The Mosquito Control Program in Tahoe (East Slope) and on the West Slope have been designated as seasonal programs with mosquito control services provided during the mosquito season, usually from late Spring into the Fall season. The Tahoe program also provides services for yellowjacket control. For services in El Dorado County, West Slope (Placerville area) call (530) 621-5300. Mosquito fish can be provided free of charge. Please report any abandoned swimming pools or possible mosquito breeding sources to our offices. For services in the Tahoe Basin call (530) 573-3197. Mosquito fish are not used in the Tahoe Basin. 

How Best to Avoid West Nile Virus .... Follow the Four (4) D's

• Drain the water. Eliminate all sources of standing water that can support mosquito breeding.

• Avoid going out during dusk and dawn. Mosquitoes are active during these times. They are especially active for two hours after sunset.

• Dress appropriately. Wear long sleeves and long pants. Also make sure that doors and windows have tight fitting screens. Repair or replace screens that have tears or holes.

• Wear a good repellant that has DEET in it. Apply according to directions.

Mosquitoes and ticks can be found throughout California, and these insects bite! You can help prevent insect bites and the spread of diseases carried by mosquitoes and ticks by using insect repellent. Click here to learn more!

Mosquito Fish (Gambusia affinis) - Defense from Mosquitoes

Mosquito fish (Gambusia affinis) are widely used to help reduce mosquito breeding. Their adaptability, hardiness, and ability to produce large numbers of young in a short time, has made them valuable biological control agents for mosquitoes.

Mature females measure two to two and a half inches long, and males from one to one and a half inches. Females can reach maturity in six to eight weeks. They bear three to four broods of young in a season; the first may number only a dozen, but later broods include 60 to 100 young. Under favorable conditions, mosquito fish live two to three years.

FEEDING: 

Mosquito fish seldom need supplementary food. During the summer there is usually enough food in the pond, and in the winter their bodies slow down so much that very little food is needed. If they are given supplementary food, caution must be exercised. Overfed fish may not eat mosquito larvae, and any excess food that the fish do not eat can cause a bacterial bloom toxic to the fish.

POND MAINTENANCE:

Mosquito fish like the shelter of rocks and plants, but they do not thrive in heavily shaded ponds, and they tend to dwell in the sunny areas. In large ponds, the fish may even avoid mosquito infested shaded areas if they find or are provided other food. Mosquito fish are cannibalistic, so plants are necessary to help shelter the very young.

Normally six to ten fish are stocked in an average home pond, more if a large number of mosquito larvae are present. In some cases, the pond may need to be drained and cleaned, or treated with safe, state approved chemicals prior to introduction of the fish.

For information concerning mosquito fish availability, please call Environmental Management Dept. at (530) 621-5300 

Marin / Sonoma Mosquito & Vector Control District 

Links to Other Information

West Nile Virus Web Page 

WNV : El Dorado County Activity

How to Avoid Mosquito Bites

Symptoms of West Nile Virus

Western Tree Hole Mosquito

The Western Treehole Mosquito, Ochlerotatus (Aedes) sierriensis, is a common pest mosquito in Western El Dorado County and the most important vector (carrier) of dog heartworm. It is found in areas where older trees have had time to develop rot cavities or pockets between limbs that can hold rain or irrigation water. It is also found in containers in which organic debris and leaves have accumulated.

Adult mosquitoes of this species are characteristically very small, dark insects with brilliant white bands on their legs. The adults are long-lived (up to six months and are found from March through June and, sometimes depending upon rainfall amounts, through August. They often appear as swarming white spots.

Treehole mosquitoes are persistent biters of man and animals. They most commonly bite in the evening although they will readily bite all day in shady areas. Both the male and the female are attracted to potential hosts, giving the impression of many more biting mosquitoes. Only the female bites for a blood meal. The male feeds on plant juices and does not take a blood meal. Occasionally the males will form a swarm in shady areas. Treehole mosquitoes are normally outdoor biters, but may enter homes on occasion.

The life cycle of this species is centered on treeholes and containers and they will not fly far from their larval sources. Female mosquitoes lay eggs on the damp surfaces just above the water line. The eggs remain dormant until the container is refilled with water by rain or irrigation. The eggs hatch into larvae which progress through four instars. Larvae transform to pupae that is the transition stage between larva and adult. An adult mosquito emerges from a pupal case in just a few days. If an adult female mosquito is successful in obtaining a blood meal, she will return to a source to lay eggs, beginning the cycle again.

CONTROL OF TREEHOLE MOSQUITOES 

The best method to control treehole mosquitoes is to eliminate treeholes and containers that hold water around your home.
Treeholes, especially those that hold water, are causing damage to the tree. Although very effective for controlling mosquitoes, filling, drilling, or cutting, may expose new wood or hold moisture causing the rot to spread. For the health and preservation of the tree, it would be ideal to keep cavities and treeholes dry by installing deflectors to prevent water entry. There are also new products on the market which when distributed into the hole, will expand and absorb any water which has accumulated (these products are more beneficial before major water accumulation has occurred). Once dry, the product then returns to its original solid form (most are in the form of crystals) until more moisture has once again accumulated. Some of these products can last through a few seasons without repeated application and there is no harm done to the tree. Old stumps can be removed, filled, or buried. It is recommended that you consult with a tree specialist or nursery advisor for information on stopping the damage caused by rot cavities. This is especially important if the affected tree is a valuable part of your landscaping.

There is no adequate, fully effective, long-term biological control for this species. There are a number of experimental techniques now being used to provide partial control.

El Dorado County uses a biological agent to treat larval sources to reduce the number of mosquitoes for one season. We treat the larval source and do not treat for adult mosquitoes. It is not possible for us to eliminate all of the tree hole mosquitoes, because of the thousands of potential sources and the difficulty in locating them.

