Where are Vermont’s worst mosquitoes?

There are only two mosquito control districts (MCDs) in Vermont: the Brandon-Leicester-Salisbury-Goshen-Pittsford District (BLSG) and the adjacent Lemon Fair District. Mosquitoes are present throughout Vermont, but the lonely presence of these two MCDs suggests there might be a lot more mosquitoes in this part of the state. Indeed, the broad floodplain of Otter Creek runs right through the two districts, and thousands of acres of wetlands and low-lying agricultural fields there can be prime breeding grounds for mosquitoes.  

Finding evidence to confirm that there are more mosquitoes there than elsewhere in Vermont is difficult. The Vermont Agency of Agriculture has been trapping mosquitoes throughout Vermont for several years and now has traps in every county and in more than 83 of the state’s 237 towns. Unfortunately, that trapping program is not designed to document how many mosquitoes are present. Its primary goal is to collect mosquitoes to analyze for mosquito-borne diseases. A preliminary analysis by the Agency of Agriculture indicates that traps in Grand Isle County captured a lot more mosquitoes than in either Rutland or Addison Counties. But there does not seem to be reliable scientific evidence that the BLSG District has either more or fewer mosquitoes than elsewhere in Vermont.

If the BLSG District is not the buggiest area in Vermont, why are the only MCDs in the state there? Could it be nothing more than an historical accident?

Mosquito trapping sites monitored by the Vermont Agency of Agriculture in 2019. The trapping program is designed to collect adult mosquitoes for arbovirus monitoring and probably can’t tell us much about which areas have the densest mosquitoes. From VAAFM.

Mosquito control in the BLSG area started around Lake Dunmore and Fern Lake in the 1960s. Locals with lake houses and the many visiting tourists appreciated some relief from the mosquitoes which could sometimes interfere with lakeside recreation, so they started spraying DDT along roads around the lakes. The effort was made official in 1978 when the BLS (Goshen joined in 1990) was formed as a quasi-municipal entity. Until 1989 the primary activity of the BLS was to spray chemical insecticide along roads (DDT was not used after 1972).

In June 1989, there were a lot of mosquitoes in the Lake Dunmore area. An explanation for the outbreak was that two previous years had been dry so mosquito eggs did not hatch. Then 1989 was a wet year and three years of eggs hatched all at once. This type of thing is known to happen, but there is no evidence that this is what happened in 1989. Whatever the explanation, an important lesson from this bad mosquito year is that roadside spraying of insecticides to kill mosquitoes was happening in the District that year. It has happened just about every year since the 1960s including some very bad mosquito years (e.g., 2004). When it’s a bad mosquito year, it’s a bad mosquito year. BLSG’s roadside spraying has never changed that and never will.

In the buggy year of 1989, Art Doty was Chair of the BLS board of trustees and deeply involved in the roadside spraying program. He lived in the area and worked as a corporate lobbyist in Montpelier. He invited Governor Madeleine Kunin to visit the lake area and see how bad the mosquitoes were. When she arrived reporters witnessed her being swarmed by mosquitoes and the scene was recounted across the country (e.g., a Los Angeles Times story from June 25, 1989). Shortly thereafter the state appropriated $100,000 for mosquito control, and a new program of larvicide treatment began.

At the time, BLS was the only MCD in the state, so I guess they got all the new funds. Apparently, no one asked whether other places in the state might also have bad years for mosquitoes, and the Governor was not about to accept invitations to see for herself. Other places in Vermont did have bad years for mosquitoes and still do (e.g., the Champlain Islands, Colchester, and the Northeast Kingdom), but to this day BLSG and the neighboring Lemon Fair District are the only MCDs and the only participants in the state funded larvicide program (which 30 years later has grown only to $140,000 per year, now split among eight towns instead of three).

So there was no study or consensus that the towns around the Otter Creek wetlands had the worst mosquitoes in the state. There was just a series of three events: a motivated community around Lake Dunmore who took it upon themselves to spray insecticides, then a municipal agreement to ensure public funding for the spraying, and then the clever lobbyist who shamed the Governor into paying for a larvicide program. It is not the geography of mosquitoes as much as these political events which gave us today’s BLSG.

That doesn’t mean there aren’t billions of mosquitoes in the District in some years. Those extensive lowland areas along Otter Creek can produce a lot of mosquitoes, and summer revelers at Lake Dunmore don’t like mosquitoes. But Lake Dunmore is five miles from Otter Creek and two miles from the nearest lowland wetlands along the Leicester River. Do the mosquitoes encountered near the lakes have anything to do with the distant Otter Creek wetlands?

There is an oft-repeated explanation that the clouds of mosquitoes rising from the Otter Creek wetlands are carried on the prevailing winds to the lakes. This type of aerial transport is probably common, but no evidence seems to exist to confirm it for this area. For example, I have not seen data suggesting that the mosquitoes bothering people on the lakes are the species that commonly breed in the distant floodplain fields or wetlands.

There is another problem with this aerial transport story. Although our weather systems typically move from west to east across the country, the winds in the Champlain Valley are almost never from the west. The mountains on either side of the valley channel the winds along the north-south axis. There are a dozen airports or runways in the Champlain Valley and every one of them is oriented north-south to avoid crosswind takeoffs and landings. Wind roses from the Burlington International Airport show this pattern clearly – in the summer the prevailing wind is from the south. A west wind that would carry mosquitoes from Otter Creek to Lake Dunmore is exceedingly rare.

Wind roses from the Burlington International Airport with average results for summer from 2009 to 2018. Left: June to August wind direction and speed. Right: May to August wind direction and speed at night (9 PM to 4 AM). The length of the petals represents the frequency of wind from that direction (the petal extends out to the direction the wind comes from). Wind from the west which could blow mosquitoes from the Otter Creek wetlands to Lake Dunmore are rare. Source.

It might be that most of the mosquitoes bothering residents and tourists at Lake Dunmore and Fern Lake have hatched in the nearby lakeside woods and wet places just as happens at every other lake in Vermont. So I don’t know for sure that the BLSG lake region, or any other part of the BLSG District, has substantially more mosquitoes than elsewhere in Vermont. I do know that where I live in Salisbury the mosquitoes are sometimes bothersome for a couple of weeks in June and after that in most years are not much of an issue.

Every now and then we have a year when an incredible number of mosquito eggs become adults (for whatever reason) and it is hard for us to imagine that anywhere in the world could have more mosquitoes than the BLSG District. There doesn’t seem to be much evidence that anything done by BLSG (or the Governor) will ever change that.

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