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Hot, dry weather could affect mosquito populations

WALPOLE, Mass. — Talk about a painstaking task.

Wednesday, Entomologist Kaitlyn O’Donnell used tweezers and a manual counter to determine how many mosquitoes had been caught in a local trap.

The answer: about an average number for this time of year.

“We will first sort them by species and decide which mosquitoes we want to send to the lab to be tested,” said O’Donnell.

Leading the count, species-wise: Coquellitidia perturbans — a typical early summer nuisance — and worse.

“They are a bridge vector for both West Nile Virus and Eastern Equine Encephalitis,” O’Donnell said. “So, typically, if there are enough of them I will submit a pool of these mosquitoes to the lab for testing.”

Wednesday, State Epidemiologist Catherine Brown, DVM, MPH, briefed the Public Health Council on mosquito-borne disease surveillance.

“The dry and cool weather that we had in the spring and early summer really likely slowed down the mosquito reproduction,” Brown said

But that weather took a turn several weeks ago. It has been hot and dry for about a month. If those conditions continue, it’s likely to impact mosquito season in good and bad ways.

“The primary vector of West Nile Virus actually does pretty well in a drought year,” said O’Donnell. This comes as the virus was found in Massachusetts mosquitos for the first time this year.

The mosquito called Culex pipiens, is a well-known West Nile Virus carrier.

“They breed in containers, in catch basins,” O’Donnell said. “In these small, temporary pools that can collect water from the little amount of rain we have had, but don’t get flushed out.”

But so far, the Culex numbers, at least in Norfolk County, have been about average, O’Donnell said.

On the other hand, the primary vector for Eastern Equine Encephalitis — Culiseta melanura — breeds in swampy areas. So it’s dependent on water.

“They live in these underground pockets of water created by the root systems in these swamps,” O’Donnell said. “We typically have an emergence of adults early in the season. Then those adults will lay eggs and then a second generation will come out later in the summer.”

In Norfolk County, O’Donnell said the numbers of Culiseta are higher than normal for this time of year.

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