Information and Advisory Note Number 29 Back to menu
1.1 What are biting midges?
Biting midges are the smallest of the flies that bite humans in Scotland. Mosquitoes, clegs and blackflies will also bite man but in large areas of Scotland the most troublesome flies are the midges. Most biting midges are about 2mm (V12") long and have wings which, when they are at rest, are folded flat, one over the other, on their backs. The biting midges belong to the family Ceratopogonidae which has 152 species in Britain. Most of the species in this Family feed on the blood of other insects such as caterpillars, craneflies, and mosquitoes. For example, one species feeds from the wing veins of dragonflies and lacewings. Another species feeds from the thorax of mayflies. Other species are predatory, feeding on small insects; the females of some species will eat the males with which they are mating. The species which bite man all belong to the genus Culicoides; most members of this genus feed on the blood of warm-blooded animals. Culicoides is a world-wide genus with hundreds of species. Of the 50 species of this genus recorded in Britain 37 species occur in Scotland (Table 1). Tropical Culicoides species spread both human and animal parasites and diseases. In Britain, midges do not transmit diseases to humans but they are vectors of several animal diseases such as sweet itch in horses.
1.2 Which species bite humans? Some Culicoides species such as C. pictipennis and C. odibilis seem to specialise in feeding from birds. The hosts of some other species are not yet known. About 20 species are known to feed from mammals and 16 species have been recorded feeding from man (Table 1). Midges are not host specific and more of these species may be recorded as biting humans in the future. Adult midges tend to be concentrated around their breeding areas and so in different habitats a different range of species will bite. For example, in damp woodland and gardens C. obsoletus can bite unobtrusively; around stables C. nubeculosus can be a problem; and in salt marshes C. halophilus can be troublesome. However, in large areas of Scotland the Highland midge, C. impunctatus, may account for 90% of the attacks on man. This species has been found in all regions and islands of Scotland but its largest numbers are in the Highlands. It is one of the smaller midges; the adult is only about 1.5 mm (V17") long and it has distinct dark spots on its wings.
Recent research has identified the three NVC vegetation communities in which most breeding sites of the highland midge C. impunctatus occur:
1. The Sphagnum sub-community of Betula pubescens - Molinia caerulea woodland (W4);
2. Molinia caerulea - Potentilla erecta mire (M25);
3. Juncus acutifolius sub-community of Juncus effusum / acutifolius - Gallium palustre rush pasture (M23).
The common features of the breeding habitat seem to be the presence of Sphagnum spp., the rushes, J. articulatus and J, acutiflorus or the purple moor grass M. caerulea. These plants characterise soils with a high enough water content for the development of the larvae. This habitat is not uncommon in the West Highlands.
Table 1: Culicoides midges in Scotland.
2.2 Life history
The eggs are laid in wet soil in boggy flushes, mires and in the transition zone at the edge of bogs. The larvae are narrow and worm-like with a distinct head, and they live in the soil. They are omnivorous and their diet includes small animals such as nematodes, other insect larvae, fungi and parts of plants. The larvae are semi-aquatic; they drown in open water and desiccate in dry soil. In suitable habitat the larvae can be found in densities of up to about 700 per square metre. The density of larvae can vary markedly over short distances depending upon the water content of the soil.
The larvae develop slowly when compared to some other species in the genus - possibly because of the nutrient-poor soil. The larvae
over-winter in their final instar and pupate in the spring.
In Scotland the adult midges begin to emerge in April and they are active on the wing until October. Within this period there are often two peaks in emergence, one in late May/early June and the second in late July/early August. There are several theories about why there are two peaks in emergence. There could be more than one generation in a year, more than one cohort in the population or climatic factors such as dry periods in the previous summer may be responsible. There is no firm evidence to distinguish between these theories, but C. impunctatus breeds in nutrient poor soil and is unlikely to develop fast enough for two generations in a year. In addition several studies have only found one peak in emergence so the pattern may not be fixed.
