One of the world's most hated insects — the mosquito — is expected to thrive this summer in Multnomah County after the region's record-breaking wet spring.
But Clackamas County isn't seeing a spike in mosquito populations so far this year, however, said Josh Jacobson, director of Clackamas County Vector Control.
The numbers of mosquitos observed in the county's traps have been average, he said.
The disparity between neighboring counties likely has to do with differences in topography, Jacobson said. The areas around Clackamas County's rivers are more hilly and less prone to being filled with stagnant water where mosquitos thrive, he said.
"We haven't quite seen a surge yet," Jacobson said. "We don't have the huge marshes that flood along our rivers, which are driving the spike in Multnomah County."
That's the case next door, where extremely high numbers of mosquitos already have been observed throughout Multnomah County, with experts warning the worst is yet to come.
"We're about halfway up that bell curve," said Bek Sudia, an ecologist who leads Multnomah County's mosquito surveillance and control team. "July is really our peak time."
While the mosquito-borne West Nile virus is rare in the Portland area, it is routinely detected in eastern and southern Oregon.
County health officials are urging people to eliminate mosquito habitats on their property and to protect themselves from being bitten.
Mosquito species in the area lay their eggs in damp soil near rivers and other water bodies during late spring and early summer. As mountain snow melts and water levels rise, those areas become covered with stagnant water and the eggs hatch.
Last year's dry spring and summer left lower water levels, fewer mosquito eggs hatching and a smaller population of adult mosquitoes, according to the mosquito control district in Clark County, Washington, across the Columbia River from Portland.
With this year's record rainfall, which caused the Columbia River to reach flood stage, hundreds of acres around local rivers flooded and mosquito eggs began hatching, including eggs that didn't hatch last year.
"All of those conditions culminating at the same time created this year's perfect storm," said Mario Boisvert, manager of the Clark County Mosquito Control District.
District officials said they were working to respond to 300 requests for mosquito control service by property owners within the last three weeks.
Multnomah County's mosquito control team sets traps in 150 places every year. The team has trapped more than 20,000 mosquitos so far this year, about four times the number in all of 2021, according to the county.
Sudia uses a microscope and tweezers to divide the mosquitoes into vials and then sends 50-bug batches to Oregon State University to test for diseases.
West Nile virus has never been detected in Multnomah County's samples. But 75 batches of mosquito samples from across eastern and southern Oregon last year tested positive for the virus.
Jacobson of Clackamas County Vector Control said counties also send samples when they're alerted to recent deaths of birds, which are reservoirs for the virus.
Clackamas and Multnomah counties haven't had a bird sample test positive since 2007, when there were three and 16 cases reported in the counties, respectively, according to Oregon Health Authority data.
Multnomah County's mosquito control team uses three methods to reduce the insect's populations because of the need to limit the risk of West Nile virus, officials said.
It provides mosquitofish, a species that eats insect larvae, for people to place in ornamental ponds. The team disperses corn cobs covered in a bacteria that kills mosquito larvae but is harmless to every other organism — Bacillus thuringiensis israelensis — in natural breeding grounds. Also, when populations spike, officials work with property owners to fog focused areas with a chemical often used to treat household pets for fleas, but at one-tenth of the concentration. They don't fog agricultural, urban or residential land, officials said.
The team also is advising people who spend time outside around mosquitoes, especially in areas where the insects carry West Nile virus, should take steps to avoid being bitten.
West Nile virus typically causes mild illness, but in rare cases, it can be fatal.
Multnomah County officials are advising people who spend time outside around mosquitoes to avoid being bitten.
They said people should take these steps:
• Wear long sleeves and pants and apply mosquito repellent.
• Minimize time spent outside during dawn and dusk when mosquitoes are at their worst.
• Prevent mosquitoes from reproducing by dumping or regularly changing standing water in kiddie pools, buckets and dog bowls.
• Repair screens in windows and doors.
• Report newly dead crows (within 24 hours of death). If the disease is in an area, crows often die before mosquitoes or humans test positive. Don't report crows that have clearly been hit by a vehicle or electrocuted (look for burned-off feet).
The need to manage mosquito populations to prevent diseases likely will become more important for the Portland area as foreign mosquito species enter the United States and as the climate continues to change.
By 2050, much of the West Coast is expected to become suitable habitat for two mosquito species known to spread more than 20 viruses that affect humans, as temperature and precipitation patterns change. A 2020 study by researchers at the National Microbiology Laboratory of the Public Health Agency of Canada shows the prediction.
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