Mosquito Prevention 101

Advice from Marin’s insect guru.

That high pitched whine in your ear disrupting your camping reverie, the bites that cause you to scratch furiously -- it just wouldn’t be summer without mosquitoes.

Is there anything commendable about this warm weather pest? “Not a bad question, just difficult to find a good answer,” sighs Eric Engh, head of education and outreach at the Marin – Sonoma Mosquito and Vector Control District

Though we don’t consider mosquitoes very savory, other things do, says Engh. Countless food chain critters eat mosquitoes, including bats, dragonflies and a variety of birds (the purple martin can apparently dispatch up to 2,000 of them a day). Also, in order to consume the sugar they need for flight, mosquitoes visit flowers and play a role in pollination (albeit a very small one). 

Given this status as a minor pollinator and major nuisance, will they eventually go away? “Not any time soon,” says Engh. Fossil evidence indicates that mosquitoes have been around for millions of years.  

Mosquitoes need water to reproduce, and warm weather to encourage them through their lifecycle

Marin had a particularly wet winter and spring (and start of summer!), and the increased precipitation will lead to more opportunities for water to collect, says Engh. “Water is where mosquitoes lay their eggs.” While it is impossible to predict how large the mosquito population will be in a given season, the warmer the weather is, the faster they will develop through their lifecycle. Very warm weather, such as last week’s heat wave can spur an egg to develop into an adult in under a week, explains Engh, whereas this process will take several weeks in cooler weather. 

Beyond being pesky, mosquitoes can carry disease, particularly the West Nile virus, which affects people as well as horses. West Nile-- common in Asia, Eastern Europe, Africa and the Middle East -- was first seen in the US in 1999. While the virus passes unnoticed in most people, some will express mild symptoms such as fever, headache, body ache, nausea and vomiting. A very few (.6 percent) will have severe symptoms including high fever, muscle weakness, tremors, coma and permanent neurological damage.

While cases of West Nile have been reported in Southern California in recent years, Northern California has been spared. Marin Mosquito and Vector Control’s constant surveillance involves analyzing mosquito pools as well as ‘sentinel’ chicken flocks, where blood is tested for various diseases, especially West Nile. While one positive mosquito pool and several dead birds were discovered in 2008, there has been no activity in 2009-10. (The virus is spread when mosquitoes bite and infect birds. The birds act as carriers, infecting more and more mosquitoes as the insects bite the birds. Eventually, a human being can become an ‘accidental victim’ when bitten by a mosquito.)

While Engh is not entirely certain of the cause of Marin’s decline, he suspects birds and other carriers may be growing resistant to the virus. While this is promising news, “we need to remain vigilant in order to prevent further outbreaks,” says Engh. “We have traps all over the county.”

Still another mosquito-borne threat is Dog Heart Worm, carried from dog to dog via mosquito bite, says Engh. (The heartworm is actually incapable of reaching adulthood without first developing in the mosquito.)

Worldwide, mosquitoes can transmit a wide variety of diseases, including encephalitis, Malaria and Yellow Fever, and more are on the radar of the CDC and other health organizations. 

“Instead of worrying about specific diseases,” Engh advises, “there are a lot of things you can do to protect yourself from mosquitoes, the diseases they may carry and their irksome bites." 

To reduce exposure in general, says Engh, avoid being outdoors during peak mosquito activity at dawn and dusk. Of course dusk is prime BBQ time, so if you are out, wear repellant. 

A number of repellants are recommended by the CDC and California Department of Public Health  and these have Deet (considered the most effective mosquito repellent), Parabin and Oil of Eucalyptus as their primary ingredients.

Products containing the highly repellent ingredient permethrin are recommended for use on clothing, shoes, bed nets and camping gear (and are recommended for this use only).

The CDC suggests following these guidelines when using these products:

  • Oil of eucalyptus is not recommended for children under the age of three
  • Apply repellents only to exposed skin (not under clothing)
  • Never use repellents over cuts, wounds or irritated skin
  • Do not apply to eyes and mouth, and use repellent sparingly around the ears
  • When applying on children, spray repellent on your hands first
  • Heavy spraying or saturation is unnecessary
  • Wash treated skin with soap and water, particularly when repellents have been used throughout the day; and wash treated clothing as well
  • Do not use Deet in products that combine the repellent with a sunscreen.  Sunscreens often are applied repeatedly, but deet is not water-soluble and will last up to eight hours. Repeated application may increase the potential toxic effects.

