In Sacramento County, the south area tends to see the majority of West Nile activity.

So far this year, 42 mosquito samples have tested positive for West Nile virus throughout the county with 32 of those samples coming from the south area.

"…it's a wide open area so I know that there's a lot of fields and potential mosquito breeding areas."

Luz Maria Rodriguez with the Sacramento-Yolo Mosquito and Vector Control District says another reason may be a high number of deserted, foreclosed homes with neglected swimming pools where mosquitoes can breed.

"In terms of West Nile virus activity, this year we're seeing a lot more activity earlier on and that's about six weeks earlier than other years. To us it indicates that not only are we seeing a lot more virus but if this trend continues we'll likely have a longer and more intense mosquito season."

The spraying is set to take place from 8 PM to midnight over about 30,000 acres. 

Opponents of pesticide spraying say the vector control district should develop non-toxic alternatives and put more effort into the prevention of mosquito breeding.