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Published September 16 2010

Cooler temps, rain boosting mosquito counts

Frequent rains and below-average daytime highs have mosquitoes swarming again in the metro area, prompting another salvo of aerial spraying.

Cass County Vector Control planned to join Moorhead in aerial spraying this evening, weather permitting. Moorhead also will conduct ground spraying.

“They’re out there, and they’re angry,” Moorhead Operations Director Chad Martin said of the persistent pests.

Vector Control ground sprayed Tuesday in Fargo and West Fargo, targeting sports fields, elementary schools and other sites that have generated complaints, Director Ben Prather said.

County residents keeping track of mosquito trap counts on Vector Control’s website may be questioning the low numbers – and with good reason.

Prather said the New Jersey light traps the county normally uses are less effective when the temperature drops. They use heat and light to attract mosquitoes, but that’s not enough in cooler temps, he said. Mosquitoes are looking for a strong carbon dioxide signature as found in human breath.

Instead of light traps, Vector Control is using carbon dioxide traps for a more accurate count.

Five traps on Wednesday yielded an average of 310 mosquitoes, compared with an average count of less than 25 in the light traps. However, carbon monoxide traps are typically four to five times as sensitive as light traps, so the count is probably comparable to a normal summertime light trap count of about 100 mosquitoes, Prather said, adding that the county plans to stop posting light trap counts after this week.

Vector Control’s seasonal workers are gone, so its six full-timers – down from a staff of 30 this summer – must try to cover a lot of ground in a narrowing window of time as the days grow shorter, he said.

“It’s a daunting task,” he said.

Frequent rains haven’t helped, creating a seesaw effect in mosquito populations. Through Tuesday, the 3.5 inches of rain that had fallen so far in September in Fargo was 2.5 inches above normal.

“It’s just been consistent rain after rain and brood after brood of mosquitoes,” Prather said.

Janet Knodel, a North Dakota State University Extension entomologist, said it’s the wet, cool conditions and the different late-emerging species of mosquitoes that seem to have the bite hitting harder at this time of year.

Fargo-Moorhead has yet to see an 80-degree day this September. As WDAY-TV meteorologist Daryl Ritchison pointed out, September averages six 80-degree days.

“With the cooler weather, it prolongs their development,” Knodel said of the skeeters. “That’s why we’re seeing quite a few of them.”

Nancy Otto, a Moorhead City Council member who pushed for the city to add aerial spraying to its arsenal this summer, said it “has been effective in most areas.” However, counts are high again, prompting today’s planned aerial spraying, said Otto, who has an official mosquito trap in her backyard.

“It’s bad right now again, and this is a dangerous time of year for that West Nile (virus) transmission,” she said.

Moorhead Operations Director Chad Martin said trap counts are low, but that’s because the city is using light traps. Moorhead doesn’t have carbon monoxide traps and is deferring to Cass County’s counts, he said.

This will be Moorhead’s third aerial application of the summer. The entire city was sprayed from the air once, and this will be the second spraying that specifically targets the Red River greenway. Truck-mounted sprayers will treat the rest of the city from the ground.

Wind speeds of 10 mph or greater or temperatures below 58 degrees would force the city to reschedule spraying to Friday or the next suitable date, Martin said.


Readers can reach Forum reporter Mike Nowatzki at (701) 241-5528