Heavy rain and mild temperatures during the Labor Day weekend probably will cause Boone County’s mosquito population to mushroom this month, putting local residents in greater danger of contracting the West Nile Virus.
Moisture and temperatures in the 80s during the next two weeks will place mosquitoes in a sultry environment perfect for breeding, said Howard Pue, a Missouri public health veterinarian. Considering that human West Nile infections often peak after Labor Day weekend, Pue warned that more human cases might develop in two or three weeks.
“They’ve got everything they need to breed now,” Pue said. “We could easily have a resurgence of mosquitoes.”
A torrid summer has kept Missouri’s mosquito population down, although it has created a unique breeding environment for female mosquitoes looking to lay their eggs in standing water, Pue said.
By mid-August, when most standing water around Boone County had evaporated, pregnant mosquitoes blindly swarmed in summer-high numbers to a few trap buckets full of standing water, known as Gravit traps.
Columbia-Boone County Health Department Director Stephanie Browning set a Gravit up near her house and said she noticed 70 or more mosquitoes trapped during some dry August nights, compared to fewer than 10 during nights in a more humid June.
Because they have already ingested blood and are ready to breed, Browning said, West Nile-carrying mosquitoes tend to be netted by Gravits, which are placed by the health department to capture test specimens.
In contrast, Browning said, some of the department’s carbon-dioxide traps, which are designed to lure hungry mosquitoes, turned up with fewer captives nightly as the temperature soared. Pue said the trend also has occurred in parts of St. Louis.
“Mosquitoes are fast breeders, but they’re not good thinkers,” Pue said.
So far, no Boone County mosquitoes have tested positive for West Nile, although infections in six birds and a horse have heralded the beginning of West Nile season in Boone County.
Although no human infections have been reported in Boone County this year, five of the six reported human West Nile cases in Missouri this year have occurred in the past two weeks, according to the Missouri Department of Health and Senior Services.
Pue said Missouri’s mosquito population will likely jump in the next few weeks and will remain high until an early winter frost kills the insects off.