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Mosquito Control Districts

This page provides an overview of mosquito control districts in Washington State, including formation, governance, annexation, sample documents, and a list of mosquito control districts.


Mosquito control districts (also known as MCDs) are special purpose districts authorized to protect public health and prevent disease by controlling and exterminating mosquitoes.

List of Mosquito Control Districts

MRSC is currently aware of 19 mosquito control districts statewide. Most of the districts are in Eastern Washington, but several have been formed in Western Washington.


  • Ch. 17.28 RCW – Mosquito control districts
    • RCW 17.28.160 – Powers of district; modified in 2013 to allow staff to enter land adjacent to district for inspections and abatement, with prior written notice to property owner
  • Ch. 70.22 RCW – Statewide program for the control or elimination of mosquitoes. Secretary of Health coordinates plans for mosquito control work and arranges for cooperation between organizations


The formation of a mosquito control district may be initiated either by resolution of the county legislative authority or by petition of the voters. A hearing is held to determine "whether the public necessity or welfare of the proposed territory and of its inhabitants requires the formation of the district," and an election is held to vote on the district's creation and on a tax levy for its first year of operation. Provisions for creating a district are found in RCW 17.28.020-.100.

Below are two MRSC resources summarizing related statutes:


Sources of funding are a one year property tax levy at the time of formation, assessments, a general purpose excess levy, and an excess levy for bonds. A mosquito control district may, prior to the receipt of taxes raised by levy, borrow money or issue warrants of the district in anticipation of revenue, and such warrants shall be redeemed from the first money available from such taxes (RCW 17.28.251).

Boundaries for Taxation

For the purpose of property taxation the boundaries of the mosquito control district must be established by September 1 and the boundary existing on September 1 of each year is the boundary used for taxing property within the district (RCW 17.28.253). Although RCW 84.09.030 (formerly RCW 84.08.160) stipulates an earlier date, AGO 59-60 No. 132 concludes that RCW 17.28.253 is controlling where the residents of a mosquito control district vote for the levy of taxes pursuant to RCW 17.28.100, 17.28.252 or 17.28.260. Note that the first property taxes or assessments will not be received until April 30 of the year after the formation.

Assessments to Finance District Operations

  • The board of trustees of the mosquito control district annually determines the amount of money necessary to operate the district and classifies property within the district in proportion to the benefits received from the district.
  • In accordance with its classification the board apportions and assesses the property within the district, and the assessments are collected with the general taxes of the county or counties. Note the basis for calculating assessments was changed to current use of property rather than market value by Laws of 2005, ch. 181.
  • The board of trustees in determining assessments must follow the procedures for county road improvement districts as outlined in RCW 36.88.090 - 36.88.120.
  • The provisions of RCW 36.88.120, 36.88.140, 36.88.150, and 36.88.170 governing the liens, collection, payment of assessments, delinquent assessments, interest and penalties, lien foreclosure and foreclosed property of county road improvement districts shall govern such matters as applied to mosquito control districts.
  • AGO 2018 No. 1 - Mosquito control districts may not assess state-owned property within MCD boundaries
  • AGO 2003 No. 9 - City-owned properties located within MCD boundaries are subject to assessment to the extent that they receive a special benefit from mosquito control operations.

See RCW 17.28.255 - .257.


One-Year General Levy

At the same time as the election on the formation of the district, a ballot measure for a one-year general levy of up to 25 cents per $1,000 of assessed value is submitted to the voters. This levy, if passed, is outside the constitutional and statutory limits. In order for the measure to pass, it must a) be approved by three-fifths (60%) of the voters, with a voter turnout at least equal to 40% of those voting in the last general election or b) receive a number of “yes” votes equal to three-fifths times 40% of the number of people voting in the last general election. (Washington State Constitution, Art. VII, Sec. 2(a)). See RCW 17.28.100.

Excess Levy for General Governmental Purposes

Per RCW 17.28.252, a mosquito control district has the power to levy additional taxes in excess of the constitutional and/or statutory limitations for any of the authorized purposes of the district, in an amount not to exceed 50 cents per $1,000 of assessed value per year. This levy is commonly called an Operations and Maintenance (O&M) levy. In order for the measure to pass, it must:

  • Be approved by three-fifths (60%) of the voters, with a voter turnout at least equal to 40% of those voting in the last general election or
  • Receive a number of “yes” votes equal to three-fifths times 40% of the number of people voting in the last general election. (Washington State Constitution, Art. VII, Sec. 2(a)).

The statute does not specify the number of years for which this levy may be made, but Art. VII, Sec. 2(a) specifically provides for a four-year levy for schools and a constitutional amendment was passed in November 2002 to allow a two-year levy by fire districts. Although the language in the section is not as clear as it could be, the remaining districts are limited to one-year levies, presumably including mosquito control districts.