2.3 Male behaviour
If they feed at all, male midges probably feed from flowers. Males form short-lived swarms in low light conditions that are easily disrupted by wind. Most swarms contain a few hundred individuals although they may range in size from tens to thousands of flies. The swarms form near the breeding sites above markers such as tussocks of grass, bracken fronds, other prominent vegetation or bare mud. Females visit a swarm to obtain a mate and the pair drop to the ground. The males rest during the day in sheltered sites such as grass tussocks and amongst moss.
2.4 Why feed on blood?
The adult female Highland midge, as with several other species in the genus, can lay her first batch of eggs using reserves built up as a larva. However, she carries many more eggs than she can mature using these reserves. To provide the yolk for the additional eggs she requires protein from a blood meat. Trapping studies have shown that perhaps less than 10% of females obtain a blood meal during their short lives. The ability to lay a first batch of eggs without a blood meal is part of the reason for the abundance of this species in the highlands.
2.5 Hosts attacked
The highland midge will feed from a wide range of mammals. Recent work has found blood in the guts of the midges from cattle, deer, sheep, humans, cats, dogs, rabbits and mice. Tests were also run for bird blood, but it was not found. The great majority of the blood meals came from cattle, sheep and deer; humans were not actively preferred over these animals.
2.6 Factors affecting biting behaviour
1. General considerations: One of the main factors affecting the biting rate by any midge is the distance from the nearest breeding ground. Midges can be blown on breezes for considerable distances from their breeding site and female midges have been found over 1km from their breeding ground In search of a blood meal. However, the .highest numbers will usually be nearer to the breeding site. The midges prefer to bite near to the ground and are uncommon more than about 3m (10*) from the ground.
2. Weather and time of day. the Highland midge is usually most active in the two hours before sunset with less activity after dark. However, several factors can modify this pattern.
(i) Light intensity: female midges are only active in low light (about 1/3 full noon sunlight). Most midge attacks, therefore, occur around dusk, although midges can also be active on cloudy days and in shaded areas,
(ii) Wind speed: midges stop flying in wind speeds greater than about 3ms (6mph). However, in sheltered situations such as hollows and in the lee of trees biting will continue on windy days.
(iii) Humidity: midges seem to have a threshold of about 60-75% humidity below which they are inactive. Peak activity occurs at 90% humidity.
(iv) Temperature: this is strongly correlated with the time of day and light levels so identifying the effects of temperature alone is difficult. However, activity is reduced below 10°C and may stop below 3°C. Midges will bite in a variety of weather conditions and they are not stopped by light rainfall.
2.7 The bite
Midges have mandibles which are relatively short and blade-like, these can only make a shallow wound. The midge then feeds from the capillary blood oozing into it, the blood flow is increased by histamines in the saliva of the midge. It takes about five minutes for the midge to become engorged and stop feeding. The histamines cause the swelling around the wound after the bite. In most people the reaction is relatively minor but in a few people allergic reactions can produce very distressing effects.
Because of the short mouthparts, biting on man is generally confined to exposed skin: midges cannot bite through cloth, although they will climb under clothes to bite.
Rainfall can be critical at several points in the life-cycle. Dry periods can reduce the numbers of eggs laid. The adult lifespan can be reduced in dry conditions and few females will fly if the relative humidity is low. In addition, the female will only lay eggs if wet soil is available, so a dry summer can reduce the number of available egg laying sites and the numbers of eggs laid. Once laid both the eggs and young larvae are vulnerable to drying-out and dry periods can cause mortality. As a result the numbers of numbers of larvae found in the soil in the autumn are very closely correlated with the rainfall during the preceding summer months. The older larvae are able to move more actively in the soil but they too can be killed if the soil dries out to any depth. The pupae have limited movement, and they are formed near to the soil surface to allow the adult midge to emerge. If the soil surface dries out during the spring and early summer when the pupae are developing they wilt die. A dry spring can significantly reduce the number of midges flying later in the year. Conversely wet weather early in the year will mean more midges later in the summer. This dependence on rainfall means that midge problems largely occur in areas where the soil is kept wet in summer by rainfall exceeding tosses due to evaporation. Because of this a good guideline is that the highland midge is. only a serious problem in areas that receive over 1250mm (50") of rain each year.
