NEW ORLEANS - Mosquito control workers can measure the recession by the number of green, cloudy swimming pools they see - algae-covered havens for mosquitoes dotting neighborhoods hit by the foreclosure crisis.

Aside from their annoying bites, mosquitoes carry West Nile virus and other diseases. With the number of foreclosures rising, it's becoming a more-important challenge to track down abandoned homes with pools from suburban Washington, D.C., to California.

In Phoenix, for example, the number of pools left untended - often because of foreclosures - rose from about 6,000 in 2007 to more than 9,100 last year, said John Townsend, division manager for Maricopa County Vector Control.

"If we keep up with the same numbers this year, we could be up to around 12,000 to 14,000," he said.

Deep South cousins of the guppy, "mosquito fish" have long been a mosquito control tool for keeping abandoned pools from becoming mosquito farms.

For years, Townsend's department could collect enough of the fish from a local wastewater plant pond where they were seeded years ago to use in abandoned swimming pools. This year, he said the department expects to buy 250,000 to 300,000 of the minnow-sized fish.

"We kind of fished them out," he said.

He wasn't among the nearly 900 professionals at the American Mosquito Control Association's meeting in New Orleans in April. Budget cuts prevented it.

About 50 people attended a session April 9 featuring entomologist Greg Thompson of the New Orleans Mosquito, Termite & Rodent Control Board. He described how his agency found and treated thousands of abandoned swimming pools and ponds after Hurricane Katrina flooded 80 percent of the city and destroyed more than 100,000 homes in August 2005.

The pools can't just be drained. For one thing, a bottle cap can hold enough water for some species to lay eggs, and a gentle rain would leave more than that in a swimming pool. And an empty pool could rise out of the ground.

Thompson's talk came as cities across the country struggle to find those ditched swimming pools.

The 1,330-square-mile Greater Los Angeles County Vector Control District said a year ago that the number of requests to treat abandoned pools was up 40 percent from 2007, mainly because of foreclosures.

The district has changed its data collection since then, and has no directly comparable figure, spokeswoman Truc Dever said. However, she said workers treated 364 pools and got 202 requests for attention to mosquito havens in March. That compares with 58 requests and 162 pools treated in March 2007 and 216 requests and 481 pool treatments in March 2008.

Starting with backyard pools

Thompson suggested that cities with records still intact - unlike New Orleans after the hurricanes of 2005 - could start by looking at swimming pool construction permits issued in the past 10 years.

Jorge Arias, the head of the West Nile virus program in Fairfax County, Va., started out with addresses for the county's 11,000 backyard pools. He's checking to see which of those match addresses of the 2,000 or so houses in foreclosure, he said.

Thompson said pools get prominent mention in for-sale ads, so real estate agents and listing services are another good source of locations, as are banks. Satellite photographs can also be useful.

"He gave really good information," said Jejbir Sandhu, a researcher for the Northwestern Mosquito and Vector Control District based in Corona, Calif. He said his department has worked with some banks to locate abandoned pools, but may need a more systematic approach.

James Burgess, manager of mosquito and disease surveillance at Lee County Mosquito Control District in Lehigh Acres, Fla., near Fort Myers, also was in Thompson's audience. He said he hadn't thought about using building permits - workers usually fly over areas in helicopters.

Foreclosures or no, calls to several fish farmers and brokers that sell mosquito fish (Gambusia affinis or Gambusia holbrooki) didn't turn up a boom in fish sales. But the fish do continue to sell in massive quantities. And they reproduce quickly, making it unnecessary to replenish pools that have been seeded.

"You have to stretch for new customers," said Daniel Suttle of Suttle Fish Farm in Laurel, Miss. He's gotten calls from as far as Nevada and California.

Fish broker Pat Church of Tempe, Ariz., said she probably sold 150,000 Gambusia in 2007, and expects the 2008-09 sales to be above 200,000.

The fish will thrive as long as no one pours in bleach or chlorine - a problem New Orleans had to deal with by posting big red signs.

The fish are by no means a perfect solution everywhere. They're native to south Louisiana, but can become a big problem outside their home territory. Townsend's department in Phoenix only uses the fish where they can't get into waterways and compete with local species.