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With Spraying Over, Dallas County Will Test Mosquitoes For Insecticide Resistance

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The West Nile outbreak of 2012 isn't officially over, at least not according to Dallas County health officials, but it may as well be. There hasn't been a death reported in more than six weeks, the number of new cases is basically nil, and for most of us, the scenes of August nights spent barricaded indoors against a blitz of pesticide-laden aircraft are distant memories.

Concern about widespread application of insecticides hasn't died away in the environmental community nor, it seems, among local public health officials. Dallas County commissioners will consider tomorrow whether to launch a program to test mosquitoes for resistance to insecticides. Such resistance has hampered global efforts to fight malaria and has also emerged in places like Sacramento, California, which routinely uses aerial spraying to battle West Nile.

"We don't think there's any resistance, but that's what the tests will tell us," said Zachary Thompson, director of Dallas County Health and Human Services. "We don't know."

Under the proposal being considered by commissioners, the county will collect mosquito larvae from two mosquito pools within Dallas County twice per year and mail them overnight to Mark Johnsen, an entomologist working for Brazos County for testing.

Johnsen's lab will then determine the concentration of two chemicals (permethrin and naled) needed to kill a sample of mosquitoes. By comparing those doses with a control population, the lab can determine the level of insecticide resistance. (If you're curious, the briefing explains how the tests are performed. Mosquitoes are knocked out with a blast of carbon dioxide, chilled to 50 degrees Fahrenheit, sorted by sex, and transferred into a vial whose inside has been fully coated with insecticide by means of a hot dog roller. The dead are counted after 24 hours.)

Thompson said the idea is to be proactive, both to know what dosage levels will be needed to effectively kill mosquitoes in 2013 and to avoid a situation in which public health officials suddenly discover that the weapons they've been using no longer work.

"I think the key is to do this type of resistance testing now so that you don't have an issue like what happened in Sacramento," he said.

But that's only one prong of the county's mosquito-fighting approach, Thompson said. He's still curious about what specific environmental factors prompted to this year's explosion in mosquito populations, particularly in Northwest Dallas.

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