In the photos below, one neglected swimming pool is only partially filled with water while the other is completely full. Which one produces more mosquitoes? If you guessed the pool with more water produces more mosquitoes, you'd be in the majority. 

Let's be clear: both pools can produce more than 1 million mosquitoes and affect people up to five miles away. Neglected pools are especially attractive to the kind of mosquitoes than can transmit West Nile virus. But, the pool with less water is the one capable of producing more mosquitoes.

Mosquitoes must breathe air so they stay mostly at the water's surface. It's not the volume of water so much as the amount of surface area of the water that enables more mosquitoes to thrive. But, that's not all. Other factors, such as the amount of food available to the mosquito larvae enable them to thrive. The shallower the water, the more concentrated the algae and other available food. Also, less water results in less water action--less waves--since the water is more sheltered from the wind. Mosquitoes prefer still, not agitated water since the eggs are fragile. Even more to the point, less water and less agitation makes survival of the young mosquitoes easier since they spend less energy swimming in order to breathe, eat, and fight wave action.

Maintaining swimming pools is the best way to fight mosquitoes. Chlorinate and filter swimming pools regularly or ensure that all the water has been removed. Even a couple of cups of water at the bottom of a pool can be a significant mosquito source. Use Mosquito Dunks to treat smaller amounts of water.  In some cases, mosquitofish can be placed in neglected swimming pools as an intermediate solution. Mosquitofish are available free from our District for Contra Costa County residents. Please contact us to learn more. 

With the recent rains, residents have been asking us about standing water and mosquitoes. A couple of common questions are, "Is it still mosquito season?" and, "Is there anything we need to do with the rainwater that's filling our house gutters?"

Good questions. 

Mosquito season in Contra Costa County is year-round. We have   23 different species of mosquitoes and so at any time of the year mosquitoes can be present with the majority presenting themselves in spring, summer and fall. In any case, standing water presents a habitat for mosquitoes. Therefore, our collective task never ends--dumping standing water, no matter how little, is important to our health year round.

Collecting rainwater is another story. There are safe ways to collect and save rainwater without producing mosquitoes. 

Specially constructed rain barrels are designed to collect rainwater while preventing mosquitoes and can be purchased from home improvement stores. For those residents who opt for a do-it-yourself option, you can use a garbage can, bungee cords and window screen for a cost-effective and mosquito-free solution. Check out our video for more information on mosquito-free rain barrels.

In either case, if you are installing a downspout to a rain barrel, attach screening to prevent mosquito larvae from flowing from house gutters into the barrel itself. House  gutters can produce tens of thousands of mosquitoes when clogged and holding rainwater. Leaves and other debris trapped in the gutters provide excellent nutrients for the mosquitoes to thrive. In addition, there are no predators to eat the mosquitoes.

Dump and drain unneeded rainwater, safely collect necessary rainwater, and clear out house gutters to ensure a healthy and happy neighborhood.