The life cycle of the mosquito consists of 3 aquatic stages - egg, larva (growing stage) and pupa, and a terrestrial adult (reproductive stage). The larvae feed on micro-organisms and their growth rate depends on water temperatures. Adult mosquitoes can subsist on plant juices and males do so entirely. Up to 300 eggs develop simultaneously in the female's ovaries and bloodmeals are taken to provide sufficient nutrient to make this possible. The rate of ovarian development is governed by ambient temperatures and, in this country, egg laying is usually more than a week after the bloodmeal. A fresh bloodmeal is required for each cycle of ovarian development. Bloodmeals are obtained from a variety of sources, amphibians, reptiles, birds or mammals, depending on availability and the preference of each mosquito species. Some mosquitoes feed on mammals, including man, and can be serious pests, especially if they occur in large numbers. Fortunately, of the 32 species of British Mosquito, only a small number bite man.
All Aedine mosquitoes are adapted to surviving long unfavourable periods by producing drought-resistant eggs. These eggs are deposited in temporarily dried out situations and will not hatch until flooded and subjected to other environmental conditioning. Not all eggs hatch at the first flooding and some require several cycles of alternate drying and soaking before they will hatch, and can remain viable for more than a year. Most Aedine species produce a single generation each year from eggs which hatch in the spring. The salt marsh species, Ae. detritus produces many generations, eggs hatching at any time if flooded. Larvae of this species which develop during the winter will not pupate until March, at the end of which mass emergence of bloodthirsty female mosquitoes may occur. Ae. detritus adults remain active until the arrival of winter, densities depending on rainfall and tidal patterns.
The pest mosquito problem at Sandwich is almost entirely due to the activities of Ae. detritus, which develops along the banks of the River Stour in situations periodically flooded by exceptional tides and/or rainfall. Although Tide Tables are published annually, actual tidal heights can deviate considerably from those predicted due to a combination of natural phenomena such as atmospheric pressure, wind strength and direction. The areas flooded by succeeding spring tides vary accordingly and, especially in rainy weather, can trigger hatching of eggs which have lain dormant for many months.
Culiseta annulata, another pest species with a wide distribution, is a large mosquito with a vicious bite but is seldom, if ever, as abundant as Ae. detritus. Its eggs, which are not drought resistant, are laid on a water surface. It may be found in a variety of waters, but has a marked preference for organically polluted water, such as is sometimes found in neglected water butts or overflows from farmyard waste. Ca. annulata 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, seldom feeding on human blood and the possibility of locally transmitted malaria is now extremely remove.
The British climate is not suited to the transmission of tropical diseases such as filariasis, yellow fever and dengue. Some mosquito-borne viruses causing 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. Nevertheless, severe infestations can cause much distress and are a valid reason for mosquito control.
Attempts to eradicate mosquitoes can be extremely expensive and invariably fail. Nevertheless, the more modest and realistic aim of mosquito control can, if the scheme is properly planned and operated, reduce even severe biting problems to acceptable levels.
Despite their evening biting habit, Aedes detritus does not normally rest in buildings, making adult control impractical and necessitating intervention in the larval sites. 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 is not the causal organism of a disease which spreads through a mosquito population, but 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 the Dover District Council.
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 either pyrethrum or synthetic pyrethroids.
In addition to Mosquito control operations, a number of measures can be taken for personal protection from mosquito bites. These fall into the following categories:
INSECTICIDAL AEROSOLS CONTAINING PYRETHROID INSECTICIDES
These quickly and effectively clear a room of insects, but do not prevent subsequent entrance of mosquitoes.
VAPOURISING 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 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.
REPELLENTS
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.
FLY SCREENS
Occupants of houses subject to regular invasion by mosquitoes can derive benefit by the installation of a special fly screen in the form of a blind set into a frame 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 ELECTRONIC GRIDS
These are less effective against mosquitoes than against flies.
ELECTRONIC BUZZERS
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 FOR INSECT 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.
