One of the absolute joys of early spring is catching sight of a fluttering, lemon yellow butterfly. Amazingly the word ‘butter-fly’ is thought to have derived from "butter-coloured fly,” because of the yellowness of the male brimstone butterfly's wings! The brimstone is a long lived butterfly, they live for up to a year after hatching into their adult form. After sleeping through the winter, the adults wake up and travel to find a mate, that’s why now is such a great time to spot them. As well as their pale yellow colour, you can tell it’s a brimstone because of their distinctive leaf shaped wings.
The brimstone is a large butterfly, with a wingspan of 55-60mm. Whilst the males are lemony yellow, the females are paler and more greenish-white with orange spots in the middle of each wing. Brimstones always land with their wings closed and rest with their wings closed, when their exquisite wing shape really pays off. They perfectly match a leaf when roosting overnight or hibernating within foliage, keeping them safe from hungry eyes. These are early to bed butterflies, settling down for the night around 3.00-4.00pm, so best to catch them in the morning.
Fertilised eggs are laid on the underside of the favourite food plants of the larvae, buckthorn and alder buckthorn leaves. Once hatched, the caterpillar goes through several development stages before pupation over the summer. Their caterpillars are green and very well camouflaged, being the same colour as the leaves of their food plant, you can discover them, often found lying along leaf margins. As you might expect the distribution of this brimstones closely correlates with that of their larval food plants.
The caterpillars are vulnerable to heavy predation and broods may be decimate by birds such as Blue Tits. If they survive live along, new adults emerge from their chrysalis in July, and as adult butterflies, they need to feed on nectar from flowers. They prefer to drink from purple flowers, and bluebells are an important early nectar source. The Brimstone has a very long proboscis (their straw like mouth part) so they can nectar on flowers which most other UK butterflies cannot, such as runner beans and teasels.