Insects on Exmoor

The habitats on Exmoor, ranging from high heather and grass moors through farmed landscape to deep valley woodlands and the coast, support an exceptional diversity of plants and animals. Insects are no exception, and on Exmoor a huge variety of species can be found. The steeply sloping terrain and poor soils mean that the National Park has been spared the extreme agricultural intensification witnessed over much of eastern England since the Second World War. Many once common species have declined westwards into the hills and Exmoor acts as a kind of time capsule demonstrating the range of
habitats and species that would have once existed across much of southern Britain.

There is no better example of this than the fritillary butterflies. Of the eight species that breed in Britain, seven still occur on Exmoor, making the area a mecca for these handsome tawny orange and black-spotted insects. A typical example is the small pearl-bordered fritillary, named because of the pattern of pearl-coloured spots that adorn the underside of its wings. The caterpillar feeds exclusively on violets in a range of habitats such as coppiced woods, bracken slopes and wet grassland. Until the 1950s, this was a common butterfly over much of Britain, but the subsequent loss of much of its breeding habitat through the grubbing out of woods and ploughing, fertilisation and drainage of unimproved grassland has resulted in its virtual disappearance from much of eastern Britain. On Exmoor it remains a familiar sight flying over its bracken and wet grassland haunts in May and June, though even here, numerous colonies have been lost. The high brown fritillary has undergone an even more catastrophic decline in fortune. During the last half century it has become extinct over 94% of its former British range. The reasons for this decline are broadly similar to those described above and the high brown fritillary is now restricted to south-facing bracken slopes. Exmoor, along with Dartmoor and the Morecambe Bay area of Cumbria, now forms one of three remaining strongholds for this butterfly.

Beetles are the largest known group of animals on earth, with 300,000 species currently being described, and an unknown number, estimated to run into millions, still awaiting discovery and description by scientists. On Exmoor over a thousand species are already known. Some beetles, such as the seven-spot ladybird,are familiar to everyone, but the majority are small, cryptic insects, rarely seen by the non-specialist. They also exhibit great variety in size and body form. One extreme is represented by the stag beetle, so named because of the massively developed jaws of the males, which vaguely resemble a pair of antlers. These are used for jousting with other males in order to win access to females. Though it is the largest beetle known on Exmoor, it is rarely seen as it spends much of its life cycle feeding up inside old tree stumps, where its huge grubs gorge on rotten wood. Beetles also exhibit a great diversity in their life cycles, with perhaps the most extreme example being the oil beetles. These are large black beetles with muted blue reflections, in which the wing cases are much reduced to reveal their bulbous body. They can be found on Exmoor's moorland fringes in March and April, lumbering slowly along path edges. The female lays a huge quantity of tiny eggs in a hole in the ground which she excavates with her jaws. When these hatch, the tiny larvae run to the top of flower heads and await the arrival of a foraging bee. When this happens, the larvae attach themselves to the bee and are carried away. If it is a mining bee then the larvae are in luck, for they will be taken back to the burrow of bee, where they drop off it and feed up on the bee grub and its pollen and nectar store. Unfortunately, oil beetle larvae will grab onto any passing insect, and many must perish by clinging to an unsuitable host. Clearly it is vital that the female lays such a huge number of eggs, given such a 'high-risk' life cycle.

The bees, ants and wasps are another of the great insect orders and one of the most important in pollinating flowers and crops. Of particular interest to humans is the familiar honey bee, and hives are still taken up onto the moors in late summer in order to take advantage of the glut of pollen and nectar provided by the heather flowers at this time. The resulting heather honey has a distinctive light flavour. Related to the honey bee are the bumblebees, some species of which are familiar visitors to our gardens. Like the honeybee, the bumblebees are thought to be of significant economic importance to man as crop pollinators. Though we still think of bumblebees as being common insects, in truth they are much less common in Britain now than at the start of this century. The primary reason is again the increasing intensification of agriculture, which has led to the loss of the wild flower-rich fields that the bees require, with the heaviest losses being in the intensively farmed east of the country. The bees, wasps and ants are a very successful group worldwide, part of the reason for this being their social habit. By living communally, with large numbers of workers, they are able to divide up labour such as foraging for food, nest-cleaning and protecting the colony in a highly efficient manner. On Exmoor, the most extreme example of this is perhaps the red wood ant. These ants make their nests from large mounds of twigs, leaves and pine needles and are quite common around the fringes of some of our larger woods. Each colony can contain up to half a million worker ants, which forage in the canopy of nearby trees where they prey upon moth caterpillars and  'milk' aphids for the sweet honeys' dew they produce. Wood ants are very aggressive, and when disturbed, are capable of squirting a jet of formic acid from their abdomen at intruders.

When we think of flies our first reaction tends to be one of distaste, and the conservation of flies is a subject that might cause some raised eyebrows. Yet on Exmoor, one of our highest priorities for conservation is the hornet robber fly, a species for which government has dictated that we are obliged to prepare a plan of conservation action. The hornet robber fly is one of our most threatened British insects, and the steep combes of the Exe and Barle valleys are one of the last refuges for this spectacular insect. It is one of our largest flies and, in its striking brown and yellow livery, is a menacing-looking beast. Closer inspection only confirms the initial impression, as the mouthparts are modified to form a razor-sharp blade that it uses to stab large insects such as grasshoppers and beetles. It lies in wait for these on the edge of bracken and bramble thickets, flying out to ambush them with a low buzz. This illustrates the point that whilst the conservation of our mammals, birds and flowers is very important, we should also be mindful of the smaller and more obscure groups as many are now rare in their own right and form a vital part of the web of life on Exmoor.

Contributed by: Bob James