Mosquitoes & Other Vectors

There are over 3,000 mosquito species in the world, more than 50 reside in California, and 25 species are commonly found throughout Butte County. Regardless of adult mosquito populations, several species in Butte County are of public health concern including Culex tarsalis, Culex pipiens, Aedes melanimon, Aedes sierrensis, and Anopheles freeborni.

Common Name: Encephalitis Mosquito
Scientific Name: Culex tarsalis
Culex tarsalis is the primary vector of West Nile virus (WNV), Saint Louis encephalitis (SLE), and the western equine encephalomyelitis (WEE). Culex tarsalis breeds in a variety of aquatic habitats ranging from clean to polluted water sources including, but not limited to flooded agricultural lands, ditches, man made containers, ponds, and urban sources. In Butte County, this mosquito breeds year round and prefers to feed on birds, but does readily attack humans, horses, and cattle.

Common Name: Northern House Mosquito
Scientific Name: Culex pipiens
Culex pipiens is a major vector of West Nile virus (WNV) and can vector Saint Louis encephalitis (SLE), and the western equine encephalomyelitis (WEE). Larvae prefer polluted or foul water high in organic content. Usually found in artificial containers, storm drains, wastewater ponds, sumps, septic tanks, fountains, birdbaths, and un-maintained swimming pools. Birds are the principal blood meal, but will attack humans and invade their homes. Culex pipiens usually breed in the early spring to late fall in Butte County.

Common Name: None
Scientific Name: Aedes melanimon
Aedes melanimon is not only a major pest in localized areas near its larval breeding sources, but is a vector of West Nile virus (WNV) and it has been implicated as a secondary vector of western equine encephalomyelitis (WEE) virus in the Central and Sacramento Valleys. This species is most commonly found in intermittently flooded areas such as duck clubs and wildlife refuges. Aedes melanimon are very aggressive mosquitoes that readily feed on mammals and humans and can emerge as soon as five days after eggs hatch.

Common Name: Western Treehole Mosquito
Scientific Name: Aedes sierrensis
Aedes sierrensis is a major nuisance mosquito usually associated with the foothill regions in Butte County. This mosquito is the primary vector of Dog Heartworm. Larvae are generally found in treeholes and containers that have a lot of leafy material. Eggs hatch with the initial fall rains and over winter as larvae. This species of mosquito is a very small aggressive mosquito that usually surfaces in early spring and will be active into the summer months. Aedes sierrensis is a vicious biter of humans and other large mammals.

Common Name: Western Malaria Mosquito
Scientific Name: Anopheles freeborni
Anopheles freeborni is one of Butte County's most abundant pests and is the primary vector of Malaria. Larvae prefer clear, fresh water in sunlit or partially shaded pools. This mosquito is most commonly found in rice fields and roadside ditches with grass. An aggressive mosquito, most Anopheles freeborni commonly feed on mammals or humans and are most active at dawn and dusk.

How You Can Minimize Adult Mosquitoes
* Properly maintain swimming pools
* Empty and invert any container that holds water
* Change water frequently in bird baths and fountains
* Stock ornamental ponds with mosquito fish
(call BCMVCD to schedule a pick-up/delivery of mosquito fish)
* Check and clean rain gutters and downspouts
* Avoid over irrigation of lawns and fields

Personal Protection from Mosquitoes
* Reduce outdoor activity around dawn and dusk
* Wear long pants and shirts with long sleeves
* Use approved insect repellent
* Seal all openings on your window screens and doors to prevent mosquitoes from
entering your home

Africanized Honey Bees (also known as "killer bees") are descendants of bees brought from Africa to Brazil in the 1950s in an attempt to improve honey production in that country. Some African queen bees accidentally escaped and began to interbreed with the local European honeybees. The resulting hybrids (AHB) have been moving north ever since and are now established in southern California.

Africanized Honeybees (AHB's) are problematic not because of their killer sting, but because of the way they respond to disturbances around their nests. AHB's respond sooner to disturbances, stay agitated and attack for a longer period of time, and follow an attack victim for longer distances than European Honeybees (EHB's). In more than 40 years of hybridization with resident European Honeybees, the AHB's behavior has not changed significantly. AHB's toxin is no more harmful or deadly than EHB's, but AHB's respond by sending most of the bees in the nest to attack. This can be thousands of bees. A victim's reaction to an AHB attack varies depending on the number of stings received, the location of the stings and any special sensitivity that may have been developed by prior exposure to bee venom. Most healthy individuals can tolerate many stings without serious effects. A victim that receives hundreds or thousands of stings may exhibit toxic effects similar to a rattlesnake bite and may even die. Stings on the mouth or throat can cause a life threatening respiratory obstruction.

AHB's and EHB's can only be distinguished by an extensive laboratory examination. So if you see or encounter a bee's nest or a swarm, stay away from it. It may be an Africanized colony. Immediately notify your county Agriculture Department. Noise and vibrations from lawn mowers, weed eaters, odor from insecticides, physical contact, or motion in close proximity to a nest may elicit a defensive response from the bees.

