Wed July 11, 2012

Boulder Finds Its Own West Nile Virus-Positive Mosquitoes

The City of Boulder announced on Tuesday that they, too, have resident mosquitoes harboring West Nile Virus. The positive test comes about a month after the first cases in Colorado.

The Boulder mosquitoes were trapped just before the July 4th holiday in three spots, including Christensen Park, Stazio Ball Fields, and Tom Watson Park. Because of pooling, the infected insects are in at least one—but not necessarily all—of those areas.

Rella Abernathy, Integrated Pest Management Coordinator for Boulder, says it's still early for them to see mosquitoes testing positive for West Nile.

“Because we’ve had this really hot weather early in the year,” says Abernathy, “everything seems to be ahead of schedule.”

The hot weather has worked especially in favor of the type of mosquito that carries West Nile Virus, known as Culex. While all mosquitoes need water for breeding, Culex doesn’t need much compared to floodwater mosquitoes, and the heat can speed reproduction.

As a result, says Abernathy, "We're having lower overall mosquito numbers, but the percentage of Culex is higher."

In recent years, cases of West Nile Virus in mosquitoes and humans have been decreasing, but predicting activity is difficult, and peaks could occur at any time.

Every year, Abernathy says, Boulder limits mosquito breeding on its 45,000 acres. Potential breeding sites are monitored and treated with a soil borne bacteria called Bacillus thuringiensis israelensis, or Bti for short. The bacteria produce a toxin that specifically kills pest larvae.

Boulder residents should also be cognizant of mosquito havens on their own property, like bird baths, flowerpots, or even little depressions in the lawn.

“Just a little cup of water can breed mosquitoes,” says Abernathy.

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