WHAT YOU CAN DO TO CONTROL TREE HOLE MOSQUITOES 

Carefully check your property for containers and treeholes.

Eliminate containers and locate water-holding treeholes.

Call the county to treat water-holding treeholes you are unable to eliminate. In most cases treatment begins sometime in March or the first part of April.

DOG HEARTWORM, YOUR DOG AND YOU 

Canine heartworm disease is a clinical condition in dogs (has also been discovered in cats) caused by the nematode parasite Dirofilaria immitis that resides within the dog or cat’s heart. This disease is a serious veterinary problem having become widespread throughout the United States and tropics and is primarily associated with dogs. Canine heartworm can only be transmitted by the bite of mosquitoes. The Western Treehole Mosquito is the most important vector (carrier) of canine heartworm.

The life cycle of the nematode parasite causing canine heartworm disease has three basic stages. The adult female worm, while residing in the animal’s heart releases immature worms, known as microfilariae, into the dog or cat’s blood stream. Further development of the microfilariae requires that a mosquito bite the infected animal. The ingested microfilariae then continue their development for the next three months, reaching a length of approximately three inches. The worm then migrates to the heart to complete its growth and release more microfilariae to start the cycle all over again.

Studies have shown that the adult worms live an average of five years and the microfilariae can persist for three years. This means than an infected dog can be a source of infection to other dogs for many years. It may be advisable to have your animal on preventative medication if you feel your animal will be at risk to vector mosquitoes or will travel to high risk areas.

Severe cases of dog heartworm result in general weakness, coughing and labored breathing. In advanced cases treatment is difficult and some animals may die from cardiopulmonary failure. Visible symptoms do not occur in the early stages of the disease, although your veterinarian can diagnose and treat this disease. It is recommended that you check with your veterinarian about early detection and treatment.

DOG HEARTWORM RISK CHART

Outdoor Dog                                                           Indoor Dog
Heavily Wooded (wet) High                                        Heavily Wooded (wet) Moderate  
Heavily Wooded (dry) Moderate                                 Heavily Wooded (dry) Moderate
Not Wooded (Urban) Low                                          Not Wooded (Urban) Vey Low

Mosquitoes in the Tahoe Area

The mosquito population in the Tahoe Basin is most active in the spring and early summer. Each mosquito species has a season when it is most active and a range of preferred hosts. Most mosquitoes will feed on more than one type of host. The female mosquito needs a blood meal in order to produce eggs. Blood meal hosts range from reptiles, amphibians, mammals, birds, to humans. All mosquito species are potential vectors of organisms that can cause disease to pets, domestic animals, wildlife, or humans.

Common Tahoe Area Mosquitoes:

Aedes (cataphylla, hexodontus, increptius, tahoensis)

These mosquitoes are known as "snow melt" mosquitoes. They occur in snow melt pools and usually do not fly far from their larval sources. They are day or night biters.

Aedes ventrovittis
The "little black mosquito of the high Sierras" is a small mosquito that breeds in shallow grassy meadow pools. Females do not fly far from larval sources, and are vicious day biters of large mammals including man.

Culex tarsalis

This is the "Encephalitis mosquito" and can transmit the encephalitis virus to humans. Immature stages develop in any standing freshwater source. Females are night feeders mostly on birds, but will switch to mammals including man.

Culiseta (inornata, incidens, impatiens)
These mosquitoes breed in ponds, basins, and artificial containers. Females actively feed at dawn and dusk mainly on large domestic animals and man.

Mosquito Borne Diseases:

Encephalitis (sleeping sickness)
Encephalitis is a flu-like illness caused by a virus. Symptoms include high fever and inflammation of the brain. Severe infections are usually marked by acute onset, headache, high fever, disorientation, and occasionally convulsions. Most common in California is Western Equine encephalitis (WEE) and St. Louis encephalitis (SLE). Mosquitoes become infected while feeding on birds that carry the virus. The mosquito can then infect humans with the virus when they bite. 

Malaria
Malaria is also a flu-like illness caused by a protozoan. Symptoms include fever, shaking chills, headache, nausea, and ending with profuse sweating. After an interval free of fever, the cycle of chills, fever, and sweating is repeated every 2 to 3 days. Man is the only important reservoir of human malaria, and mosquitos become infected while feeding on other humans that harbor the parasite. The disease is transmitted when an infective female anopheline mosquito bites a human. 

West Nile Virus (WNV) 
This is an emerging infectious disease that was first recognized in the United States in 1999 in the New York area. It was associated with a die off of birds in the area, especially crows. Symptoms in man include low-grade fever, slight fatigue, aches, and mild headache. The elderly and individuals with immunocompromised systems may experience severe headache, neck stiffness, high fever, various central nervous abnormalities, and sometimes death. This virus has been detected in California.

Canine Heartworm
This is a disease that affects dogs only. It is caused by a worm that damages the lungs and heart of a dog. Symptoms are not evident until later stages of the disease. Dogs may develop a chronic cough, tire easily, and accumulate fluid. The heartworm parasite can cause lung, liver, and kidney damage, or death. Mosquitoes attain worms by feeding on infected dogs, coyotes, or foxes.

Precautions Against Mosquito Bites:

Avoid activity outside at dawn and dusk when mosquitoes are most active.

When engaging in outdoor activities, wear long pants, long sleeve shirts and other protective clothing and apply insect repellant according to label instructions.

Keep infants indoors during peak mosquito hours. If outside, cover cribs, bassinets or playpens with suspended mosquito netting.

Identify and eliminate all sources of standing water around property that can support mosquito breeding.

Horses also are susceptible to mosquito-borne viruses and should be vaccinated each year.