Ed Dolphin from Sidmouth Biodiversity Group writes for the Herald.

Fluttering heralds of spring, hibernating Brimstone butterflies sometimes take wing on mild days in February and so are among the first butterflies seen in spring. There is a chance to see numbers of these yellow butterflies twice during the year, in late spring when all the over wintering population wake up to breed, and in late summer when the next generation emerges from their metamorphosis.

Most hibernating creatures hide away somewhere to protect themselves from the cold, but Brimstones have a remarkable ability to survive being frosted and they hibernate in comparatively open settings.

Frost kills living things when ice crystals form inside the cells and this ruptures the cell walls. In the autumn, Brimstone butterflies reduce the amount of water in their bodies and build up glycerine as a body fluid antifreeze and this allows them to survive temperatures below -10o.

Brimstone is the old English name for yellow sulphur and the bright yellow butterflies have acquired the old name. The males are a buttery yellow and the term butterfly itself may have arisen because of this. The females are paler. They lay their eggs on alder buckthorn in May or June. The generation of over-wintering butterflies then dies off.

The green caterpillars are camouflaged against the leaves and when they are resting they tend to line themselves up along the midrib of the leaf which makes them very difficult to see. They are full grown and pupate in late July and into August.

The new adult butterflies emerge in late August and then begin feeding ready for the oncoming winter hibernation. They range widely and usually settle in open woodland where they too are well camouflaged at rest, looking like pale leaves hanging on a twig. As winter sets in they begin their hibernation and the cycle moves on.

The caterpillars’ food plant alder buckthorn is a shrub that grows in the boggy ground at the head of our valley. Members of the Sid Valley Biodiversity Group and Sidmouth Arboretum are hoping to extend this by planting alder buckthorn lower down the valley to create new breeding grounds for these delightful creatures.