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July 12, 2016 (San Diego) -- San Diego County’s Department of Environmental Health is scheduled to drop mosquito-fighting larvicide for the fourth time this year on 48 local waterways Wednesday.

The County has used helicopters to drop solid, granola-sized larvicide on ponds, rivers and wetlands for more than a decade to help control mosquitoes and West Nile virus, a potentially deadly virus that can be transmitted to humans by mosquitoes. The larvicide does not harm people or pets, but kills mosquito larvae before they can become adult mosquitoes.

No one in San Diego County or the state has tested positive for West Nile virus this year. However, officials from the county environmental health’s vector control program said last week that they have already collected more batches of mosquitoes with the virus here this year than in all of 2015.

The County drops the larvicide roughly once a month from May through October.

The rivers, ponds and wetlands the County treats stretch from Chula Vista to Fallbrook and from Lakeside to Oceanside, and total just over 1,000 acres. See the complete list of the waterways online.

The County’s Vector Control Program also regularly treats roughly 1,400 potential mosquito-breeding grounds by hand, collects and tests dead birds to monitor the disease, gives out free mosquito-eating fish, tracks down and treats neglected swimming pools, and conducts a bilingual public education campaign.

Residents also play an important part in prevention. County officials are asking the public to police their own backyards and dump out standing water to keep mosquitoes from breeding.

San Diego County’s native mosquito species that can transmit West Nile virus don’t need a lot of water in which to lay eggs and can breed in people’s backyards. But the call for public help has taken on added importance in the last two years because of the appearance of two invasive Aedes mosquito species: the yellow fever mosquito and Asian tiger mosquito.

To date, relatively small numbers of those mosquitoes have been found here. And the diseases that they are best known for being able to transmit — dengue, chikungunya, yellow fever and the Zika virus — do not naturally occur here.

However, those two mosquito species love to live near people, not just in people’s backyards, but also inside their homes, and can breed in a thimble-full of water.

The Aedes species are different from San Diego County’s native mosquitoes in appearance and behavior. They’re smaller in size, have distinctive black and white markings, are known as aggressive biters and — unlike our native mosquitoes that prefer to feed between dusk and dawn — like to bite and feed during daylight hours.

County officials urged the public to follow the County’s “Prevent, Protect, Report” guidelines:


Prevent mosquito breeding sites. Every week, dump out and clean containers that hold water inside and outside homes, from equipment to toys, flower pots, old tires, anything that can collect water. Fill plant saucers with sand or fine gravel so water won’t form pools where mosquitoes can breed.


Protect yourself from mosquito bites. Wear long sleeves and pants when outdoors. Use insect repellent, preferably one containing DEET. Make sure the screens on your windows and doors are in good condition, do not have holes or tears, and are secured to keep insects out.


Report if you are being bitten by mosquitoes during daylight hours, or if you find mosquitoes that match the description of the yellow-fever mosquito or Asian tiger mosquito, by contacting the Vector Control Program at (858) 694-2888 or


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