Mosquitoes
Author: Dr Peter Jupp
Licensed under a Creative Commons Attribution license.
Introduction
Culex species are mainly bird feeders biting at night
Some culicine mosquito species are of veterinary
and feed on mammals to a lesser extent. In contrast
importance because they are vectors which transmit
Aedes and Ochlerotatus are avid feeders on livestock
certain arboviruses to livestock causing disease and
and humans that bite mainly in the daytime. Culex
mortality. These viruses are Rift Valley fever (RVF),
deposits a raft of eggs on water while the other genera
Wesselsbron (WSL), West Nile (WN), Eastern equine
deposit their eggs singly on a moist substrate such as
encephalitis
encephalitis
mud at the margin of a flooded area. Provided that the
(WEE) and Venezuelan encephalitis (VEE). All of them
humidity is adequate in this microhabitat these eggs
also cause human disease.
can survive for long periods until the next rains. Hence,
(EEE),
Western
equine
Culex must bridge the winter or dry season as either
Morphology and systematics
quiescent or hibernating larvae and/or adults while the
The mosquito vectors all belong to the subfamily
other 2 genera can survive as quiescent or diapausing
Culicinae and nearly all belong to 3 genera namely
eggs buried just below the soil surface. Shortly after
Culex (Cx.), Aedes (Ae.) and Ochlerotatus (Oc.). In the
emerging from the pupae the adults mate and the
case of Aedes, 2 different subgenera Aedimorphus
female seeks a blood-meal. Both sexes feed on floral
(Adm.) and Neomelaniconion (Neo.) are important
nectar and plant sap, the male relying entirely on these
while all the important Culex spp. belong to the
for his sustenance.
subgenus Culex (Cux.). This module on mosquitoes
describes the main features of mosquito morphology
Distribution
and shows how these are used to identify the 3 genera.
Rift Valley fever is confined to Africa and Saudi Arabia,
Identification of individual species is a specialist's job
while WSL is confined to Africa except for an
but some of the vector species have striking features
unconfirmed occurrence in Thailand. Until recently WN
that make them readily identifiable – these will be
virus was known only in Africa, Israel, India, Pakistan
described. This module also deals with how the
and France but has now spread to further parts of
morphology of the male genitalia and the larva are
Europe and widely through the USA. Of the equine
essential for distinguishing members of the important
encephalitis viruses, EEE and WEE are endemic to the
Cx. pipiens complex.
USA while VEE occurs in the tropical and subtropical
Americas. Mosquito vectors of these viruses have been
Biology
identified in all these areas and will be listed in this
All mosquitoes have “complete metamorphosis” with a
module.
life cycle of egg – larva - pupa - adult, but the biology of
Culex differs from that of Aedes and Ochlerotatus,
particularly in the egg stage.
Importance
are mainly directed at determining which species are
Arboviruses by definition multiply in both vertebrates
vectors in a particular locality. To find out which species
and
are present in an area, it is necessary to employ a
certain arthropods
including
mosquitoes
as
considered here. These mosquito vectors are therefore
variety
most important as they both play a role in maintenance
collections, as will be described. The main methods for
of the viruses and also transmit them to livestock and
collecting adults depend on either animal bait (humans
humans to cause illness and sometimes mortality. The
or sheep) or CO2 bait. Also the “gravid trap” that
transmission cycles for each of the viruses are
collects gravid Culex mosquitoes as they come to
described in this module. Vectors of RVF virus are
oviposit will be explained. Sources for obtaining traps
species
will be mentioned as will ways of killing mosquitoes and
of
Aedes
(Aedimorphus),
Ochlerotatus
(Ochlerotatus) and Culex (Culex). The interepidemic
period usually lasts for years and vertical (transovarial)
transmission may be one mechanism for virus survival
through this period. Wesselsbron virus has a very
similar ecology to RVF except Culex species are not
involved as vectors. West Nile virus is transmitted
between wild birds by Culex (Culex) species in an
endemic cycle and from this cycle infection is
transmitted to horses and humans sometimes by
additional link vectors. The equine encephalitis viruses
(EEE and WEE) are similar to WN in that they also
have endemic cycles between birds and mosquitoes
but further link vectors transmit these viruses to horses
and humans. VEE virus has a complex ecology
involving rodents and a number of mosquito species
and there are also endemic and epidemic subtypes of
this virus.
Control
There are a variety of ways stabled horses can be
protected against mosquito bites
described.
which will be
Alternatively, stables and barns can be
sprayed with carefully chosen insecticides as will be
recommended.
As to RVF and WSL outbreaks on
farms in Africa, immunization of stock should be the
first line of defence. Mosquito densities can then be
reduced, if necessary, by using an appropriate larvicide
such as the insect growth regulator Methoprene.
Collection methods
The reasons for collecting insects will be listed but they
of
collection
preserving specimens.
methods,
including
larval