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 var 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.
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 using it sparingly to make it last.
Eventually we will repace it with a paraffinic white mineral oil which will be used just as 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 2011 and
2012 led us to hire an extra assistant (Michael Darlington in 2011 and Steve O'Flaherty in 2012) who began work in May
and continued thru 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.
Barry Moats continued with us for his 3rd season. Arnica Briody is our sole assistant.in 2013 and 2014. No
assistant was needed during the low snowpack of 2015. Our current assistant is Teri Sessions.
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
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 reach
10 or more at a trap site (or 6 or more Culex tarsalis) these mosquitoes are tested using VecTest. In
2008 and 2009 all the test results were negative for West Nile virus in our district. In 2010 and 2013 there were not
enough mosquitoes for testing to be needed. There were enough in 2011 and 2012. We did not test in 2011 due to
the disease's late arrival to our state. The number of Culex in 2012 and 2015 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) . These mosquitoes
were tested and came up negative.
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
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 is staying on while we combat high Culex numbers in the area between Prowell and East Leavenworth
Road. 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.) Barry Moats served as our assistant from 2010 through 2012.. Bruce Hill of Wenatchee served
in this capacity from 2007 through 2009, while David Wood worked from 2004 through
2006. Their efforts helped to demonstrate that ground larviciding can be successful even during high water years like
2006 and 2008.
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!
larvacides are Bti, Bs, and Agnique (a monomolecular film). 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 $16 per acre. It also comes in longer lasting doughnut-shaped briquets
which are used in sites under 1000 square feet.
cheapest longer lasting material is Bacillus sphaericus (Bs). It costs about $100 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. 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.
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 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.
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
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, Bs, 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.
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.
Why does our district put so much effort into education (to reduce larval habitat) and larvaciding as opposed
to adulticiding? There are several reasons: enough to persuade those of almost any political persuasion. Because larvaciding
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 larvaciding 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 and other organophosphates is not uncommon. Finally
larvacide kills the mosquito before it has a chance to transmit disease.