Of the 23 or so species that I see regularly locally, 14 were down on the previous year’s numbers. Particularly hardest hit were Small Tortoiseshells of which I saw only four specimens in the whole of 2021 This species used to be one of our most common garden butterflies and was found in a wide variety of habitats. The last two years in particular have seen numbers drop dramatically and this butterfly is now becoming much more of a rarity. One of the reasons is likely to be an increase of parasitic fly which is extending its range northwards. This fly is common in continental Europe and is now, due to climate change and increasing temperatures, much more widespread in the south of the UK. This fly’s eggs are ingested by Small Tortoiseshell caterpillars feeding on nettles. The hatching parasite grubs feed on the inside of the host until they eventually kill it. The rapid decline of the Small Tortoiseshell should demonstrate that we should not be complacent and that an insect that we once took for granted as an integral part of the British countryside and our gardens is quickly disappearing.