This lovely butterfly, sometimes the earliest of the year to be seen, brings a splash of yellow to match our spring daffodils, and while Yorkshire is at the northern edge of the Brimstone’s British range it still puts up a good show in most years.
This is the species which some say, thanks to its striking colour, gave us the name ‘butter-fly’. The name Brimstone, too, comes from the sulphurous appearance of the males, which are more vivid than the pale green females.
The Brimstone overwinters as an adult rather than (as with most UK species of butterfly) a chrysalis. It can emerge from its hibernating place – perfectly camouflaged among ivy or holly leaves – as early as February. During a cold snap it can return to hibernation. Brimstones can be long-lived. The newly awakened adults mate from April onwards, and are still sometimes around and looking rather worn when the fresh new butterflies emerge in July. These new butterflies will not breed that year, but after feeding up on nectar to last them through the winter, will hibernate to emerge and breed the following spring. Thus a Brimstone can live for a year, unlike for example another early butterfly, the Orange Tip, which lives for only 18 days.
The Brimstone’s breeding territory depends entirely on the distribution of the plants on which the caterpillars feed: Buckthorn and Alder Buckthorn. The green caterpillars grow to 3cm and can adjust their colour to match the leaf. They lie along the leaf’s central stem and can angle themselves towards the sun to avoid making a tell-tale shadow. When they pupate they attach themselves by a silken girdle to a low stem or leaf.
In the spring adult Brimstones are a chief pollinator of primroses both in the wild and in our gardens. They have an exceptionally long tongue which enables them, in the summer, to reach down into the deep flowers of Buddleia, teasels, thistles and knapweeds, all of which we have in some abundance at St Nicks.
Brimstones are difficult to photograph in their full glory because they always rest or feed with their wings tightly closed, and in flight they move fast and often high, forever changing direction – a good defence strategy against predatory birds and eager photographers! With its veined and scalloped wings a perched Brimstone is well camouflaged, looking remarkably like a leaf. You’ll rarely see one in the evening: they are early to bed, going to roost among the leaves between 3 and 4pm even in high summer.
There are Yorkshire records of Brimstones flying in every month of the year, but on average first sightings are in mid-March, though in 2012 one was seen at Haxby on 24th February. They can still be seen as late as October (in 2012 one was watched at Nosterfield on 10th November) but most have gone into hibernation by mid-September. In the last three years at St Nicks first sightings have been on March 23rd, May 2nd and April 11th, and last year one was seen as late as 17th October.
We hope to see good numbers in the York area this year as there is usually a significant increase in the population following a warm summer such as we had in 2013. So if you are on the reserve in March or April and see one, do go into the Centre and tell the staff – all the better if you can get a recognizable picture of this beautiful harbinger of Spring.
(Our thanks to author Harald Süpfle and wikimediafoundation.org for use of photographs of pupa and caterpillar.)