According to the California Health and Safety Code, a vector is an insect or animal capable of transmitting disease or causing harm to people or animals. Vectors in Placer County include mosquitoes, ticks, yellowjackets and rodents.


Mosquitoes are common flying insects that are found in most parts of the world. There are over 3,500 species of mosquitoes worldwide, 53 species in California and over 30 species in Placer County.


Ticks are blood-sucking arthropods that can transmit a wide variety of diseases and there are multiple species of ticks in Placer County.


Yellowjackets are medium-sized black and yellow wasps that can be public health pests because they commonly nest and forage close to people.


Rodents and their fleas are capable of transmitting a variety of human diseases. The District is primarily concerned with community-wide risk of rodent-borne disease like plague, hantavirus or large community-wide infestations.

Vector Library

Type Genus and Species Official Common Name Other Names Behavior Disease Transmission Fun Facts
Mosquito Aedes aegypti Yellow fever mosquito Container breeding mosquito Aggressive day biter Capable of transmitting Zika, yellow fever, dengue and chikungunya. Invasive mosquito to Placer County. Detected in 2019 near south Placer county border. Has not been detected since.
Mosquito Aedes sierrensis Western treehole mosquito Western treehole mosquito Found in areas where older trees have had time to develop rot cavities or pockets between or within limbs which can hold rain or irrigation water. Female mosquitoes feed on a wide variety of mammals, and will aggressively try to bite people at any time of day. Warmer weather and longer days in the Placer County foothills trigger the emergence of Western treehole mosquitoes in the spring Western treehole mosquitoes are not known to commonly vector human disease, but they are the primary vector of the parasite Dirofilaria immitis that causes heartworm in dogs and cats. Western treehole mosquitoes are unlikely to transmit West Nile Virus. Adult Western treehole mosquitoes avoid sunlight and are much more likely to bite you in a shady location! Western treehole mosquitoes are widely distributed in Western North America from Mexico to British Columbia, and throughout California.
Mosquito Aedes increpitus complex (includes Ae. increpitus, clivis, and washinoi) N/A Snow pool mosquito Aggressive day biters of mammals. Do not fly far from larval sources and will readily feed on humans. Common in western Placer County in the spring/rainy season and in the Truckee and Tahoe Basin area when snow is melting. May transmit California encephalitis virus Adult mosquitoes of the Aedes increpitus complex cannot be distinguished morphologically.
Mosquito Aedes tahoensis N/A Snow pool mosquito Females are persistent, vicious biters of large mammals, including humans, but adults of this species rarely stray far from the larval development site. Capable of vectoring Jamestown Canyon virus. Aedes tahoensis is an obligatory univoltine species, meaning there is only one generation per year.
Mosquito Anopheles freeborni N/A Rice field mosquito, Western malaria mosquito Aggressive biter of large mammals. Will seek shelter inside homes and other buildings. Historically, one of two primary vectors of Malaria in Placer County. Malaria does not currently circulate in California. Especially active in west Placer including the Westpark neighborhood because of its proximity to the rice fields. One of two species likely responsible for the spread of malaria in Placer County in the early 1900s.
Mosquito Culex tarsalis N/A Western encephalitis mosquito, Standing water Mosquito Primarily feeds on birds, but also bites mammals, including humans. One of two primary vectors of West Nile virus in Placer County. Also a vector of Western Equine and St. Louis encephalitis virus. Culex eggs are laid one at a time but attach together to form a floating raft.
Mosquito Culex pipiens Northern house mosquito Standing water Mosquito Primarily feeds on birds, but readily bites mammals, including humans. One of two primary vectors of West Nile virus in Placer County. Also a vector of St. Louis encephalitis virus. Part of a species complex with Culex quinquefasciatus. These species may hybridize where territories overlap.
Mosquito Aedes bicristatus N/A Snow pool mosquito Not very aggressive but will occasionally bite humans. Not associated with known disease transmission. One of the earliest appearing Aedes species of the season with larvae maturing in January.
Mosquito Aedes cataphylla N/A Short-palped livestock mosquito, Snow pool mosquito Feeds on mammals, including humans. Capable of vectoring Jamestown Canyon virus. One of the first species of larvae to appear after the snows begin to melt.
Mosquito Aedes fitchii N/A Snow pool mosquito Will readily bite humans, particularly in shaded areas near their larval sources. N/A Lives along the crest of the Sierra Nevadas from Fresno County into the Great Basin area of Shasta County.
Mosquito Aedes hemiteleus N/A Snow pool mosquito Active in spring and early summer in higher altitude areas of Placer County. Will bite humans near larval sources. N/A Reluctant to feed on humans, except in well shaded areas.
Mosquito Aedes hexodontus N/A Snow pool mosquito Persistent day biters, particularly in shaded areas. May vector Jamestown Canyon virus, California encephalitis group virus. This species has been found naturally infected with California encephalitis (CE) group virus.
