A bounty of butterflies

The last few weeks have seen a noticeable upturn in the numbers of butterflies on the wing, with small tortoiseshell perhaps the most abundant of these. After the slow start to the year for many species, the ‘good’ summer that has followed has given our butterfly populations a real boost. The presence of so many small tortoiseshells is particularly welcome given the fortunes of this familiar butterfly over recent years. Numbers, which normally show an increase towards the end of the summer, have been lower lately and comments have been made about the possibility of a longer-term decline.

Small tortoiseshell populations here in Britain are influenced by a number of different factors, one of which is the weather. Periods of hot and dry weather during the caterpillar stage lead to faster growth rates and a larger emergence of adults. However, parasite populations also shape tortoiseshell numbers and the arrival of a new parasite was thought to have increased the pressure on British populations. Until relatively recently, small tortoiseshells were only parasitized by a number of generalist parasitic flies and wasps, parasites that also target other species. The arrival of a new parasitic fly, called Sturmia bella, just over a decade ago was thought to have changed things somewhat – at least over the short term. This species, which is more of a specialist, lays its eggs near to small tortoiseshell caterpillars. These are ingested along with the plant material on which the caterpillars are feeding. The eggs then hatch inside the caterpillar, before the resulting larvae go on to kill it. The larval fly (a white maggot) emerges from the butterfly’s chrysalis and descends to the ground on a silken thread.

Although the arrival of this new parasite may have had some initial effect, evidence from elsewhere suggests that the recent declines in small tortoiseshell populations here are not linked to the parasite but are, instead, the result of a run of poor weather. While a hot and dry summer may boost the numbers of adults seen, summer drought can reduce the availability of nettles (and in particular fresh nettle growth), used as a food plant. It will be fascinating to see how populations fare going into 2014.

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.

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.