REGION — San Diego County ecologist Chris Conlan isn’t surprised that a particularly vicious genus of mosquitos has invaded Southern California. He’s surprised it took them so long to get here.
“We’ve always had mosquitoes. What changed for us is, with the arrival of these new species, comes the possibility of new diseases that we didn’t think we had to think about,” Conlan said. “… Unfortunately, that has all changed. Now that risk is a more real possibility.”
The invaders, primarily two species of the Aedes genus, are capable of transmitting viruses to humans that cause tropical diseases such as yellow fever, dengue and Zika fever. One of them, Aedes aegypti, first established populations in southern San Diego County in late 2014 and has been spreading northward ever since; another, Aedes notoscriptus, wasn’t recorded here in significant numbers until last year.
A third related species commonly called the Asian tiger mosquito has been tormenting Los Angeles and Orange counties since arriving in shipments of bamboo plants in 2001 and being re-introduced in 2011, but so far is not well-established in San Diego County, Conlan said.
Aedes mosquitos are tenacious travelers, in part, because their eggs can remain viable in a dry state for months, easing their spread. Various Aedes species had footholds in southern Arizona and northern Baja California for years before expanding their territory to Southern California.
“With all the people and all the backyard water sources, it was only a matter of time” before Aedes mosquitos established population that made their local eradication impossible, Conlan said. “Once they get in, people don’t recognize that there’s anything different. By the time we find out that they’re here, they have to too broad a range and it’s a matter of managing them instead of eradicating them.”
Unlike mosquitos of the Culex genus native to the state, Aedes mosquitos lay their eggs at the water’s edge where they can live dormant for months. Consequently, the county’s organized mosquito control efforts that involve treating larger bodies of fresh water with larvicide are ineffective against the invaders.
Aedes mosquitos can reproduce in less than a quarter-inch of water making backyards, balconies and patios potentially fertile habitat in the form of bird baths, pet bowls or even tiny pools of rainwater collected in plant leaves or children’s toys. The county’s “Fight the Bite” education campaign emphasizes the importance of public participation in efforts to keep mosquito populations under control.
“Aedes is adapted to live with human habitation. … You can go into a neighborhood with public education and vector control and knock down or eliminate those Aedes, but if you have a single property owner in the neighborhood who refuses to let in vector control or eliminate standing water, the mosquitos just proliferate,” said William Walton, a University of California entomology professor and a past president of the American Mosquito Control Association.
In addition to public health risks posed by the invasive Aedes mosquitos, they are an irritating nuisance that appear to prefer feeding on humans; native Culex mosquitos, by comparison, prefer birds and other animals, Walton said. Aedes mosquitos also actively feed during the day whereas Culex mosquitos bite primarily at dusk and dawn.
Not that Culex mosquitos are harmless: They are capable of spreading West Nile virus and other diseases. Culex mosquitos are the primary target of the county’s use of larvicide applied by helicopter to lagoons, lakes and ponds. North County locations regularly treated for mosquitos include San Elijo Lagoon, between Solano Beach and Encinitas; the Buena Vista Lagoon, between Carlsbad and Oceanside; Lake Hodges and the San Dieguito River, between Rancho Santa Fe and Escondido; and the San Luis Rey River in Oceanside.
For that program, the Board of Supervisors last week approved a measure to designate two companies, Sanford, Florida-based ADAPCO and St. Charles, Illinois-based Clarke Mosquito Control Products, Inc. as the county’s sole vendors for larvicide. The authorization covers the annual purchase of up to $1 million of larvicide through 2024.