November 3, 2009 Report by Manager Jenny Mullins: Larvaciding for this year has ended. Larvaciding began on April 7 with the finding of pupa at a Run of the River pond.  Pupa continued to be found at various ponds in early April and a few were reared out for identification.  They appear to be Culex tarsalis and Culex pipiens, 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.  Please drain and cover or dispose of any rimless tires.  Call 548-3316 if a site in the district is not drainable and requires treatment. 2009 has been the worst year yet for West Nile virus in Washington state.  As of November 2, 341 mosquito samples have tested positive for the virus including 149 in Yakima County and 128 in Benton County.  Sampling began in late May and immediately positive samples were found in Yakima County. Since then 36 people, 71 horses, 1 dog, and 22 birds have 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, last year I acquired a stereomicroscope and learned to identify Culex pipiens and Culex tarsalis, the two Culex species present in our area.  Culex numbers remained low until mid-August. On August 18th they increased to 10 or more at two trap sites so we quickly obtained a VecTest kit and tested these mosquitoes on August 19th.  The test results were negative for West Nile virus.  On May 28th this year I refreshed my mosquito identification skill at a course put on by the state in Shoreline. This year I had access to three traps and began trapping July 6th. Most of the mosquitoes caught then were floodwater mosquitoes: 40 were caught at Run of the River July 6th and 13 were caught at Waterfront Park.  By July 15th the mosquitoes at Run of the River had dispersed and no location had more than 13 mosquitoes.  Only one Culex tarsalis was caught July 6th and two were caught on July 15th.  Numbers of Culex gradually increased so that tesing for West Nile virus was done on a regular basis in August using VecTest.  All tests were negative. Only the Waterfront Park location caught enough Culex to cause concern if West Nile virus was present.  On August 5 it caught 10 Culex including six Culex tarsalis and on August 19 it caught 11 Culex including only one Culex tarsalis.  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.  Since our testing supplies were due to expire at the end of August we tested numbers below this threshold several times. If a positive sample had been obtained then a prompt aerial spray contract would have been strongly considered.    Historically the floodwater mosquito Aedes 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.  It is an important vector 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.  The first Aedes larva were found in late April as the the river rose.  Assistants help us to deal with periods of high water flow when the water table rises to produce more larval habitat.  Bruce Hill of Wenatchee is returning this season.  Bruce joined us in 2007 and has an M.S. in entomology and retired a few years ago.  He was be responsible for most of the East Leavenworth Road area and the city parks during May and June. David Wood covered the Ski Hill area along with all of the City Parks for much of 2004 through 2006.  His efforts helped to demonstrate that ground larvaciding 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! Our main 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. Our 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. The rotation of various larvacides helps to reduce the likelihood of pesticide resistance. This year 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. In 2006 we began using methoprene, a hormone 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. 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. 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. Call us at 548-5904 if you see two or more mosquitoes in an evening. 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.