So far this year, drier than normal conditions have meant far fewer mosquitoes developing in the swaps and wetlands southwest of Boston. So officials at the Norfolk County Mosquito Control District have decided to significantly limit the spreading of larvicide from helicopters this spring.
But predicting what will happen later in the year with the pesky and potentially dangerous insects is tricky since heavier rains in the months ahead could increase the mosquito population, prompting the district to take broader action.
Last year the district did aerial distribution of larvicide in 23 of its 25 communities. But the dry weather has meant it may happen in only two this year: Medfield and Millis, according to David Lawson, the acting director for the Norfolk County district.
Lawson said that the aerial applications may come because of rain that fell April 22 and 23 that caused the Neponset and the Charles rivers to overflow their banks and put some water into flood plains.
“We’re keeping an eye on that,’’ he said.
Wider aerial operations are not in the plans right now.
“Southeastern Massachusetts has experienced the driest first three months since records have been kept,” Lawson said. “It would be irresponsible for us to conduct an aerial larvicide at this point.”
It is a decision with which state officials agree, and one that town officials contacted support, so far.
Al DeMaria, medical director for the state’s Bureau of Infectious Disease, keeps track of mosquitoes annually tested for disease because of the West Nile virus and Eastern equine encephalitis they can transmit to humans.
“Potentially heavy rains could change the picture,” said DeMaria, regarding the Norfolk County district’s decision to skip the springtime flights, but he said that the larvicide, typically a granular material dropped through a helicopter-carried spreader, “only works if it comes into contact with larvae that are there. It doesn’t hang around.”
Last week, following several more days of rain, state Health Department spokesman John Jacob added: “According to our experts, there has not been enough rain recently to significantly affect the larval mosquito populations at this time.”
And Lawson said that rain that fell last week was not enough to change plans.
“We only got about half an inch and it didn’t do much,’’ he said. “Just a blip in the big picture.”
Last year Medway was among the towns in the Norfolk district that had mosquitoes carrying disease. The state Department of Public Health recorded three positive tests for West Nile virus among mosquitos in Medway and three for Eastern equine encephalitis.
But Stephanie Bacon, town health agent, said she is confident that Mosquito Control officials are making the right decision.
“Honestly, they are the experts here and they do judge the mosquito population by what’s happening with weather patterns,” she said. “It’s their call, until and unless we have different weather. Then, I would, as a health agent, push to have them fly like they typically do.”
Warm weather is another factor.
“Normally we would start seeing adult mosquitos around mid-May,” Lawson said. “With warm weather now, we’ll probably see some adults a week or two earlier.”
DeMaria said the infectious disease bureau especially expects to see Culiseta melanura — a carrier of Eastern equine encephalitis — developing early.
“We do expect them out earlier; we’re on the lookout for that,” he said, but he added that the dry nature of the spring may otherwise suppress an early emergence of Culex mosquitos, puddle-seeking types that carry West Nile virus.
Since it costs money to put the helicopters in the air — to the tune of some $200,000 per season, said Lawson — any relatively flightless months could mean a boost for improvements to other areas of his operation.
“Our trucks are getting very old,” Lawson said, and unused aerial funding could be put toward those. “There is also a possibility that it could roll over and be available for applications next year. That would be the ideal, but it’s a little early to talk about that.”
It’s also premature to characterize the 2012 mosquito season, according to officials. Spring conditions won’t tell the whole story.
“You can’t do long-term predictions,” DeMaria said.
“A good example is the mosquito season in 2005. For the most part we had very few mosquitoes and then, in mid-August, the skies opened up and larvae started,’’ DeMaria continued.
By September we had a huge number of mosquitoes and a deep concern about EEE. We did have four cases that year. The risk had looked very low until mid August.”
Or, as Lawson puts it: “When it comes to mosquitoes, we’re always playing it week by week, that’s how we live our life.”