a glorious morning for butterflies

The first of the spring’s butterflies is always a welcome sign; not the Peacock disturbed from its winter slumbers in the woodpile or the Small Tortoiseshell that has spent the winter in the cool of the spare room but one that has truly emerged from hibernation, brought to life by the rising daytime temperatures. The other morning, venturing into a different part of the forest to check out some ponds marked on the large scale map, I found not one but many butterflies on the wing, each one making the most of the early springtime warmth. All bar one of the two dozen or so individuals that I encountered in a little under two hours was a Brimstone, invariably a brightly coloured male, sulphurous yellow in colour.

That these individuals are on the wing so early in the year stems from the fact that they have chosen to overwinter as adults, fattening up on autumn nectar before finding somewhere suitable to pass the winter; in the case of the Brimstone the site will often be deep within an evergreen shrub. Just a handful of our butterflies overwinter as adults. Along with the Brimstone, Small Tortoiseshell and Peacock there are the Comma, Red Admiral and Large Tortoiseshell, the latter now effectively extinct as a resident. Other species pass the winter as caterpillars, eggs or as pupae, with each species overwintering at the same stage each year. There is an exception to this rule in the form of the Speckled Wood, which may pass the winter as either a caterpillar or a pupa depending upon late summer temperatures and their impact on larval development times.

The Brimstone has the longest adult lifespan of any British butterfly and individuals on the wing now may have emerged from their pupal stage early last July, and they may continue on the wing through into this June or even July. The brightly coloured males emerge from hibernation before the females (hence my seeing almost entirely males the other morning) and they can be wide ranging in their habits. Over the coming weeks the males will meet and court the pale coloured females, initiating what the great lepidopterist and illustrator F. W. Frohawk described so beautifully as ‘a prolonged dalliance flight in the sunshine’. The eggs will be laid towards the end of May, each one deposited singly on the underside of Buckthorn or Alder Buckthorn leaves.

It is an encouraging sign to see so many on the wing and equally reassuring to hear of other reports from across the county charting the emergence of other species. This might suggest that the cold weather has had little impact on their overwintering success, a good omen for the coming season.

Early butterflies

The warmth of recent days has triggered the first real wave of spring butterflies. Most of the species on the wing at the moment, notably peacock, comma, brimstone and small tortoiseshell, will have spent the winter as adults, hibernating in some suitable crevice, garden shed or evergreen bush. All these early season butterflies will be seeking out suitable nectaring opportunities. Those that have overwintered as adults will have to replenish energy reserves spent getting though the long cold winter months. As such, many will range widely, seeking out nectaring opportunities with flights that may well bring them into your garden. This is one reason why gardens with spring-flowering plants are particularly beneficial to butterflies, and indeed to other insects (such as bumblebees) on the wing at this time of the year.

Butterflies do not take nectar from just any old flower but actively select those that offer the best returns. We are not just talking about the amount of nectar on offer but have also to consider the amount of energy that a butterfly has to expend in getting that nectar. Each nectar-producing flower is a bit like a petrol pump but one which a visiting butterfly will typically drain on each visit. It takes time for the fuel pump to be refilled so a butterfly does not want to be visiting a pump that has been recently emptied by another visitor. The amount of nectar left in the pump is known as the standing crop and the size of the standing crop will depend on how quickly the flower produces nectar and when it was last emptied. Flowers in which the nectar is held at the end of a long tube (known as the corolla) tend to have higher standing crops because only long-tongued visitors can access them. Flowers with short corollas can be accessed by most insects, including those with short tongues, and so tend to have smaller standing crops available.

What does this mean for our visiting butterflies? Well, those butterflies which are large in size and which therefore expend more energy just getting around, will seek out flowers with the biggest standing crop. Fortunately they can access these because of their long tongues (tongue length has been found to correlate well with body size and wing loading). Such long-tongued species also prefer to forage on plants where the flowers are clumped together rather than spaced apart. This is why you see red admirals, small tortoiseshells and peacocks feeding so readily on Buddleja davidii later in the year, while shorter-tongued species (typically smaller in size and with lower flight costs) can afford to visit solitary flowers, like bramble and dandelion, with the risk of reduced standing crops.

Butterflies emerge as sun shines

The brimstone is one of the earliest butterflies to be seen on the wing. As I write, a bright sulphur yellow male is making the most of the early warmth, seeking out nectar sources in the garden. Despite its bright colouration the brimstone is often overlooked, perhaps because it is on the wing so early in the year, relegating it to a lower tier in terms of familiarity. The species is one of just a handful of butterflies to spend the winter in their adult form, along with comma, peacock and small tortoiseshell (from the common species) and large tortoiseshell and Camberwell beauty (from the rare visitors). To these can be added red admiral, a species now able to overwinter here successfully because of our increasingly mild winters. Resembling a dying ivy leaf, the brimstone is ideally camouflaged to spend the winter tucked away in thick vegetation; only venturing out once sufficient warmth is felt. Too early an arousal can be disastrous, with the limited fat reserves quickly depleted if energy is expended in a fruitless search for nectar that is not yet available.

