Solano County Mosquito Abatement District

MOSQUITO NOTES
Scientific Name: Ochlerotatus melanimon

General Information
Life Cycle
Habits (Adult Behavior)
Economic and Medical Importance
Control Methods

GENERAL INFORMATION
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.

LIFE CYCLE
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.).

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.

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.

CONTROL METHODS
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.

BIOLOGICAL CONTROL
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.

CHEMICAL CONTROL
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.