There are over 2500 different species of mosquitoes throughout the world of
which 150 species occur in the United States. 52 species occur in California,
and 19 species occur in Alameda County. In the course of the District's
operation about 10 species are commonly found in the County. Eight of the
species account for over 99% of complaints from the public.
Each of the
species has a scientific name that is latin, such as
Culex
tarsalis. These names are used in a descriptive manner so that the name
tells something about this particular mosquito. Some species have what is called
"common names" as well as scientific names, such as
Anopheles
freeborni, the "Western malaria mosquito".
All mosquitoes must
have water in which to complete their life cycle. This water can range in
quality from melted snow water to sewage effluent and it can be in any container
imaginable. The type of water in which the mosquito larvae is found can be an
aid to the identification of which species it may be. Also, the adult mosquitoes
show a very distinct preference for the types of sources in which to lay their
eggs. They lay their eggs in such places such as tree holes that periodically
hold water, tide water pools in salt marshes, sewage effluent ponds, irrigated
pastures, rain water ponds, etc. Each species therefore has unique environmental
requirements for the maintenance of its life cycle.
The feeding habits of
mosquitoes are quite unique in that it is only the adult females that bite man
and other animals. The male mosquitoes feed only on plant juices. Some female
mosquitoes prefer to feed on only one type of animal or they can feed on a
variety of animals. Female mosquitoes feed on man, domesticated animals, such as
cattle, horses, goats, etc; all types of birds including chickens; all types of
wild animals including deer, rabbits; and they also feed on snakes, lizards,
frogs, and toads.
Most female mosquitoes have to feed on an animal and
get a sufficient blood meal before she can develop eggs. If they do not get this
blood meal, then they will die without laying viable eggs. However, some species
of mosquitoes have developed the means to lay viable eggs without getting a
blood meal.
The flight habits of mosquitoes depend again on the species
with which we are dealing. Most domestic species remain fairly close to their
point of origin while some species known for their migration habits are often an
annoyance far from their breeding place. The flight range for females is usually
longer than that of males. Many times wind is a factor in the dispersal or
migration of mosquitoes. Most mosquitoes stay within a mile or two of their
source. However, some have been recorded as far as 75 miles from their breeding
source.
The length of life of the adult mosquito usually depends on
several factors: temperature, humidity, sex of the mosquito and time of year.
Most males live a very short time, about a week; and females live about a month
depending on the above factors.
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Mosquito Life Cycle
The mosquito goes through four separate and distinct stages of its life cycle
and they are as follows: Egg, Larva, pupa, and adult. Each of these stages can
be easily recognized by their special appearance. There are four common groups
of mosquitoes living in the Bay Area. They are
Aedes, Anopheles,
Culex, and
Culiseta.
Egg : Eggs are
laid one at a time and they float on the surface of the water. In the case of
Culex and
Culiseta species, the eggs are stuck
together in rafts of a hundred or more eggs.
Anopheles and
Aedes species do not make egg rafts but lay their eggs separately.
Culex, Culiseta, and
Anopheles lay their eggs on water
while
Aedes lay their eggs on damp soil that will be flooded by
water. Most eggs hatch into larvae within 48 hours.
Larva : The larva (larvae - plural) live in the water
and come to the surface to breathe. They shed their skin four times growing
larger after each molting. Most larvae have siphon tubes for breathing and hang
from the water surface.
Anopheles larvae do not have a siphon and
they lay parallel to the water surface. The larva feed on micro-organisms and
organic matter in the water. On the fourth molt the larva changes into a
pupa.
Pupa: The pupal stage is a resting, non-feeding
stage. This is the time the mosquito turns into an adult. It takes about two
days before the adult is fully developed. When development is complete, the
pupal skin splits and the mosquito emerges as an
adult.
Adult: The newly emerged adult rests on the
surface of the water for a short time to allow itself to dry and all its parts
to harden. Also, the wings have to spread out and dry properly before it can
fly.
