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SECTION 1 - MOSQUITOES (CULICIDAE)
Intoduction
Mosquitoes are relevant to public health when their density is great enough to make them a nuisance and when they transmit disease agents. Depending on climate and the availability of breeding sites, mosquitoes can become tremendously annoying nuisances regularly after seasonal mass reproduction. To reduce the risk to health in urban areas, proper sanitation and water management are key factors. This includes preventing mosquitoes from breeding in the immediate surroundings of dwellings and housing estates, by ensuring that there are no potential breeding places. Larviciding, adulticiding or both can accomplish mosquito control. While various approaches to larviciding are possible, including biological and biochemical treatment, adulticiding is confined to pesticide treatment (indoor application of residual pesticides and outdoor pesticide fogging). Therefore, mosquito management should seek to effect control by promoting environmental changes detrimental to the development of mosquitoes, rather than treating mosquitoes with pesticides.
Classification
Mosquitoes are within the family Culicidae and there are over 3,000 species worldwide. There are about 30 species of mosquito in the UK the most common being Aedes spp, Anophheles spp and Culex spp.
Life cycle
There are four stages of life, eggs laid on water which hatch within a few hours; larva and pupa that are free swimming in water and must come to the surface to breath; and the winged adult.
Habitat
Mosquitoes occupy a range of breeding habitats including aquatic habits such as coastal salt waters, brackish inland waters, stagnant pools and water-filled hollows (including in trees and logs), marshes, artificial outdoor sites (water butts, tyres) and underground water (sumps, drains, flooded cellars). Female mosquitoes feed on avian and mammalian blood typically beginning at dusk with a peak at midnight with rain, wind and cool conditions reducing feeding. The dispersion of tmosquitoes from their breeding site depends upon species with some species (such as Aedes caspius) creating biting nuisance at up to 8km from bredding sites and others (such as Culex pipiens) only traveling a few hundred metres from the breeding site. Some species overwinter as adults in sheltered locations with others remaining in egg (in damp hollows) or larval stages (in pools and ditches).
Health Impact
There are a number of diseases carried by mosquitoes including malaria, a range of arboviruses (West Nile Virus, Yellow Fever and Dengue). Mosquito-borne infections are still unusual in Europe however, concern is rising as international travel and trade and climate change are increasingly likely to introduce both vectors and pathogens with malaria and West Nile fever the main concerns in the UK. There is currently little evidence of mosquito-borne disease and the main concern is bites that can cause severe skin eruption and localised pain, and severe infestations can cause much distress. Bites occur near coastal marshes, woodland areas and some localized urban areas. They do not damage property or pose a contamination risk to foodstuffs.
Nuisance
There are no objective levels at which a statutory nuisance may or does exist. Mosquitoes can be monitored by indirect methods such as habitat assessment or direct method such as larval monitoring in water or traps (light traps, carbon dioxide baited light traps, odour traps). Where traps are used they are usually more effective in a humid, sheltered location with long vegetation away from bright light sources. As a general guideline, an occupier might feel irritation if five or more 'flying' mosquitoes are present in a room for three successive days.
Controls
Mosquito control should be aimed at both the larval and adult stages of life cycle, although as mosquitoes do not normally rest in buildings, control of adults can be impractical. Larval control can be achieved through eliminating or changing the characteristics of larval sites. These measures include:
- sanitation, environmental modification measures and biological control, to reduce the development of vectors and to maintain their density under a minimum threshold;
- larviciding with chemical or biochemical insecticides, for a rapid reduction of the likelihood of mosquito proliferation;
- residual adulticiding, to reduce the longevity of vectors and to stop the cycles of transmission; and
- passive protection measures, to reduce host-vector contacts.
Sanitation and water management
Sanitation and water management, such as source reduction, are key to any solution of a mosquito problem. Mosquito larvae can be found in clean and polluted, fresh and brackish, and stagnant or slow-flowing waters, such as marshes. They can also be found in river deltas, rice fields, swamps, tidal floodwaters, lakes, puddles, pools, ponds, tree holes, rock holes and creeks, as well as in gutters, flowerpots, tin cans, buckets, dishes, tyres, pits and cellars.
Source reduction is therefore the only long-term solution to mosquito infestation, especially in urban areas. Preventing stagnant waters is crucial in artificial areas that may provide breeding sites by avoiding stagnation in sewers, draining stagnant water from subfloor crawl spaces (or closing air gaps with mosquito nets), covering tyre stocks (or storing tyres in warehouses), removing containers that hold water or at least regularly replacing the water.
