Wash. Rev. Code § 17.28.020
Any number of units of a territory within the state of Washington in Adams, Benton, Franklin, Grant, Kittitas, Walla Walla and Yakima counties or any other county may be organized as a mosquito control district under the provisions of this chapter.
A petition to form a district may consist of any number of separate instruments which shall be presented at a regular meeting of the county commissioners of the county in which the greater area of the proposed district is located. Petitions shall be signed by registered voters of each unit of the proposed district, equal in number to not less than ten percent of the votes cast in each unit respectively for the office of governor at the last gubernatorial election prior to the time of presenting the petition.
RCW 17.28.020