Look Out For: Brimstone Butterflies

Brimstone

For many people, it is the warm, yellow glow of a male Brimstone butterfly in flight that really signals that spring has sprung and that the new year’s bounty of butterflies is on its way.

I’m not sure why this species engenders such a strong psychological link with springtime in the human mind – after all there are other colourful butterflies (such as the Peacock, Small Tortoiseshell and Red Admiral) around on the first warm days of the year too. Perhaps it is the Brimstone’s colour that is so evocative – the colour of weak spring sunshine, of wild daffodils and primroses (indeed an early alternative name for the Brimstone was ‘the Primrose’).

BrimstoneIt is only the male butterfly that has the sulphur-yellow colour though; the Brimstone is a sexually dimorphic species, with the sexes having a different appearance. The female butterfly has much paler, cream-coloured wings and, in flight, could quite easily be mistaken at a glance for a Large White. Closer inspection, however, reveals the distinctive points on the Brimstone’s wings.

The Brimstone seemed to have a good year in 2013 and that, together with the mild winter may explain the large number of sightings reported in the last couple of weeks. The results of both the Wider Countryside Butterfly Survey and the Big Butterfly Count indicated large increases in Brimstone abundance in summer 2013 compared with 2012. However, both these surveys take place in the summer and so don’t record the springtime generation of Brimstones - we’ll have to wait for the UK Butterfly Monitoring Scheme results later in the month to get the full picture.

We do know, though, that the long-term situation for Brimstone in Britain is favourable, in stark contrast to most of our butterfly species. Brimstone national population levels have remained stable since the 1970s and the butterfly has spread northwards, colonising much of Lancashire, Yorkshire and County Durham. 

Brimstone caterpillarThe Brimstone is a highly mobile butterfly and one that can be encouraged to visit and breed in gardens. The caterpillars only feed on the native shrubs Buckthorn (Rhamnus cathartica) and Alder Buckthorn (Frangula alnus), both of which can be grown in sunny, sheltered parts of the garden (the former preferring a chalky soil and the latter a wet or acidic soil). With luck and patience you may be rewarded by sightings of female Brimstones egg-laying on the emerging leaves in springtime and of the green caterpillar which sits in a characteristic ‘humped’ posture along the midrib of a leaf.

Richard Fox
Surveys Manager
Follow me on Twitter: @RichardFoxBC