Leavenworth Mosquito Control District



Washington's Bavarian Village

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November 11, 2018 report: 

This year and last we achieved excellent control of Culex mosquitoes through the timely use of Vectomax, a combination of Bti and Bs bacteria, which is more effective than just Bti when organic matter is high. It is suitable for organic production.  No more than 8 Culex were caught overnight in any trap so West Nile virus testing was not needed.  This is the 4rd year (along with 2010, 2013 and 2017) that we have acheived this degree of control.  West Nile virus prevention will be a continuing goal for our District along with control of nuisance mosquitoes.

On April 10 and May 2 of 2017 we contracted out larviciding by helicopter using the same Bti (Vectobac GS) granules that we use by hand.  This larviciding occurred on 20 acres of snowmelt wetland east of East Leavenworth Road due to the difficulty of access to the area due to brush and continued deep water. 

In June 2016 the area between Prowell Road and East Leavenworth Road has an elevated number of Culex mosquitoes with up to 44 Culex mosquitoes in one trap.  West Nile virus testing on June 10, 2016 showed RAMP readings of 54 and 32 for Culex pipiens mosquitoes collected from two traps in the area.  This is the only time readings were over 10.  We continue to monitor this area closely.

Cool weather in May and June of 2010, 2011 and 2012 helped to knock down West Nile virus in our state. There was a bit of a resurgence in 2013 and even more West Nile virus in 2014 although still far less than in 2009.  Okanogan County had evidence of the virus (through a positive horse) for the first time in 2014 with the first positive person in 2017.  This was not too surprising given the presence of the virus in British Columbia in 2013.  2015 continued the trend with the most human cases in our state (23) since 2009 (38). 2016, 2017 and 2018 saw fewer cases but the first human case in King County occurred in 2018: the only in-state acquired human case of the disease in 2018.

In 2012 Dallas County, Texas hit the news with a large number of human West Nile virus cases and deaths.  This county did not have a nuisance mosquito problem and thus was unprepared for the high infection rate which occurred in their mosquitoes: there was no larviciding and very little adulticiding prior to the point that people started dying from the disease (which generally occurs at least two weeks after infection).  

West Nile virus requires warm weather (over 80F) to replicate within the mosquito multiplying from the gut to the salivary gland.  Once in the salivary gland the virus can be transmitted to humans regardless of the temperature when the bite occurs.  Mosquitoes themselves are more hardy than the virus.  Depending on the species, they overwinter as adults or eggs and continue their life cycle in the spring when temperatures are consistently above freezing.  Those which overwinter as eggs can hatch in large numbers when they become submerged by melting snow (these are the earliest hordes) or by water in low-lying areas along rising rivers. 

We treat mosquito larva with a variety of larvicides.  Bacterial larvicides include Bacillus thuringiensis israelensis, commonly called Bti, Bacillus sphaericus, and spinosad, the most recently available bacterial larvicide.  The growth regulator, methoprene is also a low toxicity pesticide which prevents maturation beyond pupation.  Due to its high cost it is used sparingly in situations where long-term control is desired.  Although spinosad marketed at Natular has organic certification, it kills more insects species so we have not used it yet.

Agnique MMF a monomolecular film is used where pupa are found, it lowers the surface tension of the water so that they drown.  It is no longer manufactured so we are saving the small amount left for sites with difficult access and pupa.  It has been replaced by a paraffinic white mineral oil which is used sparingly in situations with plenty of pupa.

We have not used an adulticide since 2006 due to low mosquito numbers and the absence of West Nile virus in our District.  In the past (2003 and 2006) we contracted aerial spraying of malathion at the low rates appropriate for mosquito control.  Should such a need arise again, we would likely use the same adulticide since local aerial applicators are most familiar with the use of malathion.  Spraying for the control of West Nile virus could occur in the evening to catch mosquitoes when they are most active. Such an application would be announced on our website.  Smaller areas would most likely be signed, while KPQ and KOHO would be informed of any larger application areas.  Although direct public contact is not required, we do make an effort to inform those who wish to be notified in an advance of such a spray are.  If you would like to be notified please contact us at 548-5904 and give your name, phone number and address.  Only those within 1/2 mile of the spray area will be notified.

The high snowpack in the mountains in 2008, 2011, 2012 and 2018 led us to hire an extra assistant (Chris ? in 2008, Michael Darlington in 2011, Steve O'Flaherty in 2012 and Tom Roberts in 2018) who began work in April or May and continued thru June or early July. Their work coincided well with the highest snowmelt periods.  Assistants work part-time 5 to 20 hours a week depending upon the need and thus we are well equipped to handle the fluctuating amount of standing water. Starting in 2004 we hired assistants who each served 3 years: Dave Woods, Bruce Hill and Barry Moats.  Arnica Briody was our sole assistant.in 2013 and 2014.  No assistant was needed during the low snowpack of 2015. Our current assistant is Teri Sessions who has served 3 years..

