Insecticide resistance in mosquitoes being studied

11/10/04

HOUSTON An outbreak of St. Louis or West Nile encephalitis is hardly the time for mosquito control officials to find out their pesticides aren't working. Avoiding that problem is the focus of a cooperative project undertaken this fall.

Texas Agricultural Experiment Station, Texas Cooperative Extension and the Mosquito Control Division of the Harris County Public Health and Environmental Services Department are working together to study insecticide resistance in mosquitoes and develop a strategy to overcome it.

Larvae of Culex quinquefasciatus, or the southern house mosquito, are being collected in the Houston area and raised in laboratories at Texas A&M University. Adult mosquitoes are then tested to see whether genetic mutations have made them resistant to the pyrethroid pesticides used in the district, said Dr. Patricia Pietrantonio, Experiment Station entomologist.

The southern house mosquito is the primary vector for viruses causing St. Louis encephalitis and West Nile encephalitis in Texas urban areas.

"For diseases like St. Louis encephalitis and West Nile encephalitis, the only control is mosquito control," said Dr. Jim Olson, Experiment Station entomologist. "There are no vaccines. You can only treat the symptoms in the hospital."

These two diseases may be fatal to humans, with West Nile being fatal to other mammals as well as birds.

Fragments of mosquito genes will be cloned and sequenced to determine what mutations, if any, have occurred, Pietrantonio said.

"If mutations have taken place," she said, "the insect will no longer die" when treated with insecticides.

The project is similar to one she completed in the Houston area in 1998 involving the organophosphate insecticide, Malathion. Pietrantonio found in some areas the southern house mosquito was resistant to the insecticide Malathion being used. The district switched to pyrethroid pesticides to control the pests.

In 2003, the Mosquito Control Division in Harris County sprayed 2 million acres with pyrethroid insecticides in an effort to control disease-bearing mosquitoes, said Ray Parsons, division director.

"History has shown that overuse of pesticides will lead to resistance in insects," Olson said. "Resistance can be prevented or managed if we have the knowledge about when and/or where to use and not to use them. The key here is management to maintain susceptibility."

The project was started with a federal U.S. Department of Agriculture Cooperative State Research, Education and Extension Service grant, Olson said.

"This is serving as a model for similar problems that could arise anywhere in the state of Texas or the United States," he said.

"We have like concerns, particularly for the Culex quinquefasiciastus in other areas of the Gulf Coast of Texas all the way into the metropolitan areas of the state such as Dallas where there is extensive spraying for mosquitoes going on."

"We have to develop new methods of controlling mosquitoes," Parsons said. "We know we can control them with insecticides, but it's very expensive and it only works to a certain degree. It's going to take people going out into the field and learning more about the mosquitoes: the biology of the mosquito and how to control it."

The resistance project is part of a larger program investigating the frequency of mosquito-borne diseases and other possible control methods for mosquitoes, he said.

"In many parts of the state, we don't have a clue as to what level of insecticide resistance exists," Olson said. "It's a bad time to find out you've got it in the middle of a disease outbreak. It is better you take care of it well in advance."

Source: Eurekalert & others

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