Hundreds of property owners in the region are opting out of mosquito spraying amid worries about pesticides, which is complicating efforts to combat viruses carried by the insects.

Last week, spraying began in 32 cities and towns on the North Shore and Merrimack Valley that are organized in a regional effort to control the mosquito population.

But, as in previous years, hundreds of property owners have opted out. Their concerns range from spraying’s impact on honey bee colonies and organic farms to fears about toxic pesticides.

Voters in two towns — Marblehead and Swampscott — have banned the public use of chemical pesticides entirely and instead rely on methods such as using traps to kill mosquito larvae.

Mosquito control officials said large numbers of opt-outs have made spraying ineffective in some areas, creating a public health concern.

“Mosquitoes don’t know boundaries,” said Bill Mehaffey Jr., superintendent of the Northeast Massachusetts Mosquito Control and Wetlands Management District. Its boundaries stretch from Methuen and Amesbury to Salem. “They can travel miles for a blood meal.”

Mahaffey said about 600 people opted out of the spraying program last year.

Cities and towns that join the district pay up to $45,000 every year for spraying and other preventative measures.

Homeowners who want to opt out are required to send certified letters to local officials. Some Beacon Hill lawmakers want to make the process easier.

Legislation filed by Sen. Bruce Tarr of Gloucester would allow municipalities to include an “opt out” provision on their annual census forms. His bill, which was set for a hearing Wednesday, arose from a dispute between state agriculture officials and Boxford, where hundreds of property owners opted out of the program over concerns about pesticides contaminating their drinking water wells.

“We had more than 400 families opt out on the census last year, and after that, the state said they wouldn’t accept it,” said Boxford Town Clerk Robin Phelan. “So, a lot of people didn’t bother because it’s too burdensome.”

Beekeepers also complained last year that spraying is killing their bees. Even if they ask that their own property not be sprayed, they said, the chemicals can drift onto their bees.

Massachusetts health officials are concerned about mosquitoes because they are known transmitters of West Nile virus and Eastern equine encephalitis. Both are potentially fatal.

In 2012, the state tallied a record 33 human cases of West Nile virus and seven human cases of Eastern equine encephalitis, according to the Department of Public Health. Those included two West Nile cases in Essex County that resulted in the deaths of people in Georgetown and Amesbury.

Last year, the number of people diagnosed with West Nile dropped to eight, while there was only one confirmed case of EEE, which was not fatal. Health officials attributed the drop to aggressive spraying.

Mehaffey said chemicals used in the mosquito spray — including the insecticide prallethrin, used in a brand of spray called Duet — have been approved for use by state and federal environmental agencies as safe around humans, pets and other animals.

“Some people think we’re still using DDT,” he said. “There’s a lot of misconceptions out there.”

Despite assurances, state health officials note that contact with mosquito spray has been known to cause symptoms such as nausea, shortness of breath and irritation of the eyes, ears and throat. When the spraying occurs — typically by truck or helicopter at night — health officials warn residents to stay indoors, keep windows closed, turn off window fans and keep pets inside.

As mosquito-borne illnesses become widespread, health officials say regional efforts to control the insects are vital to prevent outbreaks.

Environmental groups such as the Washington D.C.-based Beyond Pesticides have pushed state and local officials to reduce reliance on what they call harmful chemicals while promoting more organic alternatives.

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