1. Man-made containers of water such as old car tyres, empty pots, open sewers and drains containing putrid and anoxic water should, as far as is practicable, be drained and kept empty.
2. Water can be channelled to increase flow.
3. Cesspools, and cisterns, mosquito nets should be installed in aeration gaps, to ensure that covers do not leak.
4. Septic tank open discharge should not be allowed. Instead an underground purification bed should be installed; once it is linked to a sewer system, the old pit should be filled or destroyed.
5. Wells, watering and leisure pools, rainwater tanks, swimming pools not in use, construction site excavations, potted plants with saucers, cemetery flower vases and diverse containers should be emptied every 10 days or be covered with mosquito nets. Moreover, all receptacles not in use should be eliminated. Fish can be introduced in leisure ponds (as they feed on the larva).
6. Rainwater butts and tanks should have close-fitting lids.
7. For rainwater channels, all obstacles to the flow of water should be removed.
8. Water treatment plants working on a part time basis can create problems, if they are too big for the amount of water to be treated. Also, abandoned plants that still retain rainwater can create problems.
9. Wastewater treatment ponds should be clear of rooted vegetation, and the water height should be maintained at more than 80 cm.
10. Ponds that use vegetation as a purification measure should be sufficient in number, so as to allow for more than one month of total dryness a year (in winter).
11. Embankments must be covered with cement or a geotextile fabric, and ponds must be designed in a manner that avoids zones with stagnant waters.
Larviciding
Treatment efficiency and economic and ecological costs influence the final choice of intervention methods and controlling larval development is most effective. The aquatic habitats that are breeding sites should be identified accurately. There are a number of larvicides typically chemical (temephos), biochemical (such as Bacillus thuringiensis var. israeliensis and Bacillus sphaericus) or growth regulators (such as diflubenzuron and methoprene). Biochemicals and growth regulators have the advantage of being more specific, whereas chemicals are less expensive and easier to use, especially for very large habitats
Light oil or lecithin can be applied to water to reduce the surface tension and prevent larvae from obtaining oxygen. Such agents spread readily over large areas. The technique should not be used where rivers, watercourses (other than open sewers and drains containing putrid and anoxic water), lakes or ponds may be affected and the Scottish Environmental Protection Agency ( SEPA) should be consulted before use.
Adulticiding
Treating adult mosquitoes can reinforce control, if efficacy in treating breeding
sites is poor or if larviciding is not possible. To increase control efficiency it can be advantageous to do a few treatments of this type, limiting both the number of applications and the area covered and this may avoid large-scale repetitive treatments in a sensitivel environment. However, adult control, which is usually done by fogging, must be carefully applied, due to its low specificity and risks (such as allergies or damage to vehicle paintwork) that may result. As a consequence such applications are often restricted to critical situations that result in a risk to health.
Adult mosquitoes can be killed outdoors and indoors, depending on where they rest. When done indoors, it is usually through spraying residual insecticides (generally a pyrethroid) on the walls of residences thus giving a long-lasting effect (up to two months or more). Outdoor treatment of adults uses ultra low volume ( ULV) applications produced by cold (emulsion) or thermal (diesel suspension) foggers, mounted on a vehicle or aircraft. This has no long-lasting effect and must be repeated daily during either periods of high risk of disease outbreak or periods of severe nuisance biting, originating from areas inaccessible to larviciding. The active ingredients available are organophosphates (such as fenitrothion and malathion) or pyrethroids (such as deltamethrin and permethrin). Spraying operations are usually carried out early in the morning before mosquitoes become active and target the resting places of adult mosquitoes (such as hedges and groves close to human habitations).
Use of predators (biological control)
This is an area of research involving a range of bacteria, fungi, protozoa, nematodes, viruses, fish, insects, snails and plants
Passive protection
Passive methods of protection may not necessarily eliminate the pest problem, but they can limit its impact. Protection can be achieved by avoiding vector-infested areas, by using physical barriers, such as screens and nets, or using space repellents such as burning basil-type herbs, seeds of the neem tree, tree wood and resin of aromatic trees or pyrethrum dispensers.
Genetic control and transgenic mosquitoes
Diverse approaches have been considered for genetic control of nuisance or vector mosquito populations. Genetic control consists of the release of genetically modified individualsin the field, to reduce or modify the composition of natural populations of target insects. The results of studies on this method have been inconclusive.
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