Q. I have been reading
a lot about West Nile virus. Should I be concerned about mosquito bites?
A. Mosquito
bites can transmit microorganisms that cause diseases such as West Nile virus,
western equine encephalomyelitis and St. Louis encephalitis. The western
tree hole mosquito is the primary vector of canine heartworm in this area.
The mosquito life cycle varies widely by species. Some species
lay eggs on moist soil where flooding is likely. These eggs don’t hatch
until flooding occurs, possibly months or even years later. Other
mosquitoes lay eggs on water surfaces; some species lay batches of 100 or more
eggs. Eggs deposited on water surfaces usually hatch within a day.
Larvae hatch from the eggs, then become pupae. Adult mosquitoes emerge
from the pupae 1 to 2 days later. In summer the entire life cycle, from
egg to adult, may be completed in a week or less.
Mosquito adults are small, flying insects, with long
slender wings covered with small scales. Female mosquitoes can be
recognized by their long slender proboscis (snout) that is adapted for piercing
skin and sucking blood. Male mosquitoes have a shorter, thicker proboscis
that is adapted for sucking flower nectar. The immature stages are nearly
invisible to the naked eye.
Mosquitoes are most effectively controlled by public agencies that can
apply control methods over a wide area. The Contra Costa Mosquito and
Vector Control District serves all of Contra Costa County. Their web
site, www.ccmvcd.dst.ca.us, has local information. On request, they will
inspect property in their service area for mosquito problems, and provide
advice on control. The District provides free mosquito fish for Contra
Costa residents. Other Bay area counties have similar districts.
You can keep your property from contributing to the mosquito
population. The most effective control methods target the larvae.
Keep swimming pools and spas chlorinated. Any area or object that can
hold standing water for more than a day should be drained, filled with soil or
cement, discarded, treated with Bacillus thuringiensis subsp. israelensis,
or stocked with mosquito fish. Even standing water on lawns or in tree
crotches can harbor mosquito larvae. Keep rain gutters clean so water
drains.
Mosquito Fish. The
mosquito fish, Gambusia affinis, can be obtained from most vector
control districts. Do not use mosquito fish where they can escape into
the wild.
Bacillus thuringiensis subsp. israelensis. The bacterium Bacillus thuringiensis subsp. israelensis
acts as a stomach poison when eaten by the mosquito larvae. Highly
selective, it kills only the larvae of mosquitoes and a few related
flies. This pesticide is marketed in a variety of formulations for
homeowners, the most common being the Mosquito Dunk, a donut-shaped float that
lasts for about 30 days in water.
Outdoor Sprays.
Insecticide sprays or outdoor foggers may temporarily reduce the adult mosquito
population. This can be useful if the application is made shortly before
an outdoor party. However, it has no lasting effect. Burning
citronella candles or mosquito coils works best if there is little air movement.