Type: Pasture Mosquito
Scientific Name: Ochlerotatus
Habits (Adult Behavior)
Economic and Medical Importance
Ochlerotatus nigromaculis is commonly referred to as the “pasture
mosquito” in California because of its prevalence in irrigated pastures.
It is a medium sized mosquito having blackish to brown coloration.
Individual mosquitoes usually have a white band near the middle of the
proboscis (beak). A pale white stripe occurs lengthwise on the top of the
pointed abdomen. Brilliant white bands occur around the bases of each
(tarsal) segment composing the end portions of the legs. Males resemble
the females but may be recognized by the bushy antennae on their heads and
“claspers” on the tip of their abdomen.
This mosquito is
distributed from Mexico to southern Canada throughout the Western and
Central States. Although widespread in California, this species is most
abundant within the great Central Valley in association with poor
irrigation practices on pasture lands. It is also found in alfalfa, rice
fields, row crops, irrigation seepage and associated drainage ditches with
changing waterlevels. Breeding has been found from sea level to an
elevation of 6,000 feet.
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Mosquitoes have four distinct life stages: egg, larval, pupal, and adult
as seen in the illustration at the bottom. The female Oc. nigromaculis
deposits its eggs singly (up to 150) on grass stems at or near the ground
in moist places. In permanent pastures, eggs are concentrated in great
numbers at the base of clumped grasses. The eggs remain unhatched until
flooded by the following irrigation. If water does not flood the eggs,
many may remain dormant and viable for a number of years. This species is
able to produce a brood following each flooding.
The eggs hatch into larvae
(wigglers) within a short period of time after coming into contact with
water. Small organic particles and microorganisms suspended in the water
are fed upon either at the bottom or near the water surface. Breathing
takes place at the water surface and is accomplished by means of orienting
upside-down with the tip of the tail (siphon) pointing up. In the hottest
climates, Oc. nigromaculis require about 3 days minimum for larval
development. At the end of the larval stage, the mosquito molts and
becomes a pupa (tumbler). Although aquatic, the pupa can survive on damp
soil. The pupa is active only if disturbed, for this is the “resting
stage” when the transformation from the larval stage to winged adult takes
place. This usually takes about two days, but may occur in less than one.
After this transformation has been completed, the adult splits the pupal
skin and emerges. It can take as little as four days for this mosquito to
develop from egg to adult or much longer in cooler periods and localities.
This depends upon the environmental conditions such as temperature and
food quality. This species overwinters in the egg stage.
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HABITS (ADULT BEHAVIOR)
Female Oc. nigromaculis are vicious biters, attacking human beings
and other mammals such as horses, cattle, dogs, and rabbits during the
day. They are most persistent near twilight.
Newly emerged adults may
remain near their larval habitat for the first 24-48 hours before flying
elsewhere. Flights of many miles to nearby communities are common. Females
are capable of flights up to 20 miles when seeking a blood meal. Males do
not bite, instead they feed on nectar and plant juices. Females may also
feed on plant juices, but must obtain a blood meal in order to develop
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ECONOMIC AND MEDICAL IMPORTANCE
Pasture mosquitoes are considered pests and can interfere with
agricultural operations as well as the use of recreational areas. Large
numbers of pasture mosquitoes around livestock can result in weight loss
and in some areas may affect milk production. This species is not known to
be a natural carrier of disease. It may be capable of serving as a vector
(carrier) of Venezuelan Equine Encephalitis (VEE) if this disease of man
and horses should ever become established in California.
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PREVENTION AND CORRECTION:
Important methods of controlling Oc. nigromaculis that occur in
irrigated pastures and alfalfa fields include physical control methods
(also known as source reduction or environmental modification) and
improvements in water management practices. Physical control methods
include proper grading of fields, and adequate drainage capacity to
prevent irrigation water from standing long enough to allow mosquitoes to
complete their development. Water management techniques include efficient
water usage (quantity and length of time applied). These methods and
techniques provide long-term control and serve to eliminate or greatly
reduce the need for chemical application.
The most commonly used biological control agent in mosquito control is the
mosquitofish (Gambusia affinis). Due to the factors of intermittent
irrigation and temporary presence of water, the use of mosquito fish is
not practical. Other biological control methods are being studied.
Problems with drainage or prevention may develop which can make it
necessary to use chemical control. Only trained mosquito and vector
control personnel should apply chemical control agents. Control agencies
have knowledge of the proper compounds and application techniques to
assure minimal environmental side effects.
It is important to remember
that chemical control provides only temporary relief and is used by
mosquito control agencies until other measures can be implemented.
Insect repellents may be
useful if it is necessary to be in an area where large numbers of these
adults are present. Always read and follow the directions on the label
carefully when using a repellent.
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