Manage mosquito-breeding areas to help control disease, pesky biting insects

By Vicky Boyd

As the West Nile virus marches across the United States, insect experts say preventing or eliminating habitat where disease-carrying mosquitoes could breed is one of the best weapons in the battle.

Many mosquito species are closely associated with standing water, such as sumps and drainage ditches, where they lay their eggs. But another group of mosquitoes is content with laying eggs on damp ground that is intermittently flooded--like that of flood-irrigated orchards and vineyards.

Regardless of the situation, taking a few steps to eliminate those egg-laying sites will go a long way in reducing disease potential, and probably more importantly, in reducing the number of irritating biting pests that plague workers.

"Most tree fruit operations are not a problem at all," says David Farley, manager of the Fresno Mosquito and Vector Control District in Fresno, Calif. "But we have a few that are, and the difference is water management."

Watch your water
Mosquitoes in the Culex genus are the biggest culprits in spreading West Nile virus. They require water standing for at least five days to complete their life cycle.

Among the man-made structures that provide that habitat are old tires, buckets, animal troughs or other containers; sumps or tailwater collection ponds where the water isn?t circulated; and ruts or other areas with poor drainage. Draining the containers, recirculating the water or filling in holes reduces mosquito egg-laying sites.

Although Culex are typically not an orchard pest, Farley has seen problems arise after harvest when old fruit drops into water-filled ruts and creates a nutrient-rich environment.

"Any time you have water standing for a long time, you have the potential to breed mosquitoes," he says.

Mosquitoes in the genus Ochlerotatus, formerly called Aedes, are more frequently found in orchards, especially those under reduced tillage with grassy or weedy middles. They lay their eggs on damp ground and require about four days to complete their life cycle.

Like other mosquitoes, Ochlerotatus larvae require standing water, such as in flood-irrigated orchards.

Water management is critical to ensure the trees receive enough yet excess doesn't accumulate in low spots or grassy middles.

Cultivating the ground and not allowing water to stand in ruts or low spots for more than five days also helps reduce mosquito-breeding habitat, Farley says.

Ochlerotatus are often referred to as pasture mosquitoes, since they are frequently found in the moist, grassy areas, and harass grazing livestock. In orchards, they don't have the livestock on which to feed, so they go after other warm-blooded creatures, namely workers.

Although entomologists have found this genus to carry the West Nile virus, they are not the primary vector.

Zap larvae before they mature
Many of the fruit and vegetable operations in the Southeast are on higher piedmont or well-drained sandy soils. So standing water is not as much of a concern as in flood-irrigated fields, says Elmer Gray, University of Georgia Extension public health specialist and a member of the Georgia West Nile Virus Task Force.

"Typically, it's what goes on just past the field, if there are drainage ditches around the field, that would be a concern," Gray says. "Any time you have standing water, no matter how it is produced, there's a potential for mosquito breeding."

Gray says several products that target mosquito larvae are available that are not restricted use. Among the larvicides are biologicals, such as Bacillus thuringiensis (Bt) var. israelensis; insect growth regulators, such as methoprene; and Agnique, a monomolecular film that spreads over the water surface, preventing larvae from attaching.

For more information on mosquito larvicides registered in your area, contact the local mosquito district or county Extension office.

What is West Nile virus?
First identified in Africa in 1937, the sometimes-deadly disease first appeared in the United States in New York City in 1999. It has since moved westward and has been found in nearly every state, according to the Centers for Disease Control and Prevention.

So far, West Nile virus has infected more than 10,000 humans and killed more than 268, according to the CDC.

Not every mosquito carries the virus. Even if you're bitten by a virus-carrying mosquito, chances are you'll never show any West Nile symptoms. In fact, 80 percent of all those infected show no symptoms at all.

About 20 percent of people infected show mild flu-like symptoms. Only 1 in 150 people infected with the virus will ever show severe symptoms. People over age 50 are the most susceptible, and the majority of deaths so far have fallen into this age class.

The disease is particularly hard on horses, killing about 30 percent of animals that are infected.

West Nile is caused by a virus closely related to other organisms that cause St. Louis encephalitis, Eastern equine encephalitis and Western equine encephalitis. The diseases involve swelling of the brain.

Mosquitoes pick up the virus after feeding on West-Nile-infected birds. When they bite a human or other mammal, they transmit the virus.

The disease typically cannot be spread directly from human to human, except when somebody receives blood or a transplant organ from an individual who has West Nile virus. The most common transmission route involves an avian or bird reservoir as an intermediary between mosquitoes and humans.

That may be one reason why Culex mosquitoes are the primary vector?they are what entomologists call "bird biters."

Ochlerotatus, on the other hand, are "big animal biters," meaning they feed predominately on cows, horses and humans.

If you have questions about West Nile virus, contact your local mosquito district or county public health department.

8 weapons in the war against 'skeeters'

Washington State University Cooperative Extension provides these tips to help eliminate mosquito breeding places around the farm:

  1. Check irrigation and drainage ditches for leaks or seepage and maintain free flow of water.
  2. Grade newly developed land to prevent standing water.
  3. Provide drainage away from premises for excess irrigation water, or collect in storage sump and reuse on land.
  4. Manage weeds in areas where adult mosquitoes congregate, such as small man-made bodies of water, water-retention ponds, lagoons or water reservoirs.
  5. Regularly remove floating debris from ponds to reduce egg-laying sites.
  6. Promote drainage of old tires by drilling holes.
  7. Pay particular attention to aids, such as sheets of discarded plastic mulch. Since these catch water easily, they can provide an excellent, almost permanent, breeding site.
  8. Fill tree holes or develop drainage so water cannot accumulate.

Internet Hotlinks

Centers for Disease Control and Prevention West Nile virus page: