Wash. Rev. Code § 17.12.010
For the purpose of destroying or exterminating squirrels, prairie dogs, gophers, moles or other rodents, or of rabbits or any predatory animals that destroy or interfere with the crops, fruit trees, shrubs, valuable plants, fodder, seeds or other agricultural plants or products, thing or pest injurious to any agricultural plant or product, or to prevent the introduction, propagation, growth or increase in number of any of the above described animals, or rodents, the board of county commissioners of any county may create a pest district or pest districts within such county and may enlarge any district containing a lesser territory than the whole county, or reduce any district already created, or combine or consolidate districts or divide, or create new districts from time to time in the manner hereinafter set forth.
RCW 17.12.010