Mosquitoes in the Dover district
Although mosquitoes are almost everywhere within in our district, there is a noticeable population of nuisance biting mosquitoes in Sandwich. This is almost entirely due to the activities of 'Aedes detritus', which develops along the banks of the River Stour in sites that are sometimes flooded by exceptional tides and/or rainfall. The areas flooded by spring tides vary and can trigger hatching of eggs which have been dormant for many months.
The banks of the River Stour at Sandwich is the only location that we carry out mosquito spraying. We monitor the levels of lavae three times a year in key locations. If the larvae levels reach required targets we spray the areas to kill the lavae and reduce the population. If the larvae does not reach the targets no spraying takes place. The spraying only kill larvae in their early stages and will not remove adult mosquitoes.
The land at Sandwich Haven is designated an SSSI1, SPA2, SAC3 and a RAMSAR4 site, which means we need special permission to treat the area. Natural England has permitted us to treat on the site as long as certain guidelines are met.
- The first restriction is the dates we can treat.
No treatment can be undertaken between the 30th April and the 1st September. This is to reduce the impact that the treatment will have on flora and fauna
- We are limited to 3 treatments a year - Autumn, Winter and Spring
- Treatment can only take place if agreed trigger levels are reached during monitoring of mosquito larvae
'Culiseta annulata' is another mosquito with a vicious bite but is not as common as 'Aedes detritus'. It's eggs, which are not drought resistant, are laid on a water surface. It can be found in a variety of waters, but is usually found in organically polluted water, such as neglected water butts or overflows from farmyard waste. This mosquito can be active at any time of the year, even during mild periods in winter. Complaints of biting mosquitoes very late, or early in the year, are most probably due to this species.
Although mosquito-borne diseases are common in many parts of the world, malaria is the only human infection known to have been transmitted by mosquitoes in this country. Due to improved housing and related environmental factors, the malarial mosquito, Anopheles atroparvus now frequents cattle sheds and stables and seldom feeds on human blood. The possibility of locally transmitted malaria is now extremely remote.
The British climate is not suited to the transmission of tropical diseases such as filariasis, yellow fever and dengue. Some mosquito-borne viruses cause low fevers occur in Southern and Central Europe, but none have been detected in this country. British mosquitoes have a nuisance value and, in some cases, cause severe skin eruption and localised pain.
Attempts to eradicate mosquitoes can be extremely expensive and invariably fail. However, the more modest and realistic aim of mosquito control can, if the scheme is properly planned and operated, reduce the incidence of bites.
Despite their evening biting habit, Aedes detritus does not normally rest in buildings, making adult control impractical and necessitating intervention in the larval stage. Larval control can be achieved by eliminating or altering the characteristics of the larval sites or by attacking the larvae. The former method can only be done piece meal over a period of years. The only environmentally acceptable method of attacking larvae is by the application to the larval sites of formulations of Bacillus thurengiencis Serotype H14 (Bti). Bti produces a crystal which, when ingested by a mosquito larva, breaks down into a stomach poison. BTI affects only the Dipteran family Nematocara, to which mosquitoes belong. Other serotypes affect different families of insects and have been used for the protection of glass house crops. Both source reduction and larval control are used or proposed for future operations undertaken by Dover District Council.
What Prevention Measures Can You Take?
Land owners and gardeners can take preventative measures against larvae by removing habitat. Cesspools, septic tanks and drains must be sealed. All rainwater butts and tanks should have close-fitting lids. Garden ponds stocked with fish will not require treatment, but dis-used ponds and other areas of stagnant water should be drained and kept empty. Adult mosquitoes may be killed in gardens by the use of pesticides known as pyrethroids (an insecticide derived from the dried flower head of Chrysanthemum (pyrethrum) plants) or synthetic pyrethroids. However please note these can be highly toxic to fish.
In addition to mosquito control operations, a number of measures can be taken for personal protection from mosquito bites.
Aerosols containing pyrethroid insecticide
These quickly and effectively clear a room of insects, but do not prevent subsequent entrance of mosquitoes.
Vaporising mats and mosquito coils
A small electric hotplate on which is placed a vaporising mat containing a pyrethroid insecticide will give up to ten hours protection inside a room. The insecticide evaporates at the temperature of the hotplate which can be supplied to run off mains electricity or off a 12v battery. A non-electric model filled with alcohol, but without flame, is available for tents or outbuildings. Coils which smoulder and emit a pyrethroid smoke are cheaper than, but not as effective as, mats. Some people complain that the smoke causes headaches.
Nets or curtains impregnated with pyrethroid.
Whilst a demand for nets is not foreseen, it may be of interest to know that, as well as being insecticidal, pyrethroid impregnated nets afford added protection. These pyrethroids are insect irritants and mosquitoes will not try to pass through a mesh almost large enough to fly through. Impregnated curtains are reported to inhibit entry of mosquitoes into rooms.
Chemical repellents afford almost the only real protection against outdoor biting mosquitoes. Lotion, roll-on stick, cream aerosol and soap formulations are available from pharmacists. Di-ethyl toluamide (DEET) is the active ingredient of most, though some may contain di-methyl phthalate (DMP), ethy hexanedion, or citronella. Some people find them unpleasant and care should be taken to avoid eyes and lips. Repellents are not long lasting on the skin. Because of these drawbacks, it may be preferable to impregnate a cotton garment with a repellent chemical. DEET impregnated netting, jackets, socks and sheets will remain effective for weeks, especially if kept in a plastic bag when not in use.
Impregnation of a garment weighing 120gms requires 30mls (= ½ pint) of water. The resulting milky liquid is poured over the garment, which is allowed to dry and then stored in an air-tight container until required. For garments of different weights, the quantities of DEET and water are changed accordingly. Pure DEET is obtainable from the Medical Advisory Service for Travellers Abroad, London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT.
In houses subject to regular invasion by mosquitoes it is beneficial to install a fly screen in the form of a blind set into a frame. This is attached to the house window frame with runners at each side to hold the cloth netting when in use. Permanent metal fly screens are also available, but as the mesh must be fine to keep out the insect, the loss of visibility rules out this method in most domestic situations.
Ultra-violet lights with electric grids
These are less effective against mosquitoes than against flies.
Electronic buzzers are widely advertised for protection against mosquitoes, but have been shown to be totally ineffective. Some brands have been withdrawn after prosecution under the Trades Description Act.
Treatment of mosquito bites
Should you or your family receive mosquito bites, lesions may be treated by the application of an anti-histamine cream obtainable from chemists shops. In severe cases, particularly involving babies or young children, a general practitioner should be consulted as treatment for anaphylactic shock may be needed.
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