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Bacteria major weapons in W. Nile war

Jeremy Heiman
Special to the Post Independent

Garfield County will rely heavily on biological weapons in its battle against West Nile virus this year.

West Nile virus, potentially deadly to humans, is expected to peak on the Western Slope this year. To counter the spread of the disease, county officials have hired Colorado Mosquito Control Inc. of Brighton on a $100,000 contract to control the insects known to carry West Nile.

Colorado Mosquito Control’s principal efforts in Garfield County will be against mosquitoes in their larval stage, which live in still water and are easier to eliminate than adult insects.



The anti-larvae campaign will rely primarily on biological methods, using specialized bacteria to attack mosquito larvae, said Michael McGinnis, the company’s president.

The company will use a bacterium called Bacillus thuringiensis israeliensis, or Bti, as its main weapon against mosquito larvae. Bti is a unique strain of the commonly used Bacillus thuringiensis (Bt), that has been found to attack only mosquitoes and very closely related midge flies.



A second biological agent, Bacillus sphaericus, will also be used, under different circumstances, McGinnis said. While Bti is most often used early in the season and as a general larvicide, B. sphaericus is more effective in situations where a body of water can’t be treated frequently, where a longer-lasting larvicide is needed.

B. sphaericus is even more specialized than Bti, McGinnis said. It only reproduces inside a dead mosquito larva.

“I don’t think it affects other nontarget insects at all,” McGinnis said.

Weekly inspections start in May

Bti is sold in donut-shaped cakes that are placed in shallow water, said Steve Anthony, Garfield County’s vegetation management specialist, who will administer the mosquito program for the county. B. sphaericus is distributed as a granular substance that can be spread by hand or sprayed as a wet mixture.

Areas that need to be treated with larvicide will be identified by mapping areas with standing water early in the season. These include stagnant ditches, flood-irrigated pastures, natural wetlands and wet stream bank areas.

“Once we get into May, we’ll start doing weekly inspections to find larvae in the water,” McGinnis said.

A third method, a chemical called Altosid, will be used to control mosquito larvae in areas that technicians can’t easily return to. Altosid is an insect growth regulator, a type of hormone.

“It doesn’t allow them to hatch out as an adult mosquito,” McGinnis said.

Altosid is used in areas that can only be reached once a month or even once a season, and in areas where Colorado Mosquito Control finds mosquitoes breeding even after repeated visits.

Altosid, too, isn’t known to affect other organisms in the environment.

“It’s very specific to mosquitoes,” McGinnis said. “It actually mimics mosquitoes’ hormones.” The company will most likely use Altosid in the form of pellets that last 30 days or in 150-day briquettes, he said.

Spraying limited to 1 ounce per acre

Colorado Mosquito Control may have to take measures against adult mosquitoes as well, McGinnis said.

Adult mosquitoes can sometimes be numerous even when larvicide methods are successful, McGinnis said, because mountain winds can move mosquitoes from one place to another pretty easily. So it’s likely that some adult mosquito control will be needed, McGinnis said.

“But our goal is to minimize it,” he said.

Adult mosquitoes are controlled with toxic chemical spray, but the process has become less environmentally damaging in recent years.

Only areas with high numbers of mosquitoes and a significant threat of disease transmission will be sprayed, and only a minimal amount is necessary if the spraying is done correctly.

“We’re only putting out just over one ounce per acre,” McGinnis said.

The agent used to combat adult mosquitoes will be permethrin. This chemical is found in two different products, Biomist 315 and Aquareslin, McGinnis said. He’s not certain which product the company will be using this season.

Effective spraying of either of these products depends on putting out mist droplets of the correct size, 15 microns. These droplets are just the right size to kill mosquitoes and not affect other insects, McGinnis said.

Spraying is also timed for maximum effectiveness against mosquitoes and minimal effects for beneficial insects, such as honey bees and other pollinators. It’s done only at dusk, and just before dawn, when mosquitoes are active and most other insects are not.

Also, the spray doesn’t contain enough of its toxic ingredient to affect insects that are considerably larger than mosquitoes, McGinnis said.

The company’s mosquito control program will cover 50 square miles of the county, concentrating on towns and other heavily populated areas.

Carbondale, Glenwood Springs, New Castle, Silt, Rifle and Parachute cover about 16 square miles total, Anthony said. The remaining 34 square miles of coverage will be concentrated in populated rural areas.

West Nile virus infected 2,945 Colorado residents last year, and killed 55 in the state. Anthony said the disease is known to be most virulent in the second year it affects an area.

West Nile was first seen on Colorado’s Western Slope in the summer of 2003, so 2004 could be a bad year, Anthony said. The disease first affected the plains and foothills of eastern Colorado in 2002.

Contact Jeremy Heiman: 945-8515, ext. 534

jheiman@postindependent.com


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