What about toxic chemicals in repellents? Recommendations on the use of insect repellent vary, and it is best to consult the range of resources and decide what is best for you and your family. 

The nonprofit consumer information organization Environmental Working Group has expressed concern about a number of chemicals found in repellents. Parabens are widely used preservatives found in personal care products including insect repellants; phthalates are thought to disrupt the endocrine system and possibly lead to cancer. 

The American Academy of Pediatrics advises taking precautions when using Deet on children; and the CDC advises that higher concentrations of products with Deet do not mean better protection – just that the product will last longer (in other words, it is best to use a lower concentration and reapply as needed). 

For those concerned with mosquito repellent ingredients, Engh advises carefully reading labels and consulting the CDC Website, especially where it concerns children. Consumers can also call 1-800-858-7378 with specific questions. 

Natural products are thought by some to be less effective than products containing Deet, and often need to be applied more frequently. See the Environmental Working Group’s Skin Deep educational database for a listing of their recommended insect repellants.

Experts also recommend wearing clothing that provides maximum coverage (though a thick sweatshirt may not be ideal at a summer barbeque). Citronella candles can be effective too, says Engh, as long as the air is still. Mosquito netting can be used over infant carriers or eating and sleeping areas. 

Whenever possible, get at mosquitoes at their source by mosquito-proofing your yard and home.

“The ‘house mosquito’ hides during the day,” says Engh, “and when lights go out, they find you.” Mosquitoes can sense us easily through body heat, and they can smell us. Vector Control encourages people to close doors and windows, make sure to have screened windows -- and check your screens. (Engh was noticing mosquitoes in his own house and figured out that the cat had clawed holes in their screen.) 

Mosquitoes can’t grow up without water, reminds Engh. “People grow mosquitoes on their own property in ponds, pools and buckets without knowing it,” he warns. For Marin-Sonoma residents, Vector Control will provide free home visits for water sources too big to handle. If you have an ornamental pond, they can provide mosquito fish that will live in the pond and gobble up the mosquito larvae.

Marin’s abundant oak trees are home to a particularly persistent biter called the ‘Tree Hole Mosquito’ that is especially bothersome at this time of year, says Engh. In the trees many rot holes fill up with rain water and these mosquitoes lay eggs there. Fortunately, this mosquito is a weak flier. Engh recommends using an oscillating fan on your porch or deck to keep them away. (No – this trick won’t work with yellowjackets. “You’d need a pretty powerful fan!” says Engh, “and your hamburgers would blow away too.”)

“Mosquitoes grow in places you’d never think they’d grow,” according to Engh. “I was surprised to find they’d grow in rain gutters clogged with leaves where the water is nice and rich with decomposing leaves.” He advises that there may be hundreds of places around your home where mosquitoes might be breeding. If you have a lot of mosquitoes and can’t figure out where they are coming from, Vector Control can trap them and track down their nesting place. 

Contact Marin-Sonoma Mosquito and Vector Control: 707-285-2200 or 800-231-3236. Service is free of charge for any Marin or Sonoma County resident. 

Ease the Itch

Family physician Andy Ashcroft advises washing the site with soap and water, using cool compresses for swelling and calamine lotion for itching. Topical anesthetics or antihistamines are generally not recommended. If you are suffering widespread itching, oral medications such as Zyrtec may help. Consult your physician if before giving oral medications to children under six-years-old. 

Now finally for that age-old question – do mosquitoes really go for people who are ‘sweeter’? The female mosquito depends on her protein-heavy ‘blood meal’ in order to develop and lay her eggs. So she bites not to be cruel, but simply to carry on the species. Nature has equipped her with the ability to smell carbon dioxide from 100 feet away. And not just any CO2, but an array of chemicals carried only in certain people’s CO2. 

Engh says that some researchers think men’s larger body size, lactic acid production and CO2 production might make them more attractive targets. I found a host (no pun intended) of information on , indicating that mosquitoes were attracted to, among other things, Limburger cheese and feet. But researchers have not yet come up with a precise menu of alluring smells. If you get more bites, just consider it a backhanded compliment.


Marin Sonoma Mosquito and Vector Control District

State of California Fight the Bite Campaign

West Nile virus activity in California

Centers for Disease Control and Prevention insect repellent information

The Environmental Working Group  

Mosquito prevention in fish ponds 


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