Mosquito Aedes melanimon N/A Irrigated pasture mosquito, floodwater mosquito Associated with irrigated pastures and alfalfa fields. Aggressive evening biters of large mammals, including humans. In Placer County, most abundant in late summer and fall. Capable of vectoring Western equine encephalitis, California encephalitis group viruses. Will bite at any time of day if resting places are disturbed by a host.
Mosquito Aedes nigromaculis N/A Irrigated pasture mosquito, flood water mosquito Associated with irrigated pastures and alfalfa fields. Bite readily during the day but are more active at dusk. May be capable of vectoring Western equine encephalitis, California encephalitis group viruses. Development from hatching egg to adult mosquito may occur in as little as four days.
Mosquito Aedes sticticus N/A Flood water mosquito Associated with stagnant water along rivers. Vicious day and dusk biters. May travel far from larval sources to feed on domestic animals and humans. N/A Eggs will remain viable for at least three seasons in the absence of flooding.
Mosquito Aedes schizopinax N/A Snow pool mosquito Prefer to feed on mammals. Not aggressive biters of humans. N/A Found along the crest of the Sierra Nevadas from Inyo County to the Great Basin.
Mosquito Aedes ventrovittis N/A Snow pool mosquito Persistent daytime biters and annoying to humans in mountain meadows and woods. N/A Adults are among the first mosquitoes to emerge after the melting of snow in the high elevations of California.
Mosquito Aedes vexans Vexans mosquito Inland floodwater mosquito, Flood water mosquito Occurs in all types of floodwaters, particularly areas shaded by trees or shrubs. Troublesome day and dusk biters. May travel great distances to feed on large mammals, including humans. Can breed two or more generations. Minor vector of encephalitis viruses and the dog heartworm parasite Dirofilaria immitis. Aedes vexans can be easily distinguished from other Aedes species by the scalloped white bands on their abdominal terga.
Mosquito Anopheles ranciscanus N/A N/A Will sometimes bite humans, but prefer to feed on larger mammals such as horses and cows. Generally considered to have a low potential as a vector of human malaria. Immature stages of An. franciscanus can be found in shallow pools along the margins of receding streams during the dry season. Larvae are also commonly associated with abundant growth of green algae in pools exposed to sunlight.
Mosquito Anopheles punctipennis N/A Woodland malaria mosquito Females day and dusk feeders on large mammals, including humans. Historically, one of two primary vectors of Malaria in Placer County. Malaria does not currently circulate in California. May be a vector of the dog heartworm parasite Dirofilaria immitis. One of two species likely responsible for the spread of malaria in Placer County in the early 1900s. It was also likely responsible for a malaria outbreak in a Girl Scout camp near Nevada City in the 1950s. Problems with this species led to one of the first mosquito abatement programs in California.
Mosquito Culex apicalis N/A N/A Females are opportunistic night biters, feeding equally on reptiles, amphibians, and birds. Considered unlikely to bite humans. Larvae are typically found in shaded cut-off pools along streams in woodland environments. N/A This species has been successfully reared in captivity by allowing females to feed on toads.
Mosquito Culex boharti N/A Bohart’s mosquito Cx. boharti larvae prefer partly sunlit stream pools with an abundant growth of aquatic vegetation. Distributed throughout the lowlands and foothills of California. Prefer to feed on amphibians, not likely to bite humans. N/A Adult Cx. boharti feed exclusively on amphibian hosts.
Mosquito Culex erythrothorax N/A Tule mosquito Larvae prefer water with extensive tule or cattail growth. Females usually remain close to their wetland habitat and utilize a broad range of blood sources, including birds and humans. Possible minor vector of West Nile virus and other encephalitis viruses. This is one of the few Culex species which overwinter as a 4th-stage larva.
Mosquito Culex stigmatosoma N/A Banded foul water mosquito Larvae are associated with foul-water sources high in organic content (e.g. sewage, dairy, log ponds). Females feed predominately on birds but can bite humans and mammals. Secondary vector of West Nile virus, and other encephalitis viruses. This mosquito is found in greatest numbers in sewage waters.
Mosquito Culex territans N/A N/A Usually feed on reptiles and amphibians, but may occasionally bite humans and other mammals. N/A They bite cold blooded amphibians and reptiles.
Mosquito Culex thriambus N/A N/A Larvae found rock pools, ponds, and small depressions in lowland areas and riparian woodlands. Night biter that feeds predominately on birds. N/A Larvae are sometimes found in the water-filled pools of Native American grinding stones in Placer County.
Mosquito Culiseta incidens N/A Cool weather mosquito Both larvae and adults are most abundant during the cooler months of the year. Breeding occurs in a wide variety of natural and artificial small water bodies. Overwinters as adult females. Females actively feed at dusk and dawn on mammals including humans. N/A Big mosquito with dark spots on wings.
