Butterfly with leaf-like wings

With its leaf-like wings, the sight of the Brimstone butterfly is all the more rewarding among spring’s foliage

on a warm April day in a sunny cottage garden, a glimmer of yellow wafts softly over the rich purple blooms of an aubretia spilling from a drystone wall. This silent dancer is a Brimstone butterfly, its name an alternative word for sulphur and one which reflects the bright lemon hues of its leaf-shaped wings as it flutters from flower to flower. On settling, the butterfly closes its wings, revealing their greenish-yellow underside, and uses its long proboscis to feed on the energy-giving nectar before flitting onto the next bloom. After feeding, it will rest, wings open, taking advantage of the sun’s warming rays. The Brimstone, Gonepteryx rhamni, is one of the first butterfly species to appear in the spring. For the past six to seven months, this individual has been in a state of torpor, having reduced its energy levels so that it does not need to feed. It has been hiding away in a nearby woodland, behind thick green leaves. Now that the air temperature has risen, and the hours of daylight are growing longer from day to day, it is time for this hardy butterfly to resume its life cycle again. The Brimstone is one of the larger British butterflies, with an overall span ranging from 2½-3in (6-7.5cm). The wings of the males are lemon-yellow above and yellowish-green on the underside, and they have a greyish-white body. The females are a paler green, so may be easily missed when at rest among foliage. Both males and females have lines, or veins, crossing their wings and orange spots in the middle of each upper

wing, which also help to conceal them from predators. Brimstones, like other butterflies, are predated on by a range of different birds, mammals and insects, especially wasps. For this reason, they have evolved a range of different ways to camouflage themselves throughout the different stages of their life cycle: as eggs, caterpillars, pupae and adults. They are also parasitised by two species of wasp, each of which target only this particular butterfly.

Varied habitats

Brimstones are often found in woods where, on bright, sunny days, they will fly along the edge of clearings and woodland rides or wherever the sun penetrates through the forest canopy. They can also be seen along mature hedgerows, at the edge of farmland, especially where a border of grass and flowers has been left to thrive, and in rough grassland. Another good space to spot them is along roadside verges, which, because they are often left unsprayed with chemicals, have more wildflowers than much of the surrounding countryside. Like several other butterflies, the Brimstone is a frequent garden visitor, especially where there are plenty of flowering plants on which the adults can feed. The Brimstone has a fairly wide range and can be found across most of lowland England and parts of Wales, having spread northwards into northern England in recent decades, probably as a result of warmer summers caused by climate change. However, this species is rare in Scotland and thinly-distributed in Ireland.

“Though the thicket’s bushy dell Tempts thee to the foxglove’s bell, Come but once within my bounds, View my garden’s airy rounds, Soon thou’lt find the scene complete, And every flowret twice as sweet” John Clare, ‘To the Butterfly’

Like many other common British butterflies, their numbers have fallen in recent years: the UK population has almost halved since the 1970s. This is mainly due to the destruction of hedgerows. Worldwide, however, the common Brimstone has a widespread distribution, being found in suitable habitats throughout the temperate regions of Europe, Asia and North Africa, so it is not endangered on a global scale. Brimstones are able to spread because they are strong flyers, with the ability to live in a wide range of habitats. However, they are limited by the food plants on which their young feed: common buckthorn and alder buckthorn, both of which are found in damp woodland, while common buckthorn can also grow on chalk and limestone grassland, and around wetlands.

Courtship and breeding

Most Brimstones emerge from their winter hiding places in early spring, generally late March and April. But they may appear on warm days as early as January or February, and occasionally even as soon as New Year’s Day, making them one of the first butterflies to be seen each year. The males appear a few days earlier than the females, allowing them to feed and build up their strength before mating. Having emerged, both sexes will feed on the nectar of dandelions, primroses and bluebells for a few weeks, before beginning their courtship display. This involves the male flying up into the air and back down again, the two then fluttering around one another to cement their pair bond. Once they have paired, the male and female will mate, copulating for up to 48 hours. They are monogamous and, having mated, will not attempt to pair up with another partner.

