public domain image of Culex tarsalis courtesy of Centers for Disease Control
Mosquitoes, while not an unbearable problem in the Davis area, are a statewide problem largely because of the viruses they carry and spread. They are significant enough that local governments maintain mosquito and vector control units, such as the Sacramento-Yolo Mosquito Vector Control District. Areas with standing water, such as some backyard ponds, rice fields, wetlands, sewage treatment areas and irrigation ditches will have larger populations; these habitats also tend to be favored by birds and birders as areas of abundant food supply. In fact, various species of birds can serve as virus reservoirs with life-long immunity — mosquitoes acquire the virus when they feed on infected birds. Rodents such as rats and squirrels are two more carriers of arboviruses — arthropod-borne viruses.
At any given time of the year, we typically have at least one type of mosquito active — lucky us! Below is a list of those mosquito species most commonly found in our area. It is interesting to note that the state maintains flocks of chickens throughout the state as an early warning system for vector-borne disease outbreaks. The primary recommended modes of reducing human risk of vector-borne disease from mosquitoes is to reduce outdoor activities during dusk (peak feeding periods for most creatures, it seems), wear long pants and sleeves, & apply insect repellent (yes, DEET is still the most effective and reliable); it's recommended that dogs be on a heartworm maintenance program. Cases of mosquito-borne encephalitis and heartworm are not uncommon in the Central Valley. Most people infected with arboviruses are either asymptomatic or may develop flu-like symptoms, but in a small number of cases infections can cause coma or death. In summer 2005, the suggestion of spraying (by truck) insecticide over the Davis residential population caused heated arguments. (See the WNV page for details on this controversy.) In August 2005, the first case of West Nile Virus was confirmed in Yolo County and local authorities began spraying pyrethrin-based pesticides as a public health measure to control mosquito populations.
The most common method of control is the spraying of pesticides, mentioned above. Before it's ban, DDT was the pesticide of choice, being relatively harmless to people. However it was very toxic to birds, resulting in some egg shell thinning and massive infant bird mortality, and as a result, it was banned. After this the choice pesticide of use was the pyrethroids. This is based off a chemical found in a member of the Chrysanthemum family, and was a popular pesticide in it's natural form after World War II, thanks to it's quick knockdown time (because it affects the nervous systems of insects). However it had a weakness for sunlight, and quickly degraded. Chemical companies, in the interest of developing a better, and patentable, pesticide took the chemical compound and somewhat increased it's resistance to light. Pyrethroids however, are not without problems. They broadly kill many arthropods, and can have a deleterious effect on fish. In addition, pyrethroids are slowly waning in popularity because of increasing resistance found in many insects, including mosquitoes, and may be replaced by other compounds, similarly based on natural chemicals. These include neonicitinoids, which are based off of nicotine, which is a secondary chemical found in tobacco, which is quite effective, and Neem limonoids, which are based off of the compounds of the South Asian Neem tree.
Alternative methods of control, besides spraying petrochemical based pesticides, is to alter the habitat in order to discourage mosquito populations from growing. It is interesting to note that back in the late 1800s, Sacramento actually planted large groves of eucalyptus trees to control mosquito populations that spawned malarial outbreaks at the time. An effective method of habitat reduction is to reduce standing water that allows the larva to develop, including tree holes and rain barrels.
Another mosquito control is the usage of biological control. Primarily this includes the use of viruses and microorganisms like bacteria to either kill or sterilize the aquatic larva present in standing water. In some cases, it is said that microscopic cyclops protozoans can help in controlling larval populations. A popular, non-microscopic method of biological control is the release of Gambesiae fish, into water harboring mosquitoes, as these fish relish the larvae of all types of mosquito. There are even a few mosquito species whose larvae devour the larvae of other mosquitoes. Other control methods target the adult mosquitoes. These may involve generalist predators, like bats. Since implementation in Yellowstone Lake State Park bats are said to have significantly reduced the mosquito population; and also said to earn the economy of Austin, Texas $8 million/year from tourism dollars. A single brown bat can consume 600-1200 mosquitoes per hour and that the Austin bats consume an estimated 10,000-30,000 pounds of insects nightly. However, in order for a generalist predator to be an effective control, mosquito density must be high enough for random foraging to result in a drop of the mosquito population, and the height of activity for bats and mosquitoes must coincide.
Species
Encephalitis mosquito (Culex tarsalis)
This mosquito can transmit the encephalitis (sleeping sickness) virus to humans and horses, and can be found throughout California. Immature stages develop in wetlands, duck clubs, rice fields, and irrigated crops. It prefers to feed on birds in the spring and may feed on mammals in the summer.
Western malaria mosquito (Anopheles freeborni)
This mosquito can transmit the malaria parasite to humans and is common in areas with rice fields. Immature stages develop in rice fields, wetlands, duck clubs, and rain pools. It prefers to feed on mammals. This species is a pest in the Sacramento Valley from late winter until early fall.
Western treehole mosquito (Ochlerotatus sierrensis)
This mosquito can transmit the dog heartworm parasite Dirofilaria immitis, and is a severe outdoor pest feeding primarily on mammals. It is common in oak woodlands, developing in tree rot holes.
Wetlands mosquito (Ochlerotatus melanimon)
This mosquito is involved in the encephalitis virus (sleeping sickness) cycle and is a severe outdoor pest. It is common in the interior valleys of California. It develops in wetlands, duck clubs, and pastures. It prefers to feed on mammals.
House mosquito (Culex pipiens)
This mosquito creates a nuisance by disturbing sleep when it buzzes near a victim's ear. It is common throughout California. Immatures develop in foul water sources such as dairy drains and artificial containers. It prefers to feed on birds but will readily feed on humans.
Be sure to check out our Town Wildlife page to see what other non-human, non-vegetable dangers lurk about town.
