While officials warn that Americans should wear insect repellant to protect themselves, several cities, including Dallas, have taken to aerial spraying of insecticides to kill mosquito breeding sources.

In this Aug. 20, 2012 image, a Beechcraft airplane sprays the insecticide DUET over Dallas to curb the spread of West Nile virus.

The last time Dallas used aerial spraying to curb the mosquito population, Texas' Lyndon Johnson was in the White House, Mission Control in Houston was launching Gemini missions and encephalitis was blamed for more than a dozen deaths. But for the first time in more than 45 years, the city and county resumed dropping insecticide from the air to combat the nation's worst outbreak of West Nile virus.

Derek Johnson, left, environmental unit supervisor, and Jeff Everett, right, an environmental technician from Oklahoma City's Storm Water Quality division of Public Works, lower a video camera into a storm drain in Oklahoma City, Tuesday, Aug. 21, 2012, to look for evidence of mosquitoes, which could possibly carry the West Nile virus. Crews treated the manhole's mosquito infestation three days earlier with a larval inhibitor, Altosid XR. West Nile virus cases have increased this month in Oklahoma, according to the State Department of Health.

Derek Johnson, left, environmental unit supervisor, and Jeff Everett, right, an environmental technician, from Oklahoma City's Storm Water Quality division of Public Works, remove a video camera from a storm drain in Oklahoma City where they'd been searching for mosquitoes on Tuesday, Aug. 21, 2012.

A Beechcraft airplane sprays the insecticide DUET over Dallas County to curb the spread of West Nile virus Sunday, Aug. 19, 2012, in Addison, Texas. Storms interrupted aerial spraying last Thursday and Friday nights before spraying was resumed.

A Beechcraft airplane sprays the insecticide DUET over Dallas County to curb the spread of West Nile virus late Sunday night, Aug. 19, 2012, in Dallas.

A Beechcraft airplane sprays the insecticide DUET over Dallas County to curb the spread of West Nile virus late Sunday night, Aug. 19, 2012, in Addison, Texas. The state of Texas has about half of all West Nile virus cases in the country, health officials say.

Dead mosquitoes are lined up waiting to be sorted at the Dallas County mosquito lab in Dallas, Thursday, Aug. 16, 2012.

A Beechcraft airplane sprays the insecticide DUET over Dallas to curb the spread of West Nile virus early Monday morning, Aug. 20, 2012. Storms had hindered aerial spraying attempts the weekend before.

Mosquito tech Spencer Lockwood sorts mosquitoes at the Dallas County mosquito lab in Dallas, Thursday, Aug. 16, 2012. Officials expect more cases of West Nile virus through August and September.

Derek Johnson, environmental unit supervisor from Oklahoma City's Storm Water Quality division of Public Works, controls a video camera on it's trip through a storm drain in Oklahoma City, Tuesday, Aug. 21, 2012. Crews treated the manhole's mosquito infestation three prior with the larval inhibitor Altosid XR, and returned today to check for mosquitoes.

A Beechcraft airplane sprays the insecticide DUET over Dallas County to curb the spread of West Nile virus late Sunday night, Aug. 19, 2012, in Addison, Texas.

Mike Stuart of Dynamic Aviation describes the plane that's used for aerial spraying and it's difference from crop dusting planes, during a news conference in Dallas, Thursday, Aug. 16, 2012.

Dallas Mayor Mike Rawlings, center, holds a news conference in front of a plane that will be used for aerial spraying in Dallas, Thursday, Aug. 16, 2012. The last time Dallas used aerial spraying to curb the mosquito population was 45 years earlier.

Dallas County Judge Clay Jenkins, right, speaks as Dallas Mayor Mike Rawlings looks on during a news conference in front of a plane that will be used for aerial spraying in Dallas, Thursday, Aug. 16, 2012. Dallas had declared a state of emergency over the West Nile virus outbreak.

Dallas Mayor Mike Rawlings points to a map showing Dallas County and the aerial spraying against mosquitoes during a news conference in downtown Dallas, Friday, Aug. 17, 2012. Dallas County has been the epicenter of the nation's worst outbreak of West Nile virus, which has sickened hundreds.

A video camera is pictured in a storm drain in Oklahoma City, Tuesday, Aug. 21, 2012. Crews treated the manhole's mosquito infestation three days ago with the larval inhibitor Altosid XR.

The feed from a video camera on it's trip through a storm drain is pictured in Oklahoma City, Tuesday, Aug. 21, 2012 where crews searched for mosquitoes.

A plane that will be used for aerial spraying is rolled onto the tarmac for a news conference in Dallas, Thursday, Aug. 16, 2012.

A video camera is pictured after checking a storm drain in Oklahoma City, Tuesday, Aug. 21, 2012.

Dallas County Judge Clay Jenkins, left, points to a map of Dallas County showing aerial spraying against mosquitoes and West Nile virus as Dallas Mayor Mike Rawlings looks on during a news conference in downtown Dallas, Friday, Aug. 17, 2012.

Mike Stuart of Dynamic Aviation describes a plane that will be used for aerial spraying to the media in Dallas, Thursday, Aug. 16, 2012. The last time Dallas used aerial spraying to curb the mosquito population, 45 years earlier, Texas' Lyndon Johnson was in the White House, Mission Control in Houston was launching Gemini missions and encephalitis was blamed for more than a dozen deaths.