Crater Lake, on the edge of the Flat Tops, was a beautiful sight under cloudless skies Saturday. However, there are thousands, if not millions of mosquitoes in this photograph.

Photo by Tom Ross

Crater Lake, on the edge of the Flat Tops, was a beautiful sight under cloudless skies Saturday. However, there are thousands, if not millions of mosquitoes in this photograph.

Tom Ross: Entomologist rates mosquito repellents

It's true: Ravenous skeeters

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Tom Ross

Tom Ross' column appears in Steamboat Today. Contact him at 970-871-4205 or tross@SteamboatToday.com.

Find more columns by Tom here.

— Possibly the second ugliest mosquito ordeal of my life took place in the Ottawa National Forest of Northern Michigan. We were on a canoe trip in the Sylvania Wilderness that required us to portage our canoes from one small lake to another.

The portages weren't long - not much more than 100 rods, or three-tenths of a mile - and the terrain wasn't difficult. Still, each stumbling hike with the 80-pound fiberglass canoe over our heads was torture. It seemed to me that every biting insect in the North Woods had congregated under the roof created by the canoe we held aloft with aching arms.

Because our hands were fully engaged in supporting the weight of the canoe, we were defenseless. If we wanted to slap biting mosquitoes from our necks, we'd have had to put the canoe down to do it. That wasn't going to happen.

Now, 30 years later, we can pinpoint scientific explanations for why portaging a canoe in mosquito country is so unpleasant. It turns out that if your goal was to be bitten by thousands of mosquitoes in as short a time as possible, you couldn't devise a more effective means to that end than exercising with a boat over your head.

Why? Scientists have determined that mosquitoes have sensors that allow them to home in on greater concentrations of carbon dioxide in the air.

Writing in the Journal of Medical Entomology, Alan J. Grant and Robert J. O'Connell said mosquitoes have nerve receptors on their faces that are sensitive to changes in carbon dioxide concentration. Scientists also have determined that once the biting pests come within range of the source of that CO2, they are particularly drawn to chemical compounds in the lactic acid produced by humans when they are engaged in strenuous exercise. The lactic acid is contained in perspiration.

It may or may not come as a surprise that only female mosquitoes bite us. Their purpose is to acquire the blood they need to produce and lay their eggs. Mosquito dudes could care less about humans.

Here's the kicker.

Some people "smell" better to female mosquitoes than others.

If you've hiked the Rocky Mountains with a group in early July - and particularly in July 2008 - you've probably noticed this phenomenon. The mosquitoes seem to be singling out one or two people in your group.

Susan Paskewitz, a professor of entomology at the University of Wisconsin-Madison, says it's no rural myth.

The variations among different individuals in skin temperature, and the peculiar chemical compounds associated with their exhaled breath and the sweat that contains lactic acid cause some people to attract more mosquitoes than others.

We had a profound mosquito experience Saturday afternoon while hiking to a series of small lakes on the edge of the Flat Tops near Sheriff's Reservoir.

My wife used a DEET-based repellent, but I didn't use any. I'm not saying I wasn't perpetually being attacked by millions of persistent mosquitoes, because I was. But I wasn't bothered as much as she was. And I often find DEET to be more repugnant than mosquito bites.

Do we conclude that the use of the repellent was actually a disadvantage in this case? No, of course not. Instead, we can deduce that she smells sweet and the chemical cocktail linked to my lactic acid is that of a cranky old newspaper hack.

If you're curious about the effectiveness of various substances reputed to repel mosquitoes and their relative effectiveness (backed by scientific studies), Paskewitz has a summary on the UW-Madison Web site at www.entomology.wisc.edu/mosquitosite/index.html.

You'll be interested to read that the expensive backyard carbon dioxide traps now on the market probably succeed in capturing thousands of mosquitoes. However, they may not make a substantial dent in the number of times you're bitten at a backyard barbecue.

My all-time worst mosquito experience? It wasn't in the Flat Tops. It was in the Mount Jefferson Wilderness of the Oregon Cascades, which I've since learned has a reputation for man-eating mosquitoes. We literally ran down the trail and back to our vehicles to get away from them.

Saturday night in the Flat Tops was so cloudless we felt secure in leaving the rain fly off the tent. The netting-covered windows were crowded with patient mosquitoes watching us eat our steaks.

One of us smelled like DEET, and the other smelled like, well, lactic acid.

Finally, the waxing moon rose high in the sky and the temperature above 9,000 feet dipped into the high 30s. The mosquitoes buzzed off and huddled in their own sleeping bags to wait for the rising sun.

- To reach Tom Ross, call 871-4205

or e-mail tross@steamboatpilot.com

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