This species breeds in tree holes, which are water filled rot cavities or depressions found in many species of trees, especially oaks, bay laurel, eucalyptus, sycamore and elm. Any container near trees, that is partially filled with water and leafy debris, may also produce this pest. The eggs hatch when the tree hole or container fills with water. The adults emerge in March and remain in the area until early summer. This mosquito has a short flight range, is an aggressive biter, and is the primary vector of dog heartworm in Napa County.

The Western Treehole Mosquito (Aedes sierrensis) is one of 53 types of mosquitoes that occur in California. This mosquito is brightly marked with white scales which contrast with its dark body. It also has an unbanded proboscis (beak), white banded tarsi (feet) and a pointed tipped abdomen. The Western Treehole Mosquito (Aedes sierrensis) received its name because the immature stages are frequently found in water contained in rot holes of trees such as oak, laurel, madrone, eucalyptus and other local species. This mosquito is found in most California counties and is the primary vector of Dog Heartworm disease in the Western United States.

Diseases Transmitted

Western Treehole Mosquitoes are a serious pest problem when they are present in large numbers. This mosquito is the primary vector of Dog Heartworm Disease in the coastal and foothill communities of California.

Life Cycle

Western Treehole Mosquitoes have four life stages: egg, larva, pupa, and adult. The immature stages need standing water to complete their life cycle.

After an adult female lays her eggs they hatch into larvae (wrigglers), which feed on small organic particles and microorganisms in the water. Feeding occurs when they hang from the water’s surface by the tip of their tail (siphon) or by browsing along the bottom of their habitat. Because they are air breathing organisms they must return to the water’s surface to breathe.

Larval development varies from ten days to five months depending on weather conditions with developmental completion occurring around the spring equinox (late March). At the end of the larval stage, the mosquito molts and becomes the aquatic pupa (tumbler). The pupa is active only if disturbed, for this is the resting stage where the larval form is transformed into the adult. This can take four or more days during which time feeding does not occur. When the transformation is completed, the new adult splits the pupal skin and emerges.

Adult Daily Activity

Adults begin to emerge with the advent of the spring equinox, requiring a 12 hour day length to trigger emergence. Males tend to hover around potential hosts of the female, seizing her in flight when she approaches, to mate with her. Male mating swarms also occur in the shaded areas of this mosquitoes habitat. Adults can live up to several months depending on temperature, humidity and other climactic factors. They are frequent pests in residential and recreational areas April through August where large numbers of trees are present.

Adult Flight Range

This mosquito has a limited flight range staying very close to its breeding site.

Adult Feeding

Adults feed predominantly on small mammals but will feed on large mammals and humans when available. Peak feeding activity occurs at dusk, although host feeding does sometimes occur during the day and night. Treehole Mosquitoes prefer to feed outdoors (but sometimes enter homes) during the mid-morning and late afternoon. Like all species of mosquitoes, only the female mosquito takes a blood meal (bites).

Eggs and Larvae

Eggs are laid individually inside moist treeholes, crotches of trees, and containers with damp leafy debris. Eggs usually hatch the next season following flooding with early winter rainfall. It should be noted that the eggs of this species can survive for many years before hatching, with only part of each batch hatching during a single season. Larval development can take ten days to five months to complete, depending on quantity of rainfall and other environmental conditions.