Planes dropped insecticide in Dallas County on Monday night in a second round of aerial spraying for mosquitoes.
Five planes were spraying more than 360,000 acres after the successful completion of spraying on Sunday.
A spokesman for Dynamic Aviation, the company flying the planes, told NBC 5 the pilots would be using a flight pattern with less turns. The planes are able to cover more acres per hour, but it also must refill on chemicals every hour and a half.
The planes should be finished spraying by 2 a.m.
Officials said they hoped to cover on Monday night every city participating in aerial spraying. Planes took off at about 8:30 p.m.
A second application is recommended to kill the larvae that have hatched since the first spraying event, according to a statement from the Centers for Disease Control and Prevention released by the county.
"Until the results of tonight's spraying mission are known, it is undetermined whether aerial spraying will occur Tuesday evening. CDC representatives will be in North Texas this week, surveying and analyzing the region's efforts to combat the West Nile virus and will make recommendations on how to proceed," the city said in a news release.
Zachary Thompson with Dallas County Health and Human Services said the aerial battle so far seems to be working and that the Duet adulticide is knocking down mosquitoes.
"Now that the aerial assault is showing positive preliminary results, we need to expand our assault on the ground and enlist residents to join the fight by draining standing water to cut off mosquito breeding grounds," said Rawlings. "It only takes a little soil, compost or a few leaves, and water that stands stagnant for one, two weeks, to breed mosquitoes."
Dallas County Judge Clay Jenkins said county health officials are setting traps to see if the spraying is working. He said he is optimistic about the early results.
"[It's] very encouraging," he said. "It's the state's information and Clarke's information, so I don't want to release it, but it's as encouraging as it can be, but it's a tiny snapshot."
Dallas Mayor Mike Rawlings asked citizens to take responsibility for standing water after a weekend of heavy rain. If residents see any standing water that they cannot get to, county officials urge them to call 311 for assistance.
Dallas County also stated that the city of Seagoville has confirmed it wants to participate in the aerial-spraying operations. No spraying schedule was released with the initial announcement. Duncanville also approved aerial spraying Monday.
Highland Park is participating in the second round of spraying.
After three nights of interruptions, pilots were able to spray all of the targeted area in Dallas County without interruption from Mother Nature. Severe thunderstorms interrupted aerial spraying Thursday and Friday nights, while Saturday's spraying was canceled because of rain before planes even got off the ground.
The Environmental Protection Agency has said that the insecticide used in the spraying, Clarke' Duet, poses no significant threat to humans or animals.
NBC 5's Omar Villafranca contributed to this report.