The program, initiated Thursday, was called off both Thursday and Friday nights due to rain, CBS News reported.
Joseph Conlon, a technical adviser to the American Mosquito Control Association, said the effectiveness of the spraying will only be known after some time has passed.
"There's really no way of positively determining that until the season plays itself out," said Conlon, a retired Navy entomologist with extensive worldwide experience in mosquito control.
The Centers for Disease Control and Prevention says 26 people have died nationwide from the West Nile virus and there have been nearly 700 cases reported across 43 states.
Texas is considered the outbreak's epicenter; 10 people have died of West Nile there, and more than 200 have gotten sick.
"This is just the beginning of the West Nile season," Conlon said. "The last five years by this time, we had an average of about 172 cases of West Nile" nationwide.
Conlon said people should be less concerned about whether the pesticide that's been sprayed over Dallas could itself harm people's health.
He said he "can understand people's concerns" but noted "the insecticides they're using have been fully evaluated by the EPA [Environmental Protection Administration] and have been deemed to not pose an unreasonable risk if they're utilized according to the label."