On the Mosquito Trapline

Vermont State Entomologist Alan Graham vacuums mosquitoes out of a resting box trap. Photo courtesy of Alan Graham.

I’m out hunting an unlikely target: mosquitoes. At each stop along the road, I pull on a bug net and gaiters, wrestle a cylindrical vacuum and battery pack onto my shoulder, and head to my assigned swamp. I return with my bounty (there’s no bag limit, incidentally) and store it on dry ice in a giant cooler in the trunk of my car. The fog rolling onto the road combined with my Ghostbusters-like getup is surely a strange scene to cars passing by.

Each summer in Vermont, field technicians from the Vermont Agency of Agriculture visit swamp sites across the state to get a representative sampling of two mosquito-borne illnesses: West Nile Virus and Eastern Equine Encephalitis (EEE). The study has helped put together maps detailing the distribution and spread of these viruses.

Less commonly talked about than West Nile Virus, EEE can be fatal to humans and horses. First identified in the 1930s, the virus is present mainly along the eastern coast of the United States, with sporadic occurrences along the Gulf Coast and in the northern Midwest. It is rarely contracted, but is often lethal if it is, and can cause severe brain damage in survivors. In 2014, a total of eight human cases were reported across the United States. No one in Vermont became ill, but there were three cases in New Hampshire, two in New York, and one in Maine. Why this disease is so rare is not well understood.

The EEE virus is primarily found in birds, particularly passerine, or perching, birds. Mosquitoes catch the virus when they feed on birds and can then transmit it to people, horses, deer, and moose. These are all “dead-end hosts,” which means they cannot transmit the virus themselves. So, if a horse contracts the disease, its owner need not worry about catching it.

Monitoring this disease is trickier than it may seem. Testing mosquitoes for EEE and West Nile Virus requires that they be kept at sub-zero temperatures. They must be alive until the moment they are put on ice, otherwise the virus denatures and cannot be detected. Samples are kept chilled, even while under the microscope. And since each lab test is an expense, only those species most likely to catch the disease are tested. (Of the 45 species of mosquitoes in Vermont, only 10 are tested.) Each collected mosquito must be identified and sorted, which in turn means the mosquito samples must be handled very carefully so as not to disrupt any identifying features. The loss of antennae, legs, wings, or even the delicate scales on the sides of the thorax and abdomen can render an individual mosquito unidentifiable. For example, one of the mosquito species most likely to test positive for EEE, Culiseta melanura, is identified by microscopic bronze scales hidden where the wing meets the body.

All these steps happen after the mosquitoes have been caught – but the first challenge is simply to create a trap that will attract mainly mosquitoes (especially those species in which we are interested). While traps using light are available and effective, those traps collect many different types of insects, and sorting through them would waste valuable time in the lab.

So we use three different types of traps that don’t rely on light. The first uses carbon dioxide, in the form of dry ice, to attract the mosquitoes, tricking them into thinking that there is an animal exhaling nearby. The ice is put into a small thermos that hangs from a tree, and the carbon dioxide gradually releases through a hole. When the mosquitoes buzz by for a tasty meal, they are gently sucked up into a net by a battery-powered fan.

The second type of trap also uses an automated fan and net, but attracts the mosquitoes with a different appeal. Certain species like to lay their eggs in dirty water, especially puddles in cow pastures. By mixing manure with water, one can create an attractive breeding habitat. When pregnant mosquitoes land on the water, the fan again captures them in a closed net, where they will buzz around until a field technician collects them the next morning.

The third kind of trap is called a resting box trap. Once a female mosquito has gotten her fill feeding on birds, she searches the forest floor for a dark, sheltered space. We set up black plastic boxes with one side open and use a gentle, cylindrical vacuum with a nylon stocking on the end to suck up those satiated mosquitoes. The resting box trap and the carbon dioxide trap are set up in prime habitats for Culiseta melanura: lowland acidic swamps with primarily softwood trees.

Though Eastern Equine Encephalitis can be deadly, it is rare. (The State tested 3,245 batches of mosquitoes last year; each batch ranged from 1 to 50 mosquitoes; of those batches, 8 tested positive for EEE.) Risk increases in swampy areas of the Northeast and the northern Midwest. Simply applying an effective insect repellent will significantly reduce that risk. Monitoring programs are conducted to keep people alert to the presence of insect-borne diseases and to track changes over time. Check out the U.S. Geological Survey’s disease maps to learn more about the frequency and geographic distribution of EEE, West Nile Virus, and other similar mosquito-borne diseases.


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