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Insecticide Resistance

	Person in lab holding a glass bottle.

The use of insecticides to kill mosquitoes that spread Zika, dengue, and chikungunya viruses, is one part of an integrated mosquito management program. Insecticides may be used by professionals and by homeowners. Insecticides can be applied by hand (indoor and outdoor sprays and foggers), by truck, or by aerial (airplane) spraying.

Over time and repeated use, insecticide resistance can occur in mosquito populations. Insecticide resistance is an overall reduction in the ability of an insecticide product to kill mosquitoes. This means that, when used as directed, the product no longer works, or only partially works. Insecticide resistance can be product specific, or it can develop to a certain class(es) of product.

In order to delay or prevent the development of insecticide resistance in vector populations, integrated vector management programs should include a resistance management component (Florida Coordinating Council on Mosquito Control 1998). Ideally, this should include annual monitoring of the status of resistance in target populations to:

  • Provide baseline data for program planning and pesticide selection before the start of control operations.
  • Detect resistance at an early stage so that timely management can be implemented.
  • Continuously monitor the effect of control strategies on insecticide resistance.

How Insecticide Resistance is Measured

Monitoring for resistance in the vector population is essential and is useful in determining the potential causes for control failures, should they occur. CDC has developed an assay to determine if a particular insecticide formulation (combination of the active ingredient in the insecticide and inactive ingredients) is able to kill mosquito vectors. The technique, referred to as the CDC bottle bioassay, is simple, rapid, and economical compared with alternatives. The results can help guide the choice of insecticide used for spraying.

How the Bottle Bioassay Works

  • A bottle is coated with a known amount of insecticide. Mosquitoes are then put into the bottle and observed until for a pre-determined period of time (diagnostic time).
  • Resistance is determined by the percentage of mosquitoes that die (mortality rate) in the diagnostic time.

Insecticide Resistance Testing in Puerto Rico

Beginning in February of 2016, CDC Entomologists located at the Dengue Branch carried out Bottle Bioassays tests to determine the presence of insecticide resistance of Aedes aegypti to the most commonly used EPA-approved insecticides for mosquito control. CDC entomologists collected mosquitoes from many locations throughout Puerto Rico and brought them to the laboratory. Bioassay testing was performed in the laboratory with laboratory reagent grade chemicals and all organisms used were discarded in the laboratory waste at the Dengue Branch.

Results from testing show that the mosquitoes collected from 38 locations in 23 of the 78 municipalities in Puerto Rico are either resistant or partially resistant to many insecticides tested. Resistance has been observed in permethrin and malathion, two commonly used insecticides in Puerto Rico.

Results are presented below as maps showing whether Aedes aegypti were susceptible, partially resistant, or resistant to specific insecticides from the municipalities shown.


	•	Map of insecticide resistance to Alpha-Cypermethrin  in Puerto Rico.


	Map of insecticide resistance to Bendiocarb in Puerto Rico.


	•	Map of insecticide resistance to Bifenthrin in Puerto Rico.


	•	Map of insecticide resistance to Deltamethrin in Puerto Rico.


	•	Map of insecticide resistance to Etofenprox in Puerto Rico.


	•	Map of insecticide resistance to Lambda-cyhalothrin in Puerto Rico.


	•	Map of insecticide resistance to Malathion in Puerto Rico.


	•	Map of insecticide resistance to Naled in Puerto Rico.


	•	Map of insecticide resistance to Permethrin in Puerto Rico.


	•	Map of insecticide resistance to Phenothrin in Puerto Rico.


	Map of Puerto Rico showing municipalities in Puerto Rico where mosquitoes are resistant to tetramethrin.  Those municipalities are: Carolina, Canovanas, Humacao, and Humacao