Our district historically has had large numbers of Aedes vexans mosquitoes.  Their larva are dominant during high water periods since the females lay their eggs on land in anticipation of flooding. However Culex tarsalis and Culex pipiens are the species of greatest concern for the transmission of West Nile virus. These species lay their eggs on standing water and will increase in numbers through August so residents are urged to police their yards and drain any standing water in tarps, tires, buckets and the like weekly.  Please drain and cover or dispose of any rimless tires.  Call 548-5904 if a site in the district is not drainable and requires treatment.

2009 was the worst year to date for West Nile virus in Washington state.  346 mosquito samples tested positive for the virus that year.  Sampling began in late May and immediately positive samples were found in Yakima County.  During 2009, 38 people, 72 horses, 1 dog, and 22 birds also tesed positive for the virus. Most of the human cases were serious including symptoms such as meningitis, encephalitis, paralysis and one death.  In 2008, West Nile virus was found in 57 mosquito pools, 3 people,  41 horses,  1 dog and 24 birds.

Since the Culex mosquito is the main carrier of this disease, in 2008 I acquired a stereomicroscope and learned to identify Culex pipiens and Culex tarsalis, the two Culex species present in our area. When Culex numbers reached 10 or more at a trap site (or 6 or more Culex tarsalis) these mosquitoes were tested using VecTest.  We acquired a RAMP Reader testing system in 2014 since VecTest was no longer available.  All the test results have been negative to date for West Nile virus in our district. In 2010, 2013, 2017 and 2018 there were not enough Culex mosquitoes for testing to be needed.  We did not test in 2011 due to the disease's late arrival to our state.  The number of Culex in 2012, 2015 and 2016 was a cause for concern (up to 39 in a trap at the south end of the District in 2012 and many more at the west end of the District in 2015). The south end of the district was a concern again in 2016 with the first two RAMP results over 10 (but below 60). Residents of the area were warned to avoid mosquitoes although the RAMP testing results did not warrant an aerial spray block.

Culex tarsalis mosquitoes feed equally on birds and mammals while Culex pipens prefer birds.  Since Culex tarsalis is more likely to transmit the virus to humans we normally use a lower threshold for West nile virus testing: six Culex tarsalis mosquitoes in one trap versus 10 of either Culex species.  The numbers required are most likely to occur in August when the population of Culex peaks. 

If a positive sample is obtained then a prompt aerial spray contract would be likely.   

Historically the floodwater mosquito Aedes vexans has been the predominant mosquito in our area.  This mosquito lays its eggs on land in anticipation of flooding due to snowmelt or a rising water table.  Due to its preference for mammals it is only an incidental carrier of West Nile virus.  Other species of Aedes are important vectors of malaria, dengue fever and other diseases which do not require birds for their amplification but these diseases are not expected in our area in the near future. 

Assistants help us to deal with periods of high water flow when the water table rises to produce more larval habitat. All assistants possess a pesticide applicators license so that they can work independently.  Teri Sessions is our current assistant and stayed later in the season than previous assistants: while we combatted high Culex numbers in the area between Prowell and East Leavenworth Road in 2016 and while manager Jenny Mullins recovered from an injury in July 2018.. In 2013 and 2014 Arnica Briody served as our assistant and helped us to a achieve a record low in the number of mosquitoes in 2013: the most mosquitoes in a trap overnight was just 6 (at Waterfront Park) and the most Culex caught was just 4 (at the Wheeler woodland.) 

We encourage residents to call 548-5904 to report sightings of two or more mosquitoes within the Leavenworth District (within 2 miles of Leavenworth). This allows us to track down small backyard breeding sites for the Culex mosquito which lays its eggs on water and is most likely to carry the West Nile virus because it feeds on both birds and mammals.  Such sites have the potential to produce thousands of mosquitoes so residents are urged to check their property regularly for standing water. Barrels, buckets, tires and tarps are common culprits.  Be especially aware of containers which will be refilled by your sprinklers and make sure that no site has standing water for more than a week.

Any time is a good time to make sure that screens are in good repair.  As summer arrives be sure to use an effective repellent during those times when contact with mosquitoes is likely. Do not allow mosquitoes to bite you!

Our main larvacides are Bti, Bs, and pupicides Agnique (a monomolecular film) and Cocobear (a dilute mineral oil).  We use Bti (Bacillus thuringiensis israelensis) the most: it works well on both Culex and Aedes and usually kills larva overnight by dissolving their midgut after they ingest it. Other strains of Bt are used to control a variety of forest and agricultural pests and are commonly used by organic farmers. In its granular form Bti is our cheapest pesticide at about $18 per acre. It also comes in longer lasting doughnut-shaped briquets which are used in sites under 1000 square feet (they are the most expensive larvicide per acre).

Our cheapest longer lasting material is Bacillus sphaericus (Bs). It costs about $120 per acre and works in a similar manner to Bti but is not effective on Aedes the floodwater mosquito which lays its eggs on dry land and then hatches in great numbers when flooded. Today this bacteria is combined with Bti in the organic certified Vectomax granule which we now use in small areas and in areas where organic matter may make the Bti less effective.

Agnique, a monomolecular film drowns pupa by reducing the surface tension of the water.  It is no longer being manufactured and so has been replaced by Cocobear a dilute mineral oil. The rotation of various larvacides helps to reduce the likelihood of pesticide resistance. 

In 2006 we began using methoprene, a growth regulator which prevents the completion of metamorphosis in mosquitoes. The formulation we use provides control for up to 30 days. We use it in storm drains and hard to reach areas (such as hawthorne thickets and islands which will become inaccessible) where the presence of pupa does not bother us.  We may substitute the newest bacterial larvicide, spinosad, for it in the future.

In 2009 we added a combination larvicide, Vectomax to our arsenal.  Vectomax combines Bti and Bs to allow long-lasting control of Culex with the added benefit of Bti to control Aedes and to prevent the development of Bs tolerance in Culex. 

Storm drain treatment is a very important part of our disease prevention program and that of many other cities. The storm drains are treated once a month by bicycle along with other small sites in Leavenworth using methoprene, or Vectomax.

Most of our district has not been sprayed with adulticide (malathion) since 2003. About an eighth of the district was sprayed in 2006.

Complaints help us to track down mosquito breeding areas. Complaints helped us to locate untreated storm drains, tiny ponds, tires and easily drainable tarps, buckets, boats and the like. Call us at 548-5904 if you see two or more mosquitoes in an evening.  In your message please leave your phone number, the location of the site and a description of the problem (adult mosquitoes or standing water). 

Residents need to be sure to check their yards for any standing water: standing water collected by any container will attract the Culex mosquito which lays its eggs on water. Drain the water if possible, otherwise call us at 548-5904 for treatment.  In 2005 at least three complainants were inadvertantly breeding mosquitoes on their own property: in an old bathtub, a container for cuttings, and in a tarp. In other cases, the neighbors were breeding mosquitoes: in an untreated swimming pool, a horse trough and a large cooking pot. If you have a swimming pool, then make sure it is empty or chemically treated.  If you have a horse trough, make sure the water is fresh and if unable to do this or if you will be away on vacation make sure to get Bti briquets from us for treatment every 3 to 4 weeks.

Horse owners are reminded to vaccinate their horses: a two shot series is needed the first year, followed by a booster shot each year thereafter. For the best protection these shots should be completed in the spring. West Nile virus is fatal in about 30% of those cases diagnosed in horses.

Although West Nile Virus is not usually deadly in humans, its effects can be quite debilitating.  All ages can be affected but the average age (median and mean) of all cases is in the late 40's with  an average in the late 50's for the worst symptoms.  In order to avoid the disease, avoid contact with mosquitoes. This can be done by maintaining home screens and by avoiding mosquito laden areas and wearing long sleeves, particularly in the evening. There are several effective repellents on the market now: products containing DEET have endured the test of time, the military uses a 33% DEET formulation: higher concentrations may irritate more sensitive skin, formulations between 7 and 20% are quite effective as well. For those who are apprehensive about the use of DEET, products containing Picaridin or oil of lemon eucalytus also offer long-lasting protection, but oil of lemon eucalyptus should not be used on small children due to its higher oral toxicity.

Why does our district put so much effort into education (to reduce larval habitat) and larviciding as opposed to adulticiding? There are several reasons: enough to persuade those of almost any political persuasion. Because larviciding is applied when mosquitoes are most concentrated, it is cheaper and more likely to be effective. (Where possible, habitat reduction is by far the cheapest and most effective route.) The materials used for larviciding are much better targetted to the mosquitoes and thus have much less impact on other species and much less risk to humans. At present there are no documented cases of mosquitoes becoming resistant to Bti, whereas resistance to malathion, other organophosphates and pyrethroids is not uncommon. Finally larvicide kills the mosquito before it has a chance to transmit disease.