General Obligation Bonds - Excess Levies

  • A mosquito control district may issue voted general obligation bonds for authorized capital purposes, and provide for the retirement of the bonds by excess property tax levies.
  • A proposition authorizing both the issuance of such bonds and the imposition of excess levies must be approved by the voters of the district, at an election held pursuant to RCW 39.36.050, by three-fifths of the voters with a voter turn out of not less than 40% of the total number of votes cast within the area of said mosquito control district at the last preceding county or state general election. (Note that district officials most probably do not have the choice of using the voter turnout in a county election to validate the vote. Art. VII, Sec. 2(b) of the Washington State Constitution dictates the voting requirements for voted general obligation bonds. It states that the voter turnout must be 40% of the number voting in the last preceding general election, where it is understood that it is a state-wide general election.)
  • Mosquito control districts may become indebted for capital purposes up to an amount equal to one and one-fourth percent of the value of the taxable property in the district, as the term "value of the taxable property" is defined in RCW 39.36.015.
  • Such bonds shall never be issued to run for a longer period than 10 years from the date of issue and shall be issued and sold in accordance with Ch. 39.46 RCW.

See RCW 17.28.260.

Financial Administration

Duties of County Treasurer

  • The county treasurer collects all mosquito control district assessments. The collection and disposition of revenue from such assessments and the depository thereof shall be the same as for tax revenues of such districts as provided in RCW 17.28.270.
  • All taxes levied under this chapter shall be computed and entered on the county assessment roll and collected at the same time and in the same manner as other county taxes.
  • When collected, the taxes shall be paid into the county treasury for the use of the district. If the district is in more than one county the treasury of the county in which the district is organized is the depository of all funds of the district.
  • The treasurers of the other counties shall, at any time, no more than twice each year, upon the order of the district board settle with the district board and pay over to the treasurer of the county where the district is organized all money in their possession belonging to the district. The last named treasurer shall give a receipt for the money and place it to the credit of the district (RCW 17.28.270).

See RCW 17.28.258 and Ch. 84.56 RCW on collection of taxes.

Fees Charged by County Treasurer

  • Mosquito control district assessments authorized under RCW 17.28.255 are "special assessments, fees, rates, or charges" and a county treasurer may charge the district a service fee not to exceed four dollars per parcel each year for collecting such assessments. See AGO 1994 No. 24.
  • The county assessor of each county lying wholly or partially within the district is to certify annually to the board the aggregate assessed valuation of all taxable property in their county situated in any mosquito control district as it appears from the last assessment roll of their county (RCW 17.28.310).
  • Funds of the district shall only be withdrawn from the county treasury depository upon the warrant of the district board signed by its president or acting president, and countersigned by its secretary (RCW 17.28.280).

See RCW 36.29.180.

Annexation, Consolidation and Dissolution


  • Any territory contiguous to a district may be annexed to the district.
  • Property within a city can be annexed if the city agrees by resolution and the resolution is attached to the annexation petition.
  • The annexation process is begun by the submission of a petition to the board of trustees. The petition must contain a description of the territory sought to be annexed, be signed by registered voters of the territory equal in number to at least 10% of the number of votes cast in the territory for governor at the last election prior to the time the petition is presented. Thereafter the board sets the petition for hearing.
  • The annexation is subject to approval of the electors of the territory proposed to be annexed.
  • The expenses of the annexation election are borne by the district regardless of the election outcome.
  • Certified copies of the order of annexation must be filed with the secretary of state, county clerk, and county auditor of each county in which the district is situated.
  • After annexation the district board consists of the number of members appointed in the manner prescribed by statute for the board as though the district had been formed originally embracing the annexed territory. However the original board members continue to serve the remainder of the term for which they are appointed.

See RCW 17.28.320 - .350.


  • A consolidation of mosquito districts may be achieved through a special election.
  • The process begins by adoption of a resolution by a two-thirds vote of the district board, and concurred in by the adoption of a concurrent resolution by a two-thirds vote of other district boards seeking consolidation.
  • Consolidation requires a two-thirds vote of each mosquito control district proposed to be consolidated.
  • After the consolidation, the board of the consolidated district shall consist of the number and shall be appointed in the manner prescribed by statute.
  • The terms of the members of the district boards of the several districts consolidated who are in office at the time of consolidation terminate at the time the consolidation becomes effective.

See RCW 17.28.360 - .410.


  • Election: The district may at any time be dissolved upon the vote of two-thirds of the qualified electors in the district at a special election called by the district board upon the question. See RCW 17.28.420-.450
  • Inactive Districts: Dissolution of inactive districts occurs through county legislative action. See Ch. 36.96 RCW.

Examples of Mosquito Control District Documents

General Code Provisions

District Formation

  • Chelan County:
    • Resolution No. 2010-29 (2010) – Submitting two ballot measures to voters: one to create Idlewild MCD, and one authorizing a one-year excess levy to fund operations
    • Resolution No. 2010-85 (2010) – Making initial board of trustees appointments


Job Descriptions

Other Documents

  • Clark County Mosquito Control District:
    • 2017-2018 Work Plan – Includes proposed strategy for addressing the threat of West Nile Virus, Zika, and other diseases
    • Interlocal Agreement with Clark County Public Health (2017) – County public health department provides clerical and other support services to MCD, including payroll, board support, annual report preparation, budget preparation assistance, and website support

Recommended Resources

Last Modified: February 23, 2024