The egg, larvae and pupae stages depend on temperature and species
characteristics as to how long it takes for development. For instance,
Culex tarsalis might go through its life cycle in 14 days at 70 F
and take only 10 days at 80 F. Also, some species have naturally adapted to go
through their entire life cycle in as little as four days or as long as one
month.
The following pages show a typical mosquito egg raft, larva, pupa,
and adult, and explains more about each stage.
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Mosquito Egg Raft
Culex mosquitoes lay their eggs on
the surface of fresh or stagnant water. The water may be in tin cans, barrels,
horse troughs, ornamental ponds, swimming pools, puddles, creeks, ditches, or
marshy areas. Mosquitoes prefer water sheltered from the wind by grass and
weeds.
Culex mosquitoes usually lay their eggs at night. A
mosquito may lay a raft of eggs every third night during its life
span.
Culex mosquitoes lay their eggs one at a time,
sticking them together to form a raft of from 200- 300 eggs. A raft of eggs
looks like a speck of soot floating on the water and is about 1/4 inch long and
1/8 inch wide.
Tiny mosquito larvae emerge from the eggs within 24 hours.
Notes: Anopheles
mosquitoes lay their eggs singly on the water, not in rafts.
Aedes
mosquitoes lay their eggs singly on damp soil.
Aedes eggs
hatch only when flooded with water (salt water high tides, irrigated pastures,
treeholes, flooded stream bottoms, etc.).
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Mosquito Larva
Mosquito larvae, commonly called "wigglers" or
"wrigglers", must live in water from 7 to 14 days depending on water
temperature.
Larvae must come to the surface at frequent intervals to
obtain oxygen through a breathing tube called a siphon. The larva eats algae and
small organisms which live in the water.
During growth, the larva molts
(sheds its skin) four times. The stages between molts are called instars. At the
4th instar, the larva reaches a length of almost 1/2 inch.
When the 4th
instar larva molts it becomes a pupa.
Note :
Anopheles are
unlike
Culex and
Aedes larvae since they do not have a breathing
tube, they must lie parallel to the water surface in order to get a supply of
oxygen through a breathing opening.
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-
Mosquito Pupa
-
Mosquito pupae, commonly called "tumblers", must live in water from 1 to
4 days, depending upon species and temperature.
The pupa is lighter than
water and therefore floats at the surface. It takes oxygen through two breathing
tubes called "trumpets". When it is disturbed it dives in a jerking, tumbling
motion and then floats back to the surface. The pupa does not eat.
The
metamorphosis of the mosquito into an adult is completed within the pupal
case.
The adult mosquito splits the pupal case and emerges to the surface
of the water where it rests until its body can dry and harden.
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Mosquito Adult
Only female mosquitoes bite animals and drink blood. Male mosquitoes do not
bite, but feed on the nectar of flowers.
Aedes mosquitoes are
painful and persistent biters, attacking during daylight hours (not at night).
They do not enter dwellings, and they prefer to bite mammals like humans.
Aedes mosquitoes are strong fliers and are known to fly many miles
from their breeding sources.
Culex
mosquitoes are painful and persistent biters also, but prefer to
attack at dusk and after dark, and readily enter dwellings for blood meals.
Domestic and wild birds are preferred over man, cows, and horses.
Culex
tarsalis is known to transmit
encephalitis (sleeping
sickness) to man and horses.
Culex are generally weak fliers and do
not move far from home, although they have been known to fly up to two miles.
Culex usually live only a few weeks during the warm summer months.
Those females which emerge in late summer search for sheltered areas where
they "hibernate" until spring. Warm weather brings her out in search of water on
which to lay her eggs.
Culiseta mosquitoes
are moderately aggressive biters, attacking in the evening hours or in shade
during the day.
Anopheles
mosquitoes are the only mosquito which transmits
malaria to man.
Mosquito adult, larva and pupa drawings modified from
Aquatic Entomology , by W. Patrick McCafferty; Illustrations by Arwin V.
Provonsha; Jones and Bartlett Publishers, 1983
