Marin/Sonoma Mosquito & Vector Control District
800-231-3236 (toll free)
707-285-2200 (office)
595 Helman Lane, Cotati, CA 94931-9736

CALIFORNIA COAST & OCEAN
WINTER 1998-1999

San Francisco Bayshore's Most Numerous Resident

Whole buzzing areas of Heaven will have nothing but mosquitoes
- - from "Heaven," by Jack Kerouac (1958)

Wes Maffie

      Anywhere and anytime you restore a wetland, you can have mosquitoes. It's an unfortunate fact that we all have to deal with. When people wax ecstatic about vast flocks of birds and shimmering fishes in the huge tidal marshes of more than 100 years ago, they seldom think of the insects, especially the mosquitoes, San Francisco Bay's most numerous residents. As far back as 1772, Father Juan Crespi wrote about their fierce attacks. This humble creature, brilliant gold, black and white, or reddish-brown in color, made living near the Bay a real challenge until the advent of mosquito control programs.
      Up through the late 1970s, the accepted "solution" was to drain "swamps" or spray organophosphate pesticides into them. Few people understood the value of marshes or the damage that such activities could have on the sensitive ecosystems within the Bay's wetlands. The desire to fill or drain wetlands for human habitation or agricultural and industrial uses also required that something be done about the mosquito menace. Now, as we try to restore some of the wetlands that were diked or drained decades ago, mosquitoes also return.
      If mosquitoes were only a minor nuisance, control might not be such a big issue. Unfortunately, however, mosquitoes also carry diseases, such as malaria and encephalitis, which historically were serious problems within this state. In 1996, there were 55 reported cases of malaria in Alameda County. Between 40 and 60 cases have been reported in Santa Clara, Los Angeles, and San Diego Counties in recent years. Fortunately, local mosquitoes did not transmit the disease in these cases.
      Unfortunately, the mosquitoes that can carry these diseases do breed here, making the risk for the reestablishment of these diseases quite real.
      Today's mosquito control programs use strategies that are crafted to be environmentally friendly. Instead of pesticides that primarily target adult mosquitoes, or methodologies that could significantly modify a particular habitat, the focus now is on preventing adult emergence. Bacteria, mosquitofish, and species-specific insect growth hormones are now the mainstays of mosquito control programs. Should the need arise to control adult mosquitoes, sprays that contain pyrethrin, a substance derived from chrysanthemum flower heads, or synthetic pyrethroids are used. By targeting immature stages, smaller areas can be treated, thereby reducing the impact on non-targeted organisms and the environment.
      Public education and public involvement in wetlands enhancement or restoration projects is also a critical component of modern control programs. This aspect is most valuable, since it can be an effective, long-term control methodology that enhances the environment and helps to keep costs to a minimum.
      Mosquitoes are aquatic in their immature stages and terrestrial as adults. Their life span ranges from two weeks to several months, depending on the species involved, the time of year, and environmental factors such as temperature, humidity, and competition for food.

Few people appreciate the beauty of the mosquito, or know that it pollinates flowers.

      The summer salt marsh mosquito, Ochlerotatus dorsalis, can go from egg to larva to pupa to adult in less than one week, while other species, like the winter salt marsh mosquito, A. squamiger, can take up to four months to complete its life cycles.
      The eggs are laid singly or in rafts, on the water surface or in places where water will ultimately cover them. Most species' eggs hatch into larvae within 24 to 48 hours, but some can last for several years in dry places, hatching only when conditions are just right. The larvae, also known as wrigglers, feed on microorganisms or decaying plant material. The pupae, or tumblers, are the last stage to occur before the adults emerge. They swim with a tumbling motion and do not feed. This stage lasts about 24—72 hours. It is the adult female mosquito that most people know best. Only the female bites, needing the protein of a blood meal to produce up to as 600 eggs during her lifespan.
      Both males and females pollinate flowers, using the nectar as a source of energy. Mosquitoes also serve as prey for some of the other insects and birds that reside within the baylands. So, for all of the difficulties that they might cause humans, they can also be beneficial.
      When a wetland is being enhanced or restored, local mosquito abatement districts usually work with other agencies on the project design and implementation. The Ora Loma Marsh, in Hayward, was recently restored for a number of sensitive species, including the endangered salt marsh harvest mouse. Planning for this project began almost ten years ago. The design was altered to improve tidal flow, thereby reducing the amount of stagnant water in which mosquitoes thrive. Although it has only been a couple of years since the restoration occurred, mosquito breeding has been markedly reduced and it is quite apparent that the health of a degraded marsh is now returning.
      Local abatement districts are aware of the wide range of views concerning mosquitoes, public health and comfort, and the value of wetlands. It is their goal to develop consensus and still be an effective, environmentally oriented service to the constituency they serve.

Wes Maffei is the manager of the Napa County Mosquito Abatement District. He spends much of his spare time studying marsh insects and their interrelationships with plants and other organisms within the San Francisco Baylands.