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It’s important for the District to know whether the adult mosquito control treatments we conduct have the intended effect: an immediate reduction in the number of WNV-infected adult mosquitoes in an area.

To ensure that our adult mosquito control treatment was effective, we compare the number of mosquitoes trapped in the immediate area prior to treatment to the number of mosquitoes trapped in the same area after the treatment. This is reported as an overall reduction in the mosquito population.

We also test the effectiveness of the product used for the adult mosquito control treatment by placing cages of laboratory-raised mosquitoes in the area to be treated. After the treatment, we pick up the cages and count the number of mosquitoes surviving. This tells us if the product works well on the mosquitoes in that particular area.

After the treatment, we test mosquitoes trapped in the area to ensure that no WNV-infected adult mosquitoes remain. This ensures that the immediate human health risk has been eliminated, and it is safe to return to conducing preventative treatments for mosquito larvae.




When the District treats a neighborhood to control adult mosquitoes carrying West Nile Virus, caged mosquitoes are placed inside and outside of the fogging zone in the evening prior to the fogging.  These cages are filled with 20-25 female Culex pipiens mosquitoes that have been reared from the District mosquito colony. 

The colony mosquitoes are known to be susceptible to pyrethroid pesticides.  As the truck-mounted ULV fogger passes by, the mosquitoes in the cages should die.  They are collected at dawn, and the numbers of dead mosquitoes are counted. 

All or almost all dead mosquitoes indicate that the pesticide material was effective.  The cages placed outside the fogging area serve as a control, to make sure that something else, such as a weather anomaly or the procedure to insert mosquitoes in the cages, was not the cause of the mosquito mortality. 

The results from these cages are reported as “Efficacy” in the table.  Percent reduction is the amount of decline of adult mosquitoes in carbon dioxide traps set in the area before and after the fogging operation.

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