Nearly 50 people gathered in the Bolinas Firehouse’s cramped conference room on Tuesday night to hash out the future of mosquito control in West Marin.

Since 2005, the Marin/Sonoma Mosquito and Vector Control District has maintained and periodically renewed a contract with a local mosquito council that has set forth policies specific to West Marin, where a diverse ecosystem of waterways, endangered species and organic farming makes pesticide use a touchier subject than it might be in more urban parts of the county.

The contract is set to expire in March, and many West Marin residents are worried the district will revise its terms so that certain toxic chemical pesticides that were previously restricted will be made permissible.

“We feel, for the most part, that the agreement [with the district] has worked very well,” said Liza Goldblatt, the council’s co-chair. “We profoundly hope that this agreement will be renewed.”

Despite a lengthy presentation detailing its work in West Marin, the district did not give any indication of whether or not it intends to revise the council’s contract.

“We are in active listening mode,” said Phil Smith, the district’s staff manager. “We have no articulated position.”

Currently, the contract ensures that only low-toxic pesticides can be used to combat mosquito populations in West Marin. (Over the hill, the district is free to use a much broader range of pesticides, some of which are suspected of causing long-term environmental damage.) Representatives from East Marin present at Tuesday’s meeting praised the council’s contract, calling it a model for the entire county.

“The contract is appropriate for all of Marin County,” said Frank Eggers, the Fairfax representative on the district’s board. “There shouldn’t be any pesticide use around where children congregate to play.”

Last fall, negotiating committees from the council and the district met to deliberate the pros and cons of using a chemical pesticide known as methoprene. During these discussions, district staff members urged the council to consider using Altosid, a product marketed in the form of methoprene briquets.

“Preliminarily, when we looked at it, we thought [Altosid] could be suitable,” Mr. Smith said. “We haven’t found any other product suitable for killing adult mosquitos.”

Studies have shown that methoprene is toxic to estuarine and marine invertebrates. According to the U.S. Fish and Wildlife Service, it “is slightly to moderately toxic to fish.”

According to Doug Karpa, the legal program co-director for the Turtle Island Restoration Network, how exactly methoprene would interact with West Marin’s ecology is unknown. What is known is that many pesticides, including methoprene, have demonstrated adverse effects on a variety of species.

“The one thing you don’t want is to find out is that they have tremendous effects because you’ve observed a species go extinct. Then it’s too late,” Mr. Karpa said.

Methoprene is prohibited in West Marin solely by virtue of the mosquito council’s special contract with the district, but it is widely used elsewhere in Marin and in Sonoma County.

The preliminary discussions between the council and the district came to a stalemate over the methoprene issue, at which point the council contacted the utility district to set up Tuesday’s meeting. (The utility district originally contracted with the district to set mosquito-control policy in 2006, and its members are staunch opponents of methoprene.)

“We’ve released some nasty things into the ecosystem over the last 50 years,” said Don Smith, a utility district director. “There’s really no need for a chemical

The biggest culprits in mosquito breeding in West Marin are septic tanks, where the larvae breed.

Septic tanks are pervasive throughout West Marin, but tanks in Stinson Beach pose the greatest risk for a mosquito outbreak, according to the district and the council. Unsealed, improperly sealed or leaking tanks placed in close proximity to Bolinas Lagoon create ideal breeding grounds for mosquitos; when open or opened, they can release a plume of tens of thousands of the pests.

Some residences in Stinson Beach have received repeated visits from the district for not properly sealing their septic tanks. Last year a single property was visited six times; four others had multiple visits.

These repeat visits could be reduced, the council argues, if the district would immediately notify the council and other local agencies when a treatment visit happens. Instead, the district only reveals this information in an annual report, by which time it is too late to prevent repeat mosquito offenses.

On Tuesday, Stinson Beach representatives called for a renewed contract that will more thoroughly address the use of pesticides in residential septic tanks.

“We would like the council to have input on what is used in Stinson Beach septic tanks,” said Jim Zell, a director of the Stinson Beach County Water District. “We would also like to see greater communication between the council and the district.”

According to Ms. Goldblatt, there is less communication between the West Marin Mosquito Council and district staff than there used to be. “That’s what needs to be enhanced,” she said. “And if we really focus on education of the public, and if we focus on prevention, there should not be an issue.”

The council requested that the renewal of the contract be placed on the agenda for the district’s February board meeting. If a decision cannot be reached during that meeting, the council will request that the district grant an additional extension on its contract.

Read more at the Point Reyes Light.