3.2 Predators and parasites
The larvae are eaten by generalist invertebrate predators in the soil and can be parasitised by nematodes or infected by bacteria or ciliate protozoans. There is little data on the impact of these on the larval population.
The adults are eaten by a wide range of generalist predators including empid flies, dragonflies, fish, amphibians and bats. They are also caught by sundews and butterworts. Again, there is little data on the impact of such predation on the populations.
4.1 Pressure for control.
Midges can have an impaction the economy of areas where they are common.
1. Forestry workers and crofters are particularly susceptible because they often work low to the ground in sheltered and shaded areas where the midges are active throughout the day. Work can be impossible on some particularly midgy days, and even when the biting rates are lower the midges can make work difficult and unpleasant.
2. Tourism can be affected by midges, visitors may be deterred by the thought of midge bites and the memory of being attacked by midges may discourage some from returning to the Highlands. People on holiday can be less tolerant of midge bites than they are at other times.
3. Businesses may be deterred from locating in parts of Scotland by the possibility of staff being attacked by midges.
Starting in the 1940's, such concerns lead to several government-funded investigations into the practicality of controlling midges.
4.2 Methods of Control and Management
1. Chemical control:
No effective chemical control exists at present Sprays miss most of the adult midges because they rest in sites such as the underside of leaves and in crevices. Even if they could be killed, more midges emerge throughout the summer and move on the wind. Any spraying would have to be repeated throughout the season if biting rates were to be reduced significantly. Control of the larvae is equally difficult: they live buried in the soil in relatively diffuse breeding sites. Chemical control agents would have to be persistent enough to penetrate the soil to some depths. Persistent pesticides would have a severe impact on other aspects of the natural heritage and mosquito control work has shown that resistance evolves more quickly to persistent insecticides. The larval habitat is so common and the flight range so large that the cost of treating all breeding habitats in an area would rapidly become prohibitive. Treatment would also have to be repeated because the cleared area would be rapidly re-colonised.
2. Biological control:
No biological control agents have yet been found. Inundative control of the larvae by spraying with native micro-organisms and nematodes is being investigated but it is liable to suffer the same problems outlined for chemical control. The control agents would need to persist in the midge habitat, infect a high proportion of the larvae, be specific to the midge and be cheap enough to allow large areas to be treated repeatedly. The inoculative introduction of additional natural enemies from abroad is not likely to be
successful because C. impunctatus is a native species, it is unlikely to have suitable natural enemies abroad that are not already present.
Repellents are effective but not perfect: they will only reduce the numbers of bites. Extracts from several plants repel midges for example; citronella, an extract from an asian grass, was widely used before the last war and several herb oils such as thyme are said to be effective. Smoke also repels midges. Most commercial repellents are now based on the synthetic compounds DEET or DMP which are more effective than the older repellents. However, they do melt some plastics and should not be used on babies' skin, nor should they be used over all the body for prolonged periods. They are less effective if the person is active than if they are resting because perspiration increases the attractiveness to midges and removes some of the repellent.
4.4 Avoiding being bitten
The best method to avoid being bitten in the Highlands is to use a repellent and to recognise the conditions when midges are likely to be most active and avoid going out in them. The following 'rules of thumb' may help.
Midges are liable to remain a problem in the Highlands for the foreseeable future, and developing better avoidance strategies and repellents is likely to be the most productive way forward.
Hendry G. (1989) Midges in Scotland. Aberdeen University Press
Dr David Phillips, Invertebrate Ecologist
Scottish Natural Heritage
2 Anderson Place
Edinburgh, EH6 5NP
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