Mosquito control officials say they need to start aerial spraying in Davis and Woodland now to protect public health and welfare. 

"There’s no other known way of quickly reducing an infected adult mosquito population and that’s what our spraying is designed to do."

David Brown – the head of the Sacramento-Yolo Mosquito and Vector Control District – says this is the last step after several months of tracking the virus through dead bird reports, mosquito samples and human cases.

And Brown says aerial spraying in Sacramento’s urban areas is also a possibility. 

"We’re evaluating other areas of Sacramento County to see if they’ll meet that threshold and if they do we’ll respond appropriately." 

The spraying in Davis and Woodland is set for sundown on Thursday and Friday. Opponents of aerial spraying question its effectiveness.