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Season for marsh mosquitoes coming to an end

CARLSBAD — The large number of marsh mosquitoes that surprised residents who live in northern Oceanside and parts of south Carlsbad earlier this month are not expected to be as much of a problem as the summer begins to wind down.
According to Greg Slawson, senior vector ecologist for San Diego County Vector Control, the months of July and August are peak times for the hatching of the eggs, found in the marshy areas of the local lagoons and estuaries. The salt water is what is needed to make the eggs hatch, experts said.
An unusually high tide July 21 helped hatch a type of mosquito known as the Aedes taeniorhynchus, or black marsh mosquito, found in the mouth of the San Luis Rey and Santa Margarita rivers in Oceanside and the Batiquitos Lagoon in south Carlsbad.
And although aggressive biters, the black salt marsh mosquito does not carry the West Nile virus.
About 10 days after the high tide, residents noticed an unusually high number of mosquitoes as the larvae turned into adults and traveled inland.
“We had a huge high tide that pushed water a little further back than usual,” Slawson said. “We run our show around the high tides, and we gear up at this time every year.”
Slawson said environmental concerns place restrictions on how the larvicide is applied, noting that is usually done by wading into the marsh, as is the case at the Batiquitos Lagoon, or using helicopter drops on freshwater bodies of water and the San Luis Rey River. There are 40 such drop locations scheduled this year.
“We treat with larvicide in the low depressions, but getting the pesticides to the right spot can be difficult,” Slawson said. “It is hard to predict how far back the tides will go. With something like this it is hit or miss, and unfortunately this year we missed it. It is unfortunate, but the silver lining is that they can’t vector disease.”
Slawson said that there are reportedly a couple dozen types of mosquitoes in the county, some of which do carry the West Nile virus.
The numbers are down so far this year, compared to last year, of the number of birds found with the West Nile Virus, he said. Only a couple dozen have been diagnosed compared to a couple hundred at the same time last year.
And, according to information released by the vector control department, only two human cases were reported so far this year, and both of those cases were thought to be contracted outside the county.
“We are still learning,“ Slawson said. “But apparently it is cyclical.”
But the black marsh mosquito is seasonal, Slawson said, so when the weather starts to cool, so will the mosquito population.