There are four genera of mosquito in the Yukon. The one that causes the most aggravation, and keeps Randy Lamb busy in the spring, is the Aedes sp. or "snow-melt" mosquito.

Lamb is Mosquito Control Program Officer for the Government of the Yukon. He spends much of the spring traveling to Yukon communities, searching ponds for mosquito larvae, and tracking larval development.

Snow-melt mosquitoes hatch in large numbers, usually in May, from shallow, warm ponds of standing water left by the melting snow. Lamb's job is to kill off as many of the larvae as possible, in and near some Yukon communities, before they turn into the flying, biting insects that we know far too well.

The Mosquito Control Program uses a biological larvicide that contains heat-killed bacteria. The bacteria are fatal to a few species of biting flies, including the snow-melt mosquito, but have no effect on birds, mammals, amphibians, or other insects like dragonflies and damselflies.

"The bacillus was found in nature, in Israel, in a pond where they noticed there was a high mortality among mosquito larvae," says Lamb. Now it's the standard for mosquito control across North America.

The larvicide used in the Yukon is packaged as a coating on crushed corn cob fragments, a bit smaller than kernels of unpopped popcorn, which are sprinkled over standing water. The larvae eat the bacterial spores along with their food, become sick, and die within 4 to 24 hours.

The larvicide can be sprinkled by hand over smaller ponds, but to cover large areas the corn-cob granules are loaded into a hopper that hangs below a helicopter.

"It's the size of a small hot tub," says Lamb. The granules empty through the bottom of the hopper, passing through a rotary blade that scatters them at a controlled rate. A mere four kilograms of granules is enough to cover a hectare.

Lamb estimates there is close to 100 percent mortality in mosquito larvae exposed to the larvicide. The trick is to catch them while they are still larvae. Once they turn into pupae, and then into adults, they don't eat, so the larvicide has no effect.

That's why Lamb spends much of April and May on the road, sampling ponds, checking larvae numbers, and recording larval development. His goal is to treat a pond after most of its mosquito eggs have hatched and before the most advanced larvae pupate, in order to keep larvicide use to a minimum.

"We have to be really on top of monitoring development rates," he says.

He generally visits each community in the Mosquito Control Program two or three times through the spring. The first visit is to pinpoint mosquito breeding areas and to check larval development. By early May, he says, all the communities have been inspected once.

On the next round of visits, in the middle of May, Lamb usually takes along enough larvicide to hand-treat ponds where the larvae are unusually advanced.

"It doesn't take much. A handful will do a fair-sized pond."

He also delivers larger stocks of larvicide to store in the communities, in preparation for the main assault on the mosquito larvae later in the month.

Generally, mosquito larvae in all the communities develop on roughly the same schedule, Lamb says. There is usually a period of seven to ten days when most ponds are ready for treatment.

"That's the big whirlwind week," he says. "Usually in that week we're doing every community."

Once the whirlwind week is over, the Mosquito Control Program enters its monitoring phase. Since snow-melt mosquitoes generally have only one major hatch a year in the Yukon, the rest of the summer is spent checking the program's effectiveness by monitoring ponds, conducting bite counts both inside the mosquito control area and outside it, and trapping mosquitoes to identify species and estimate numbers.

The last step in the program is the season-end report that assesses the effectiveness of the summer's program and serves as the beginning of planning for the next year's assault on Yukon mosquitoes.

For more information about the Yukon government's Mosquito Control Program, call 667-5269. For information about pest control and pesticides, contact Environment Canada's Environmental Protection Branch in Whitehorse.