Written By Michael “Doc” Weissmann, Ph.D., Chief Entomologist

Culex_tarsalis_p_2Western_Encephalitis_Mosquito_250.jpgCulex tarsalis is one of “…a number of species of which little is known and which are not, as a rule, common or troublesome….” This is how Evelyn Groesbeeck Mitchell described this species in her 1907 book, Mosquito Life. By this time, it was already established that malaria was transmitted by Anopheles mosquito species, and that Aedes aegypti was the vector of Yellow Fever. The link between mosquitoes and some forms of encephalitis was not yet known.

Today, Culex tarsalis is also known as the Western Encephalitis Mosquito. It is a species of concern throughout much of the United States, especially west of the Mississippi River, for its competency when it comes to vectoring several forms of encephalitis. In many areas, this species is the primary vector of Western Equine encephalitis, St. Louis encephalitis, and more recently, West Nile virus. Although usually preferring to seek birds as a blood meal source, they also readily bite other animals, transferring these diseases from the bird reservoir to some mammals, including humans.

As mosquitoes go, the Western Encephalitis Mosquito is one of the more easily recognizable, with its distinctive scale patterns. The legs have white banding on each side of the joints, and the proboscis is adorned with a bright white band of scales in the middle. The purpose of these bands is unknown, but they may help the mosquito recognize potential same-species mates, or assist with orientation when flying, perching, and feeding on nectar-rich flowers.

“Feeding on nectar-rich flowers? But I thought mosquitoes drank blood!” This is a common misconception about mosquitoes. For most species, their main source of food as adults is nectar from flowers and other plant juices. Males live exclusively on nectar or similar fluids, while females need more protein to produce eggs and will take a blood meal to supplement that need. The preferred species of flower visited is usually poorly documented and varies with the mosquito species, as well as the season and geographical location. In situations where a mosquito species specializes on only certain flowers, such knowledge could be useful in controlling the mosquito. Unfortunately, most nectar feeding occurs after sunset, making it difficult to study this behavior, such that even for medically important species like Culex tarsalis, little is known about their nectar preferences.

Species in the genus Culex are known as “standing-water” mosquitoes. Unlike their “floodwater” relatives (such as Aedes vexans) that lay eggs above the water line, standing-water mosquitoes must lay their eggs directly on the water’s surface. Culex eggs are laid one at a time, but attached together to form a raft of 100 or more eggs. The structure of the individual eggs and the way in which they are attached together make the egg raft able to float on the water surface until they hatch, usually within a couple of days after being laid. These mosquitoes must have standing water for egg-laying and larval development. In the case of Culex tarsalis, which is highly opportunistic when it comes to seeking a water source, they usually occur in natural or man-made swamps, or other semi-permanent waters, often with high organic content. In the arid regions of the west, such watery habitats were uncommon prior to large-scale agriculture and urban development, making this species much more common today than it would have been historically.

Western Encephalitis mosquitoes overwinter as adults, historically in caves or tree hollows. Today they can be found mid-winter in human structures like mine shafts, storm sewers, culverts, sheds, cool basements, and garages. All these places are cool enough that the overwintering mosquitoes do not burn up all their fat reserves, yet protected from the elements and warm enough that they do not experience a deadly hard freeze.

Prior to the spread of West Nile virus into the western USA, at the beginning of the century, Culex tarsalis was ignored for the most part by mosquito abatement professionals, who were at the time more focused on reducing mosquito annoyance from more common floodwater species. Encephalitis outbreaks were relatively rare. West Nile virus changed that. Today, reducing the incidence of disease has become the primary focus of most mosquito control programs in the west, and Culex tarsalis has become the primary target species in those efforts.

VDCI is committed to public education and spreading awareness throughout the U.S. about the dangers of mosquito-borne diseases and their preventability, with the overarching goal of reducing illness and fatality statistics. Our dedicated and experienced staff works tirelessly to prevent the spread of mosquito-borne diseases in all of the contracts we service. If you would like more information about any aspect of an Integrated Mosquito Management (IMM) Plan, including mosquito surveillance, disease testing, or adult control, please contact Vector Disease Control International (VDCI) and we will help you get started immediately.

Michael_Doc_WeissmanPhD.jpgMichael “Doc” Weissmann is the Chief Entomologist at Colorado Mosquito Control, a VDCI company. He has been identifying mosquitoes since the mid-1980s, and has been in charge of the Surveillance Laboratory at CMC since 2003. Doc received his B.A. and M.A. in Biology from the University of Colorado, and his Ph.D. in Entomology from Colorado State University. He can be reached through the VDCI website or by calling 800.413.4445.


Contact the professionals at 800.413.4445 for all of your integrated tick and mosquito management needs.

Since 1992, Vector Disease Control International (VDCI) has taken pride in providing municipalities, mosquito abatement districts, military bases, industrial sites, planned communities, homeowners associations, and golf courses with the tools they need to run effective integrated tick and mosquito management. We are determined to protect the public health of the communities in which we operate. Our tick and mosquito mangement professionals have over 100 years of combined experience in the field of public health, specifically vector disease control. We strive to provide the most effective and scientifically sound mosquito surveillance and control programs possible based on an Integrated Mosquito Management approach recommended by the American Mosquito Control Association (AMCA) and Centers for Disease Control and Prevention (CDC). VDCI is the only company in the country that can manage all aspects of an integrated tick and mosquito management program, from surveillance to disease testing to mosquito aerial application in emergency response situations.

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