Mosquitoes
About Mosquitoes
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. In
the UK, 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 found
in Britain, only a small number bite humans.
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. 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, Aedes. detritus produces
many generations. The eggs may hatch at any time if flooded. Larvae
of this species which develop during the winter will not pupate
until March. This may result in mass emergence of bloodthirsty
female mosquitoes. Aedes detritus adults remain active until the
arrival of winter, densities depending on rainfall and tidal
patterns.
Mosquitoes in the Dover district
Although mosquitoes are prevalent almost everywhere within in
our district, there is a noticible 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 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 Aedes 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. Culiseta 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.
Health aspects
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. Nevertheless, severe
infestations can cause much distress.
Anti-mosquito measures
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 Chyrsanthemum (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.
Aerosals containing pyrethroid insecticide
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
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.
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
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
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.
Contact
Environmental Protection Team
Dover District Council
White Cliffs Business Park,
Dover, Kent CT16 3PJ
Tel: 01304 872428
Email: envhealth@dover.gov.uk
Contact Customer Services
Team Telephone: 01304 872428
E-mail: customerservices@dover.gov.uk