Alongside the bright yellow males, whose colouration is thought by some to have given rise to the word ‘butterfly’, female brimstones may also be seen on the wing. Their colouration is a pale yellow green, reminiscent of the primroses that are in flower at the same time. These adults will remain on the wing through into June, possibly even early July, mating and then laying their eggs singly on buckthorn or alder buckthorn, the favoured food plants of their caterpillars. Unlike certain other species, which form distinct colonies, brimstones range widely and females will travel considerable distances in order to find suitable sites for egg-laying. The generation of adults that emerges from these eggs may be on the wing from late July or early August and will remain on the wing as the warmth of summer begins to fade through into autumn. This means that, successful hibernation allowing, some of these adults will reach 11 months of age, making the brimstone one of our longest-lived butterflies. Because it overwinters as an adult, the brimstone has to spend many hours in late summer nectaring, laying down the fat reserves that will get it through the winter ahead. Watch one feeding in your garden and you will quickly notice that it is able to utilise those flowers with particularly deep nectarines, like buddleia, teasel and runner bean.

So, while this butterfly may be seen on the wing in every month of the year, it is only really now, during early spring, that we tend to notice them; a real shame for such a spectacular insect.

Springtime buzz

A visit to Thetford Warren Lodge the other lunchtime not only produced my second Brimstone butterfly of the year but also my first bumblebee. Although queen bumblebees that have overwintered successfully can be seen on warm days earlier in the year (as early as February in species like the familiar buff-tailed bumblebee), the cold weather of recent weeks has held things back somewhat. Queens of the six most common and widespread species are likely to have emerged from hibernation by now and will be searching out early flowers in order to replenish energy supplies. Over the coming weeks, the flowers of both red and white deadnettle will prove attractive to many species. However, if the weather deteriorates (and at night) the queens will seek shelter, often in deep leaf litter.

One pattern that you may begin to notice over coming weeks is for queens to fly very low, just above the contours of the ground as they search out a suitable place for their nest. Many species will seek out disused small mammal burrows in which they will lay their first batch of eggs. These eggs, usually 8-14 in number, produce the first generation of offspring, smaller females that are usually infertile and who serve as a worker caste, helping to raise later broods as the colony begins to grow in size. One quite common species, known as the common carder bee, is more adaptable when it comes to nest site selection and may be found using compost heaps. Not all bumblebees make their own nest. There is one group of bumblebees, known as cuckoo bumblebees, which will invade an already established nest, often killing the incumbent queen in the process, prior to laying their eggs.

Some of our bumblebee species emerge later in the year, including the rather localised red-shanked carder bee. There are records of this species from the north Norfolk coast and Breckland but it appears to be in decline. This species, like many other carder bumblebees, emerges in late May or early June. This means that there is quite an extended period of emergence across our 25 or so species, something that provides plenty of scope for the interested naturalist. Those individuals on the wing at the moment face many challenges if they are to successfully found a colony from which fertile males and females will be produced later in the year. Mortality rates of queens are very high (perhaps 80%) because of predation, inclement weather and a nematode worm. The queens that you are seeing on the wing now will be busy getting into condition before producing the larvae which will form the basis of their colony over the coming months.

HIbernating butterflies

Although winter can be a difficult time, there are species that manage to avoid the worst of its ravages by going into hibernation. Many are familiar to us: newts that retreat into the spaces under log piles, hedgehogs that bury themselves in last autumn’s leaves or ladybirds that squeeze into the cracks of a window frame. Most of these creatures will spend the winter unnoticed by homeowners and gardeners, but others may be chanced upon as logs are brought in from the woodpile or something is fetched from the garden shed.

This winter, a small tortoiseshell butterfly has chosen my shed and it is interesting that this butterfly should choose to overwinter as an adult. Very few of our resident butterflies overwinter as adults; the majority spend the winter as caterpillars, although nine species overwinter as eggs and eleven as pupae. It is also interesting to note that, with the exception of the speckled wood, the hibernating phase is always the same in a given species. The speckled wood can hibernate as either a caterpillar or a pupa. Along with the small tortoiseshell, the other species to overwinter as adults are comma, peacock and brimstone. To these can be added large tortoiseshell (though now thought to be extinct as a resident species), red admiral (though only in the extreme south of the country does overwintering appear to be successful) and Camberwell beauty (which overwinters occasionally). Many of these species will spend the winter in hollow trees or outbuildings but the beautifully camouflaged comma prefers to overwinter on exposed branches or among dead leaves and the brimstone hibernates deep within an evergreen shrub. During hibernation these butterflies must be vulnerable to predation by birds and this may be why many have evolved their camouflaged underwings. The peacock goes a step further and has evolved a warning display. If disturbed, the peacock will open its wings to their fullest extent to reveal the large ‘owl-like eye’ pattern on its wings. At the same time it produces a hissing sound by rubbing its wings together, something that is usually enough to scare off a would-be predator.

Once we receive a run of several days of warm, spring-like, weather many of the butterflies will emerge from hibernation to seek out nectar sources to replenish reserves lost over winter. None of the species is able to mate immediately on waking but needs to feed and bask in the sun to develop the eggs or sperm needed for the reproductive act. The butterfly can always re-enter hibernation if spring has not really arrived, although it will have lost vital reserves and be less likely to survive the remaining winter period.