If you are attacked, leave the area quickly. Cover your face to protect your eyes and mouth. Get into a shelter where bees cannot enter, such as a car or house. Do not dive or go underwater. AHB's have demonstrated tremendous patience in that they will wait for a victim to surface for air and then deliver stings to the mouth area. Individual bees can only sting once. After stinging, a bee leaves the stinger and poison sac imbedded in the victim. The poison emits an alarm pheromone (odor) that stimulates other bees to direct their sting in the same area of the victim's body. It is important to remove these poison sacs as they continue to pump poison into your body after it has detached from the bee. Don't pull the sac out with your fingers; that will squeeze more venom into you. The sac should be scraped out with a dull edge by dragging the edge along your skin. A credit card works well for this. Victims of multiple sting attacks need immediate medical attention.

Yellowjackets and hornets are made up of several species of wasps in the genera Vespula and Dolichovespula. These different species build nests in vastly different places. Vespula pensylvanica is a ground nesting species, often called the meat bee. Vespula vulgaris commonly builds its nests in rotting tree stumps in higher elevations. Vulgaris germanica is found frequently in urban areas, sometimes nesting in houses. Dolichovespula maculata, the bald-faced hornet, builds paper nests attaching them to eaves of buildings or limbs of a tree. Colonies of these species reach much higher numbers than the paper wasps. They can achieve populations between 1,500 and 15,000. Yellowjackets and hornets get involved with human activity, as they are attracted to some of the same kind of foods humans are. Other meetings can occur when people inadvertently disturb a ground nest or nest in a wall or tree. Yellowjackets and wasp are most problematic in the late summer or fall, when colonies reach their highest numbers. In Butte County, they usually involve one of five species: Vespula pensylvanica, Vespula vulgaris, Vulgaris germanica, Dolichovespula arenaria or Dolichovespula maculata. The first two are by far the most common in the Butte County area, and the third has appeared recently from the eastern US.

The paper wasp is made up of mostly the Polistes genus. They are easy to distinguish from other bees, hornets and yellowjackets as they are less aggressive and they build a hexagonal, open paper nest. Markings and colors vary but include yellows, browns and blacks. The nest is constructed in protected areas above the ground. Common areas their nests can be found include on walls or under eaves of homes and other buildings. Nest construction begins in the spring and construction and maintenance continues as long as the colony continues to grow. Wasps gather fibers from old decaying wood or dead, dry plants, chew them up and mix the debris with water to make their gray paper nest. Populations in these nests rarely ever exceed 200.

In the United States, seven kinds of hard ticks and five kinds of soft ticks carry diseases, are a nuisance, or cause paralysis. Often these diseases are transmitted by the tick's saliva during feeding behavior. However, some diseases, such as tularemia, can enter through the skin if a person comes into contact with a crushed infected tick. In recent years, Lyme disease has become the most reported arthropod borne disease in the country. Many experts feel that if it were not for AIDS, Lyme disease would be the number one infectious disease in the United States. The most common ticks found in Butte County are: the Western black-legged tick, Ixodes pacificus, the American dog tick, Dermacentor variabilis, the Pacific Coast tick, Dermacentor occidentalis; and the Brown dog tick, Rhipicephalus sanquineus.

Ticks are found wherever their hosts are found. Some ticks feed on only one type of host, while others suck blood from many different animals. When not attached and feeding on their hosts, most hard ticks live on the ground in vegetation, such as grassy meadows, woods, brush, weeds, leaf litter, etc. Most ticks will crawl to the tips of grasses, brush, leaves, or branches and wait. With their front legs outstretched, they will wait for a host to brush up against them. This behavior is called questing (see photo left). When the tick does come into contact with an animal, it will grab on and crawl to an appropriate area on the animal to feed.

The house mouse (Mus musculus) is one of the most frequently encountered rodent pests found near people and buildings. It quickly can establish itself in your home. This mouse is believed to be the second most populous mammalian species on Earth, after Homo sapiens. Mice nest in any undisturbed location, often in wall cavities, behind cabinets, in attics and garages. Their nests are made of shredded fibrous material such as cloth or paper. The house mouse usually weighs no more than one ounce. Its body is 2-3½" long with a tail measuring 3-4". Mice are primarily nocturnal, meaning they are active and feed mostly during the night. They get the water they need from the food they eat but will drink water when they find it. They'll eat many foods, but prefer seeds and cereal grains. On average, a mouse ventures only 10-30 feet from its nest. Mice can jump over 12" high, climb up the sides of buildings, and cross cables and wires. They also swim.

Mice have litters of 3-14 young. The unborn mice develop in 19-21 days. One female can have 5-10 litters per year. The newborn are blind and furless. Females reach sexual maturity at about 6 weeks and males at about 8 weeks, but both can breed as early as 35 days. House mice usually live under a year in the wild.