Mosquito Culiseta inornata N/A Large winter mosquito, Winter marsh mosquito Larvae occur in a wide variety of natural and artificial habitats. Adults most abundant fall through spring. Females feed from dusk to dawn on large mammals, including primarily cattle and occasionally humans. May be a very minor vector of some arboviruses. One of the largest mosquitoes in Placer County.
Mosquito Culiseta particeps N/A N/A Larvae occur in foothill environments in shaded clear pools containing algae, leaves and other debris. Females feed on large mammals and humans near breeding sources. N/A Populations are seldom abundant enough to present a nuisance to humans except in isolated rural settings.
Ticks Ixodes pacificus Western black-legged tick Hard tick Active from October to July. Larvae and nymphs feed on small animals like rodents, birds, and lizards. Adults feed on larger mammals including deer. This tick is the primary vector of Lyme disease in the western United States. When a tick bites a lizard, the lizard’s blood destroys the bacteria that causes Lyme disease and prevents further spread of the pathogen.
Ticks Dermacentor occidentalis Pacific Coast tick Hard tick Active from November to June. Larvae and nymphs feed on small rodents while adults feed on large mammals, especially deer. Capable of transmitting Rocky Mountain spotted fever, tularemia. Ticks are actually classified as arachnids, or relatives of spiders, scorpions and mites.
Ticks Dermacentor variabilis American dog tick Hard tick Active from May to August. Larvae and nymphs feed on smaller mammals while adults feed on larger mammals like dogs. Capable of transmitting Rocky Mountain spotted fever, tularemia. Ticks require a blood meal for sustenance.
Ticks Ornithodoros hermsi Relapsing fever tick Soft tick These soft ticks are found in the nests of their rodent hosts. Humans are most likely to be bitten while sleeping in mountain cabins that also house chipmunks or other rodents. Tick-borne relapsing fever. This tick looks different than the others because it is a member of the soft-tick family. Soft ticks do not have the hard shield and they are shaped like a raisin.
Rodents Rattus rattus Roof rat Black rat Roof rats are agile climbers and usually live and nest above-ground in shrubs, trees and dense vegetation such as ivy. In buildings, they are most often found in enclosed or elevated spaces such as attics, walls, false ceilings and cabinets. The roof rat has a more limited geographical range than the Norway rat, preferring ocean-influenced, warmer climates. In areas where the roof rat occurs, the Norway rat may also be present. Roof rats are dangerous vectors of many human diseases such as plague, typhus, leptospirosis, rat-bite fever, trichinosis, and salmonellosis. In Placer County, roof rat problems often occur when rats feed on fruit trees and pet food.
Rodents Rattus norvegicus Norway rat Brown rat Their burrows are found along building foundations, beneath rubbish or woodpiles and in moist areas in and around gardens and fields. Nests can be lined with shredded paper, cloth or other fibrous material. When Norway rats invade buildings, they usually remain in the basement or ground floor. Norway rats live throughout the 48 contiguous United States. While generally found at lower elevations, this species can occur wherever people live. Norway rats are carriers of several serious diseases of humans including leptospirosis, rat-bite fever, cowpox virus, trichinosis and salmonellosis. This species gave rise to domesticated rats commonly used for research and kept as pets. Domesticated rats are no more likely to spread disease than other common pets like cats and dogs.
Rodents Mus musculus House mouse N/A House mice thrive under a variety of conditions. They are found in and around homes and commercial structures as well as in open fields and on agricultural land. House mice can transmit diseases such as lymphocytic choriomeningitis virus, leptospirosis, and salmonellosis. A mouse can squeeze through a hole as small as a dime!
Rodents Peromyscus maniculatus North American deermouse Deer mouse Deer mice prefer forests, grasslands and agricultural crops and most often come into contact with humans in cabins, sheds, and outbuildings bordering natural or agricultural land. Vector for the rare but often fatal Sin Nombre virus, the cause of hantavirus pulmonary syndrome. Deer mice shed the virus in their urine, droppings and saliva. In areas, especially indoors, where these rodents may be found, take special precautions to prevent hantavirus transmission like wearing protective goggles and a mask, spraying a disinfectant on areas with droppings, urine or nest materials and wet mopping to avoid sweeping or vacuuming. Deer mice are jumpers and runners that receive their name due to their agility.
Yellowjackets Vespula pensylvanica, germanica, atropilosa, acadica, alascensis, sulphurea yellowjackets wasps, meat wasps, meat bees, sweat bees Yellowjackets can be very aggressive and cause serious injury when swarming to defend their nest. Yellowjackets can be public health pests because they commonly nest and forage close to people. Because of their territorial behavior and affinity for human food, yellowjackets can restrict or prevent outdoor activities in areas such as campgrounds, picnic areas, and backyards. Yellowjackets do not transmit disease, but stings are painful and can be life-threatening to those who are allergic or who receive large number of stings. Unlike honeybees, yellowjackets do not leave a stinger behind, allowing them to sting multiple times.