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He pulls over and grabs a computer-printed map of the area. Red circles mark spots where MMCD records indicate mosquito breeding grounds. Computers have made the job easier, says Pennuto, who remembers the days when "it used to be all gut feeling and going through years and years of paperwork that would tell you which sites to treat each year."
Pennuto was pretty good at the gut-feeling part of it. He has learned to see a wetland the way mosquitoes would, to look for shallow water shaded by dense shrubs, preferably with some floating leaf litter. And while he lacks bulbous, compound eyes, Pennuto has the advantage of memory--countless trips to countless wetlands, over what amounts to hundreds of mosquito generations. "It's a gift that some people have, to remember one wetland over another," he says matter-of-factly. "I remember each site."
This time of year, Pennuto is after spring-hatching mosquitoes. Controlling them early on, he explains, is critical to keeping the year's crop of skeeters down: "There's only one hatching a year, but the adults can survive all summer, so we try to stay on them."
In all, says Pennuto, some 50 species of mosquitoes live in Minnesota, but the District focuses on the 15 kinds most likely to sting people. "There are lots of mosquitoes out there that never bite humans," he explains. "Some get their blood meals from reptiles or amphibians or chickens--birds in general."
"Blood meal" is mosquito-fighter talk for what a female mosquito will suck out of its prey, using six moving mouth parts that scissor and saw their way into the skin. One blood meal is about a drop, and, contrary to conventional wisdom, the mosquitoes don't eat it. "They generally feed on plant nectar," Pennuto explains. "They use the protein in blood to develop eggs."
Like most insects, mosquitoes are obsessed with eggs and reproduction in general. That infernal buzzing? It's a mating song: Researchers who made recordings of mosquito noises in the Forties found them "so distinctive in character that an experienced observer can not only distinguish one genus from another, but can also distinguish males from females of the same species. The noise of a single female will cause the males of the same species to burst into an answering chorus."
Mosquito sex varies from species to species, with males typically swarming before grabbing females. Some species prefer to swarm over fresh dung, others in the calm air on the leeward side of chimneys and steeples; thick swarms have prompted observers to call 911 and report a fire. Few will mate in the lab; researchers speculate that this is because copulation often occurs in flight, and the couples need ample vertical falling space. And in the mosquito version of Lolita, the male of a New Zealand species, Opifex fuscus, has been observed skimming across the water's surface, ready to mount a nubile female fresh from her pupa.
But Pennuto is interested in an earlier stage of the mosquito life cycle. His waders squeak and squish as he crosses the gravel road and wades into the wetland, bearing a long-handled scoop. He dips out a ladleful of water and inspects it closely. "Lots of larvae here," he says. "I can see five right off the bat." He wades out and proffers the sample, which to the untrained eye seems to contain nothing but water, dirt, and floating weeds.
"Look for movement," he instructs, and suddenly the little wigglers become visible; they're mostly head, but the tail shimmies around in the scoop. "Mosquito larvae don't have gills," Pennuto explains. "They need to rise to the surface of the water to breathe--there's a breather siphon on the tail, which is why their heads are down." He pulls a glass vial out of the pickup's cab, deftly pours the water in, and labels the container with the wetland's number. At day's end he'll drop this and other samples off at the MMCD lab, where staffers will identify the larvae's species and genus.
In this vial, Pennuto figures the lab won't find anything but garden-variety spring hatchers--eight of them, it looks like. He does the math. "If, according to typical averages, half of the mosquitoes grow to adulthood, that's four adults. And if half of them are females, then that's two females able to lay 250 eggs at a crack, which they will probably do three times before they die. So right here are 1,500 potential mosquitoes."
But right now those imaginary insects are trapped in a vial treated with a little bit of alcohol. A few strategic scoops dumped on the gravel road could kill tens of thousands of potential mosquitoes. It's heady stuff to the amateur--though, Pennuto points out, even 100,000 would amount to just a fraction of the mosquito population that will eventually fly from this particular wetland.
Or rather, that would fly, were it not for Pennuto. He returns to his truck and opens a plastic canister containing a yellow, granular material made from ground-up corncobs. The granules are inoculated with bacteria, called Bacillus thuringiensis israelensis or Bti, which infect the digestive systems of mosquito larvae and kill them. Pennuto fills up an apparatus that looks like an oversized flour sifter and returns to the wetland. "Fish and dragonfly larvae can eat mosquito larvae," he says as he walks, shaking the sifter to spread the corncob granules around. "But the problem with this type of wetland is that mosquitoes have figured it out. Where you find mosquitoes, you don't find predator fish or insects, because this type of wetland dries up and so they wouldn't survive here."