In May, the female lays her eggs, one by one, on the undersides of the leaves of buckthorn and alder buckthorn plants, placing a single egg on each leaf to minimise the chances of them being eaten by birds. She will try to choose only undamaged leaves, as any holes may indicate the presence of other Brimstone caterpillars, which could, in turn, attract predators. The eggs are approximately 1.3mm long, 0.5mm wide and shaped like a skittle. When laid, they are greenish-white. Over the next week or two, they gradually change colour, turning first yellow, then brown, which indicates that they are ready to hatch. When the caterpillars emerge from the egg, they are just 1.7mm long. However, they grow very rapidly, as they feed voraciously on the buckthorn leaves, and undergo up to five ‘instars’, the period between moultings, when they shed their outer layer to enable them to grow. By the time they are fully grown, after a further five to six weeks, they will have reached a length of 1½in (3.5mm). Brimstone caterpillars are mainly green in colour, with white hairs and small black markings, which help to break up their outline and conceal them as they feed on the top surface of the leaves. Once the caterpillars reach their full length, they pupate, a stage that lasts for approximately two weeks. The pupae are just under 1in (25mm) long, and green in colour, so that they resemble a curled-up leaf. They hang from the stems and leaves of the buckthorn plants by a thin strand of silk. The adult butterflies emerge from July to August, by which time their parents will have died, their life’s purpose, to create a new generation of Brimstones, having been fulfilled. These fresh adults feed on the nectar of summer flowers through to September, preferring red, purple and blue blooms, such as thistles, and can also use their long proboscis to probe into teasels.

Winter shelter

As the temperature begins to drop, and the days get shorter towards the autumn equinox, Brimstones seek out places to hide away for the coming colder months. Unlike other overwintering butterflies, which often spend the season in man-made outbuildings, such as sheds, Brimstones usually prefer the dense evergreen foliage of holly or ivy, or a clump of brambles. This may be because their colour and shape blends in more closely with those plants. However, because they will sometimes emerge to feed on unseasonably warm winter days, Brimstones may be seen in any month of the year. For many summer butterfly species, life is short at only a few weeks or a couple of months at most. But the Brimstone has an adult lifespan of up to a year, making this yellow herald of the springtime among the longest-lived of all British butterflies.

“To regret the exchange of earthly pleasures for the joys of heaven, is as if the grovelling caterpillar should lament that it must one day quit the nibbled leaf to soar aloft and flutter through the air” Anne Brontë, The Tenant of Wildfell Hall

A downland flower meadow, one of many habitats where the wandering Brimstone can be spotted. The spring blooms provide an abundance of nectar, and the butterflies, which are more often found in the generally warmer southern half of the UK, can be seen basking in the gentle sun.

Newly-emerged adult Brimstones spend much of their time feeding. They settle, wings closed, showing a preference for purple flowers. Their long proboscis allows them to take nectar that is beyond the reach of many other species.

Two Brimstones in flight, their veined wings illuminated in the April sunshine.

The female’s wings are less striking but more leaf-like with a whitish-green hue.

The male Brimstone has primrose-coloured fore wings, although its hind wings are lighter with a greenish tint.

The green chrysalis is extremely difficult to pick out on a twig. After 10-14 days, the adult butterfly emerges.

The caterpillar finds a suitable place to pupate, usually a leaf or stem in low undergrowth. It hangs itself by tail hooks before securing itself with a silken girdle.

On hatching, the caterpillar is a yellowybrown, but during the first moult becomes green, to blend with the lush foliage. It hangs from the leaf by a silken thread.

A freshly-laid egg on alder buckthorn, Frangula alnus. Initially, the eggs are white, turning yellow in a few days, then grey, before hatching into a caterpillar which will feed on the fleshy leaf.

Two feeding Brimstones mirror each other with their angular, leaf-like wings.