Western tree hole mosquitoThe Western tree hole mosquito (Aedes sierrensis) receives its name because the immature stages of this mosquito species are frequently found in water contained in rot-holes of trees, such as oak, laurel, madrone, eucalyptus, and other local tree species. They will occasionally breed in artificial containers such as roof gutters, tires, cans, and buckets. They are frequently pests in residential and recreational areas March through August, where large numbers of trees are present. As vicious biters, they can be a severe nuisance and are the vector of dog heartworm disease in California. Tree hole mosquitoes prefer to feed outdoors, but sometimes enter homes during the mid-morning or late afternoon. Like all species of mosquitoes, only the female bites.

What does a Western tree hole mosquito look like?

The Western tree hole mosquito is small, dark-bodied, and has white markings on its back and legs.

What is dog heartworm disease?

Canine heartworm disease is a clinical condition in dogs caused by a roundworm, Dirofilaria immitis, which resides within the dog’s heart and lungs. This disease, a serious and possibly fatal veterinary problem, is associated with dogs, coyotes, and foxes. Canine heartworm is transmitted by the bite of an infected Western tree hole mosquito.

What are the symptoms of dog heartworm disease?

The outward symptoms of the disease are not noticable in most cases, until reduced blood flow caused by adult worms damages the heart, lungs, liver, and kidneys. Advanced symptoms of heartworm may include rapid tiring, shortness of breath, chronic soft or dry cough, listlessness, and weight loss.

How can I prevent my dog from acquiring heartworm?

Check with a veterinarian regarding treatment options and prevention. Drugs are available to prevent the disease and it is curable if diagnosed in the early stages.

In California, the time of highest risk for dogs to contract heartworm is March through August, however unseasonable rain may extend this period.

How does a Western tree hole mosquito transmit dog heartworm disease?

A dog already infected with heartworm is bitten by a tree hole mosquito (a). That mosquito, now infected with the microfilariae (very small worms), can then go on to bite a different dog (b). Now that dog is infected. The microfilariae work their way to the dog’s heart and adjacent large blood vessels and through the blood stream where they continue to grow. As adults, they produce more microfilariae and the heartworm life cycle continues.

How can I prevent Western tree hole mosquitoes?

Residents should examine trees on their property for holes, cavities or crotches. These can hold water and provide mosquito habitat. Adding water absorbing polymer crystals available at local nurseries to the holes are often a sufficient remedy, however, residents may want to contact their local nursery or tree surgeon to determine the best way to correct the problem.

Residents should also:

* Check and clean out gutters that collect plant debris and trap water.

* Empty, turn upside down, or throw away containers that collect water, such as cans, buckets, and old tires.

* Use an insect repellent if it is necessary to be in an outdoor area during mosquito activity.

Residents of Contra Costa County can also contact the District by phone: (925) 685-9301 or make an online service request.