Roof rats (Rattus rattus) are usually gray to slightly brown in color. Adults are typically 8 inches long, with a 9 inch long tail. The tail is long, dark, and scaly. Roof rats get their name because they spend about 90% of their time above ground. They live in trees, run on power lines, the tops of fences, and love to set up shop in attics. They live within a range of 300–500 feet, frequenting sometimes several homes in a neighborhood. Like the house mouse, roof rats are also nocturnal. Females have 4-6 litters per year, with 6-8 young per litter. They are fully weaned within a month, and sexually mature in as little as two months. After three months of life, they are independent and on their own. They seldom live more than a year.

The Norway rat (Rattus norvegicus) can grow to twice as large as the Roof rat. They have a thicker, heavier body, a shorter tail and they have a more blunt snout. They are usually browner in color than Roof rats. Norway rats prefer to live at ground level, hence their presence in the sewers. They range 50-150 feet in their search for food and shelter. Females produce 6 to 12 young in each litter and have 4 to 6 litters per year. Like all rats, they have poor vision, and rely primarily upon their sense of smell and their highly specialized senses of feel and balance to survive.

As with all rodents, their teeth grow continuously, and thus they gnaw on many surfaces to wear down their teeth. They can chew through almost any material including masonry, steel, plastic, electric cables, water pipes, etc. posing a threat of flooding or fire.

Rodents urinate continuously, contaminating everywhere they go. A rat expels up to 5.5 liters of urine per year. Rats produce about 40 droppings per day, mice about 80.

Cutting off sources of food and water for rodents is the first step towards elimination. Store food, birdseed, pet food, garbage, compost, and recyclables in secure metal, glass, ceramic, or heavy-duty plastic containers with tight-fitting lids. Feed pets at scheduled times. Put unfinished food in the refrigerator. Promptly clean up spills and crumbs. Keep bird-feeding areas clean of spilled seed. Move firewood, garbage cans and debris piles away from the house.

Next, entry points need to be identified and sealed off. Poke steel wool or wire mesh into entry holes with a screwdriver or attach flashing. Prune branches away from the roof. Seal openings underneath and behind appliances with latex caulk. Seal gaps around water, gas, and heating pipes, heat registers, air ducts, electrical chases, and false ceilings.

Rats tend to be very shy and cautious animals. When confronted with a new object in their environment, such as a trap, they will often avoid it for days until they are used to its presence in the area. They have poor eyesight so they leave behind a strong pheromone odor telling them where to run. Set traps at night, when mice and rats are most active, and check or remove them in the morning. A dab of crunchy peanut butter on the trigger is an enticing lure. Live trapping with box traps or glue boards may be used for monitoring and for removal. It's better to trap intensively for a few days than to set only a few traps for a long time. Snap traps can be attached to rafters with nails and to pipes with wire. The trigger should snap towards the wall.

Rodenticides are sold in many different forms, including pellets, powder, blocks, and meal. If you choose this option, follow the manufacturer's label carefully or work with a professional. They are very effective in eliminating domestic rodent pests. Pesticides can be hazardous to children, pets, and wildlife. In addition, animals that eat poisoned mice could be poisoned themselves. Poisoned rodents may die inside walls and sub floors where they cannot easily be removed. Their slow decomposition creates foul odors and attracts other pests, such as flies and beetles.

When cleaning rodent droppings, urine, or nest materials, wear rubber gloves and a facemask with a HEPA filter. Ventilate the area. Don't stir up dust by sweeping or vacuuming. Instead, thoroughly wet contaminated material with a 10% chlorine bleach solution (1½ cups bleach in 1 gallon of water) or household disinfectant. Wipe up with a damp sponge and place in the garbage. Spray dead rodents and nests with disinfectant, then double-bag and dispose of them. Disinfect toys, silverware, or other items that may have been contaminated. Discard contaminated foods, drinks, napkins, paper plates, and cups.

Both rats and mice carry a number of potentially dangerous diseases, viruses and parasites. These cause a range of illnesses many of which can have serious consequences, most commonly food poisoning.

Hantavirus pulmonary syndrome (HPS) is a deadly disease transmitted by infected rodents through urine, droppings, or saliva. Humans can contract the disease when they inhale aerosolized virus.

Murine typhus occurs worldwide and is transmitted to humans by the bite of rat fleas.

Rat-bite fever (RBF) is a systemic bacterial illness caused by Streptobacillus moniliformis that can be acquired through the bite or scratch of a rodent or the ingestion of food or water contaminated with rat feces.

Salmonella causes a typhoid-like disease in mice. In humans S. Typhimurium causes severe disease. The disease is characterized by diarrhea, abdominal cramps, vomiting and nausea, and generally lasts up to 7 days.

Leptospirosis is a bacterial disease that affects humans and animals. Humans become infected through contact with water, food, or soil containing urine from these infected animals.

Eosinophilic meningitis is an infection of the brain. In a complex life cycle between rats, slugs and snails, humans become infected by accident when they eat an infected snail or food that has been contaminated by slugs or snails, particularly salads.