General Contact Number: (530) 621-5567

Vector Control

Mosquito 3Western 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.


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.


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.


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.


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