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Treehole Mosquitoes  & Dog Heartworm

Spring is the prime time for the emergence of the western treehole mosquito, also known as Aedes sierrensis. As the name suggests, these mosquitoes lay their eggs in the cavities of oak, bay, and several other types of trees. Treehole mosquito larvae can also be found in discarded tires or containers where plant debris and water has accumulated. Residents living in areas with dense populations of trees or adjacent to recreational areas and open space may experience large numbers of treehole mosquitoes. These mosquitoes can bite aggressively during the day and evening. 

As far as we know, treehole mosquitoes don’t transmit pathogens to humans, but they can spread canine heartworm, which is a serious and potentially fatal disease in dogs. Treehole mosquitoes become infected when they feed on dogs or coyotes that harbor the heartworm parasite. Once the mosquito is infected, it can transmit the parasite to other animals.

When an infected mosquito feeds, the heartworm larvae drop out of the mosquito’s mouthparts and enter the animal through the puncture wound. The larvae then live in the tissues under the skin for an average of two months before migrating to the animal’s lungs and pulmonary artery of the heart, where they multiply.

Medications are available to prevent dog heartworm. Contact your veterinarian for more information.

 Ways you can minimize treehole mosquitoes on your property:

  • Inspect trees for holes, including holes as small as half an inch that lead to cavities that may hold water. Contact a licensed arborist to discuss measures to prevent water from accumulating in tree holes.
  • Remove buckets, toys, tarps or other items holding water.
  • Check your gutters for clogs and standing water.
  • Contact the District for advice on reducing mosquito populations on your property.
  • For temporary relief from treehole mosquitoes in your backyard, try using one or more oscillating fans pointed away from seating areas. Treehole mosquitoes are weak flyers and have difficulty flying against the wind generated by the fans.


For more information or to report mosquito issues call 707-285-2200 or visit


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