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This essay intends to outline the ecology and control of the highland midge Culicoides impunctatus. It will give an overview of the Habitat and life cycle of the highland midge as well as discussing some of the problems caused by this species such as effects on tourism and forestry. It will also discuss the efficacy of various control methods that may be employed in an attempt to control this pest species.
Culicoides impunctatus is a member of the family ceratopogonidae and the insect order diptera (true flies), is approximately 1.4mm long and weighs approximately 0.5 µg. The highland midge can be differentiated from other midge species such as the garden midge (Culicoides obsoletus) due to characteristic dark blotches on their membranous wings. Culicoides impunctatus are holometabolous insects, as they go through various stages of growth (known as instars), which are separated by moults. Culicoides impunctatus can occur at huge population densities in Scotland, e.g. landing rates of 10-635 midges/min on an exposed human arm (Carpenter et al, 2005).
The highland midge's preferred habitat is boggy, acidic soil, such as mires and the transition zones on the edge of bogs. These habitats are often characterised by rushes such as Juncus articulatus and Juncus acutiflorus, as well as the presence of Sphagnum spp., and the purple moor grass, Molinia caerulea. These plants characterise soil with enough water content for the development of midge larvae, this habitat is quite common in the north and west highlands of Scotland. Culicoides impunctatus use these areas as breeding sites, as their larvae are semi aquatic and will drown in open water, but are also vulnerable to desiccation in dry soil. In a suitable habitat midge larvae can be found in densities of up to 700 per square metre (SNH)
Fig 1 - The typical habitat of Culicoides impunctatus, containing peat bog, water and shrubbery.
Culicoides impunctatus is bivoltine, with 2 generations of adults beginning in early may and extending to late September (Blackwell et al, 1992) The first generation begins its life cycle as an egg, which is laid during the summer months in areas of moist soil, these eggs are generally laid in batches of 30-100 approximately 5 days after the female is fertilised. The larvae hatch within a day and burrow into the top layer of soil. The larvae of Culicoides impunctatus are both omnivores and detritivores (consuming detritus is the soil) and feed on other larvae, protozoa, green algae, and nematodes. Triggered by an increase in temperature and hours of daylight the larvae enters the pupal stage, this phase generally lasts for 1-2 days. Once metamorphosis has taken place the midge emerges fully formed and typically has a life span of between 20-30 days. The larvae of second generation females survive over winter by entering a fourth instar phase, living as larvae in the soil (Blackwell et al, 1992). Larvae of Culicoides impunctatus develop slowly when compared to other species in the genus; this may be due to the nutrient poor soil in which they develop (trees for life).
The adult female highland midge has the ability to lay a first batch of eggs without the need for a blood meal. This is done using nutrient reserves built up in the larval stage. This may be one of the reasons for the high numbers of Culicoides impunctatus in the highlands. The adult female however carries many more eggs than can be matured using these nutrient reserves. It is only after the first batch of eggs is laid that the female requires protein from a blood meal to provide the yolk for additional eggs. This bloodsucking behaviour is known as haematophagous. This blood meal can be obtained from a large number of mammals, recent studies have shown that the gut of Culicoides impunctatus contained the blood of cattle, deer, sheep, humans, cats, dogs, mice and rabbits (SNH). It was also noted that humans were not actively preferred over other animals, and were only bitten opportunistically (Blackwell, Brown, & Mordue, 1995)
Research evidence has shown the adult female Culicoides impunctatus are attracted to dark coloured moving objects, especially when these objects are associated with the release of carbon dioxide (CO2) which is released when mammals exhale. This well developed olfactory sense is also used to detect other chemicals which are secreted in the body odour of mammals such as acetone and lactic acid (Mordue, 2003). After locating the host the highland midge bites using short blade like mandibles, these are used to make a shallow wound, the midge then feeds on the capillary blood from the wound. After around 5 minutes the midge becomes engorged and ceases feeding. Once the adult female midge has finished feeding, she releases a pheromone, which acts to attract other females.
The highland midge is a major pest species in the highlands of Scotland, causing severe annoyance to both man and livestock due to their attacks, research suggests that Culicoides impunctatus are responsible for around 90% of bites on humans. Culicoides impunctatus also cause major disruption to outdoor industry and tourism during the summer months (Hendry and Goodwin, 1988) Forestry workers can be particularly vulnerable to midge attacks due to the fact they often work low to ground and in shaded areas where Culicoides impunctatus are active throughout the day, it is estimated that up to 20% of summer working days in outdoor jobs such as forestry are lost due to midge activity (trees for life). Tourism may also be affected with people being deterred from visiting the Scottish highlands due to high levels of highland midge activity, or being deterred from returning to the Scottish highland on holiday due to the thought of being attacked and bitten by the highland midge (SNH). The Scottish tourism industry is estimated to be worth £2 billion pounds per annum(Hendry 1996), the loss of income through visitors not visiting or returning to the highlands is currently undocumented and can only be guessed at (Blackwell & Page, 2003)
For the reasons stated above research is being carried out to find and implement some kind of control for the large population of Culicoides impunctatus. This essay will now go onto discuss some of the potential methods of controlling the highland midge.
