Stephanie L. Richards, PhD, Medical Entomologist
WHEN ARE INSECTICIDES USED IN MOSQUITO CONTROL
An integrated mosquito management approach uses a variety of methods such as reducing/removing egg laying sites, biological control (e.g. mosquito fish), larvicides, and/or adulticides to control mosquito populations. Surveillance-based targeted mosquito control methods should be used to protect public health.
WHAT IS INSECTICIDE RESISTANCE?
Since mosquitoes have such short life cycles (approximately 7-8 days from egg to adult, depending on environmental conditions), it is possible for them to adapt to environmental pressures over time. There are many types of insecticides that are commonly used to control adult mosquitoes. The active ingredients in different types of insecticides act in different ways (i.e. different modes of action) to incapacitate mosquitoes.
After mosquitoes in a population are exposed to doses of insecticides that are too weak to kill them, mosquitoes may develop resistance to the active ingredient(s). This makes the use of surveillance-based targeted mosquito control of the utmost importance.
"RESISTANCE” CAN MEAN DIFFERENT THINGS:
- Behavioral changes where mosquitoes may avoid insecticide-treated areas.
- Changes in the metabolism of the mosquito that help it survive insecticide exposure.
- Genetic changes or adaptations that prevent the insecticide from acting on its target within a mosquito.
- Decreased penetration of insecticides into the mosquito body.
- An increase in the rate at which the mosquito excretes the insecticide.
The World Health Organization classifies a mosquito population as resistant when less than 80% of mosquitoes exposed to an insecticide die after a certain period of time. That period of time is called the diagnostic time. If an insecticide kills 98% to 100% of mosquitoes exposed at the diagnostic time, those mosquitoes are classified as susceptible to the insecticide.
HOW IS INSECTICIDE RESISTANCE TESTED?
Mosquito control programs should check for insecticide resistance/susceptibility periodically (e.g. beginning, middle, and end) during the mosquito season to ensure that the most effective control measures are being used.
The Centers for Disease Control and Prevention (CDC) Bottle Bioassay is commonly used to monitor insecticide resistance for adulticides. Before beginning this procedure, bottles should be calibrated using known susceptible mosquito populations and/or for each regional mosquito population by determining the appropriate diagnostic dose and time for each insecticide active ingredient.
Briefly, glass bottles are coated with the appropriate dose of insecticide solutions mixed with acetone or ethanol. Control bottles are also prepared that contain only acetone or ethanol (no insecticide). Bottles are allowed to dry before use and the acetone or ethanol evaporate, leaving only insecticide residue in the bottles. Live mosquitoes are introduced into the bottles using an aspirator and their mortality is checked at predetermined time points over a two hour period of time.
The mortality rate at the diagnostic time is recorded and the degree of resistance/susceptibility is noted. This bioassay can be repeated over time with mosquito populations from the same geographic area to determine if any changes in susceptibility/resistance occur over time.
The World Health Organization Pesticide Evaluation Scheme (WHOPES) is another method for evaluation of insecticide resistance in adulticides.
SHOULD THE PUBLIC BE CONCERNED ABOUT INSECTICIDE RESISTANCE?
With regular monitoring of insecticide resistance, these issues can be managed to some degree and the most effective control measures can be used.
Evaluation of control measures should be a component of a mosquito control program. However, not all control programs have the expertise and/or funding to do resistance monitoring, hence populations may go unchecked. In order to maximize mosquito control, programs should periodically rotate their insecticide use based on data gained from resistance monitoring.
Since different insecticide active ingredients may use different modes of action to kill mosquitoes, new insecticides are continually being developed with novel modes of action. Manufacturers such as chemical companies develop insecticide active ingredients and insecticide formulations and these products must be evaluated by and registered with the Environmental Protection Agency.Dr. Stephanie L. Richards is an Associate Professor of Health Education and Promotion in the Environmental Health Sciences program at East Carolina University. She received a B.S. in Biology and M.S. in Environmental Health from East Carolina University. She completed her Ph.D. in Entomology with a minor in geographic information science at North Carolina State University. She completed her post-doctoral work in Arbovirology at University of Florida, Florida Medical Entomology Laboratory.