Mosquitoes are usually thought of as a summer phenomenon, like barbecues or days at the beach. But have you ever wondered what happens to them during the ‘off season’? The answer may be more complicated than you think because it differs by species (and we have 23 known mosquito species in Contra Costa County).

Culex mosquitoes, like the ones that are the primary vectors of West Nile virus, and Anopheles mosquitoes (potential vectors of malaria) go into diapause (a form of hibernation) during the coldest part of the year. In late October or November, adult females seek out sheltered locations like caves, culverts or underground storm drains where they will be protected from extreme temperatures. They remain there until the first sign of warmer weather, often in late February or early March, when they emerge from their hiding places, take a blood meal from a bird or other warm-blooded host, and produce eggs to start the mosquito life cycle once again. One of the leading theories for how West Nile virus gets started every year is that female mosquitoes that were infected the previous fall pass the virus on to birds when they emerge to feed in spring.

Other mosquitoes, like the ‘winter mosquito’ Culiseta inornata are actually most active during the winter months and absent or rare during the summer. These are our largest mosquitoes and will sometimes bite people, although they are not known to transmit disease and generally don’t fly far from where they emerged, so they rarely cause significant problems.

Certain Aedes mosquitoes, like pasture mosquitoes, rainwater mosquitoes, and salt marsh mosquitoes, spend part of the year as eggs on plants or soil in areas that are periodically flooded by rains, tides, or irrigation. Depending on the species and when their habitat is exposed to water, they may hatch during the winter, early spring, summer, or fall. Since the growth rate of mosquito larvae is temperature dependent, species that hatch during the winter may not become adults until spring so they often go un-noticed (except by our sharp-eyed field inspectors whose goal is to treat the larvae before they emerge as adults).

A variation of this pattern is seen in the tree hole mosquito, Aedes sierrensis, the carrier of dog heartworm, which spends most of the winter in the larval stage. Eggs hatch after the first heavy rains in the fall when their tree-hole habitats first fill up with water, but the larvae develop very slowly and only develop and emerge as adults with the advent of warm spring weather (usually in April or May). A tip for dog owners: although the adult mosquitoes that carry heartworm are only around for part of the year, some veterinarians recommend keeping your dog on anti-heartworm medication year-round since it takes a while after the start of treatment before the dog is fully protected. Some Aedes species exhibit ‘installment hatching’. Not all the eggs hatch immediately after a single flooding and some may remain dormant for several years. This is thought to be a sort of ‘insurance policy’ against floodwaters that may dry up too quickly for the larvae to complete their development. It turns out, that even mosquitoes know not to put all their eggs in one basket.