At present no effective chemical control exists for the control of Culicoides impunctatus. Spraying of sites would be ineffective as adult highland midge often rest in crevices or the undersides of leaves. Even if a significant numbers of Culicoides impunctatus could be killed, repeated spraying would be required as more midges emerge throughout the summer months. Control of highland midge larvae is equally difficult. Any insecticidal control used would have to be persistent enough to penetrate the deep soil the midge larvae live in. Persistent insecticide use would also lead to rapid resistance development. Furthermore the larval habitat is so common in Scotland that the insecticide costs would rapidly become prohibitive. This is evidenced by the fact that Culicoides impunctatus was the target of several unsuccessful insecticidal trials by the department of health for Scotland in the 1940's and 50's (Kettle, 1996). The spraying of chemical insecticides is also highly non specific and would reduce overall numbers of insects, including some species which may prey on Culicoides impunctatus. This may actually lead to an increase in highland midge numbers. Due to the complete lack of predation, Culicoides impunctatus may become a resurgent pest.
Biological control agents are currently being investigated, such as the control of larvae by spraying with nematodes, but these measures are likely to have the same problems as chemical control methods. The control agents (nematodes) would have to persist in the midge habitat and also be cheap enough to allow repeated treatments of a large area of habitat. Inoculative control such as using natural enemies are likely to be unsuccessful as Culicoides impunctatus is a native species and is unlikely to have any natural enemies abroad which do not already exist in the highlands of Scotland. In addition to this, although the highland midge is so numerous, it is not a major food source for any insectivorous animals or birds. Bats such as pipistrelle's (Pipistrellus pipistellus) do eat insects such as the midge, but this is a very minor part of their diet. Birds such as warblers (Phylloscopus spp.) and swifts (Apus apus) will also occasionally feed on the highland midge. Midges are also a food source for insectivorous plants such as sundews (drosera spp.) and butterworts (Pinguicula vulgaris). These plants flourish in the acidic, boggy soil where midges live, but the numbers caught by these plants are insignificant when you consider the huge numbers of Culicoides impunctatus. The midge larvae are occasionally fed on by the larvae of other dipterous flies, as well as the larvae of coleopterous species, but as with the plant species, the numbers consumed are insignificant. It has been theorised that the large numbers of Culicoides impunctatus may be due to the deforestation of some areas of the Scottish highlands. Deforestation could have caused increased water content in the soil, creating a perfect breeding ground for the highland midge. This theory may also explain why no species take advantage of the large numbers of the highland midge, as deforestation was a recently modern development, this would not have allowed significant time for a specialised midge predator to evolve. Recent studies in Skye and Argyll have found that the larvae of 2 species of mites (Parafeiderium stuarti & centrotrombidium blackwellae) are parasitic on the adult midge; this research suggests that these larvae could be used to provide a biological control for Culicoides impunctatus.
Recently much research has been done to investigate the effect of midge scent traps on controlling midge numbers, these have been fairly successful. Devices which are known as midge magnets and midge eaters have recently been marketed as an effective control method. As stated earlier Culicoides impunctatus detect hosts to bite by using their sense of smell to locate both carbon dioxide and natural chemicals given off by mammals such as acetone and lactic acid (Mordue, 2003). In addition to this it has been found that the highland midge is attracted to a chemical known as 1-octen-3-ol (Blackwell et al, 1996);(Bhasin, Mordue, L, & Mordue, W, 2000) this chemical with the addition of carbon dioxide is exhaled by mammals, and thus is a powerful attractant for midge species. Studies have shown that Culicoides impunctatus are sensitive to this chemical at a release rate of 0.11 mg/day (Bhasin, Mordue, L, & Mordue, W, 2000). These midge catching devices give off a mixture of certain chemicals (1-octen-3-ol, acetone, lactic acid and CO2), and have the potential to catch thousands of Culicoides impunctatus per day. It should be noted however that the placement of these traps is important, traps should be placed to draw midges away from human habitats rather than towards them, also the success of these traps improves over time, catching more midges a few weeks after placement than within the first few days. These midge traps are highly specific, and therefore do not harm any other insect species.
Fig 2 - A portable midge magnet
Despite never being tested on Culicoides impunctatus, insect growth regulators (IGRs) that disrupt metamorphosis have been shown to be effective against the larvae of Culicoides sonorensis. Laboratory studies in the USA using field collected larvae showed that similin in quantities of 0.5 p.p.m were effective in controlling emergence of the midge by 90% (Apperson & Yows, 1976). This method may have some potential to control midge numbers in the Scottish highlands.
Midge repellents are also often used in an attempt to stop Culicoides impunctatus from biting humans. Most modern insect repellents contain a chemical called di-ethyl toluamide (DEET), this chemical has been proven highly effective in deterring the highland midge from biting, and is the active ingredient in most of the midge repellents on sale today. DEET however has been proven to be toxic, and as such should not be used frequently (Selim, 1995). Recently however a midge repellent (known as smidge) has been developed which interferes with the midge's ability to locate its prey by confusing the midge's sense of smell. Smidge is completely DEET free and according to manufacturers can be used safely on adults and children older than 30 months. This is however is a recent development, and as yet no field trials have been carried out. It is therefore difficult to determine the efficacy of this repellent.
To conclude the high population of Culicoides impunctatus is still a problem in the north and west highlands of Scotland. However a large amount of research is being conducted with a view to controlling the numbers of this particular pest species. The continuing development of new and improved control methods such as insect growth regulators (IGRs), scent traps such as the midge magnet, and midge repellents such as smidge show that progress is being made in tackling this problem pest.