Scientific Name: Ochlerotatus
Habits (Adult Behavior)
Economic and Medical Importance
The female Ochlerotatus melanimon is a medium sized mosquito that
varies in color from brown to tan. The abdomen is predominantly dark
scaled transversed by a medium longitudinal whitish stripe and whitish
scales that form narrow bands across each abdominal segment (visible from
above). The end segments of the legs (tarsi) are dark with the exception
of broad white bands, which overlap the joints. The wings have narrow pale
and dark scales, which give them a “salt and pepper” appearance. The
markings of Oc. melanimon are almost identical to those of Oc.
dorsalis. Oc. melanimon are generally darker while Oc.
dorsalis are paler. There are differences in certain microscopic
characters in the adults and larvae of both species.
This mosquito is found
throughout the interior areas of California, especially the Central Valley
and the Sierra mountain range. Outside California, it is found in the
other western states and southwest Canada.
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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. melanimon
deposits its eggs singly (up to 150) on damp soil or at the base of
grasses that will be inundated at a later date. Suitable habitat for this
species includes irrigated pastures, alfalfa fields, duck clubs and other
seasonal waterfowl areas. In pastures also producing Oc. nigromaculis,
Oc. melanimon appears first in spring, is either replaced or
overshadowed by Oc. nigromaculis during the warmer weather, and
reappears late in the season. Oc. melanimon prefer the deeper,
cooler water in pastures and develop more slowly. Duck clubs and waterfowl
areas provide habitat for Oc. melanimon either alone or in
association with Oc. dorsalis (in brackish water areas of the
Sacramento River Delta). At a concentration of 1 percent salt, equal
numbers of both species can be found. As the percentage rises to 2
percent, Oc. melanimon disappears (Bohart, 1956).
Winter is usually passed in the egg stage. The eggs hatch during the first
irrigation of spring or during the late summer and fall. Multiple
generations are possible during the season (March-Nov.) in agricultural
sources. Eggs can withstand considerable drying and remain viable for a
number of years. They do not all hatch with the next flooding or
irrigation. The eggs hatch into larvae (wigglers) within a short period of
time after coming into contact with water. Small organic particles and
micro- organisms suspended in the water are fed upon either at the bottom
or near the water surface. The larval stage can last from 3 to 7 days
depending upon the water temperature and food quality. 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. 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” (1-2 days) when the
transformation from the larval stage to winged adult takes place. After
this transformation has been completed, the adult splits the pupal skin
and emerges. It takes from 4 to 10 days for the newly hatched larvae to
emerge as adults, depending upon the environmental conditions. Adults
disappear with the first frost during the fall or from November until
March or April (Solano Co.).
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HABITS (ADULT BEHAVIOR)
Female Oc. melanimon are vicious biters, attacking human beings and
other mammals such as horses, cattle, dogs, and rabbits during the day,
evening and especially at dusk if disturbed.
Newly emerged adults may
remain near their larval habitat for the first 24-48 hours before flying
elsewhere. 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 their eggs. The females are strong fliers,
capable of flights 10 or more miles from a source when assisted by
prevailing winds. Marked females have been recaptured up to 28 miles from
a breeding site.
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ECONOMIC AND MEDICAL IMPORTANCE
This species is considered a pest and can interfere with agricultural
operations as well as recreational activities in wildlife areas. It is
also a secondary vector (carrier) of the Western Equine Encephalitis virus
(sleeping sickness). Infection occurs while the mosquito is feeding on
viremic blacktail jackrabbits. This species is also the primary enzootic
vector of California (CE) group virus in the Central Valley. Recently it
has been shown to be a potential vector of West Nile Virus.
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PREVENTION AND CORRECTION:
Important methods of controlling Oc. melanimon 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 the elimination of depressions, 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.
Oc. melanimon that
occur in duck club and waterfowl habitat can be controlled by the
installation and maintenance of ditches and water control structures that
will facilitate rapid flooding and draining. Levees should be maintained
to prevent unwanted flooding.
The most commonly used biological control agent in mosquito control is the
mosquitofish (Gambusia affinis). Due to the nature of some of the
source types that Oc. melanimon is found in, i.e. irrigated
pastures; the use of mosquitofish is not always possible. The combination
of intermittent irrigation and the temporary presence of water make it
unsuitable habitat for mosquitofish. Field observations have shown that in
waterfowl areas with naturally occurring high populations of mosquitofish,
satisfactory levels of control may be possible.
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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