Pyrethroids are synthetic chemical insecticides that act in a similar manner to pyrethrins, which are derived from chrysanthemum flowers. Pyrethroids are widely used for controlling various insects. Permethrin, resmethrin, and d-phenothrin (Sumithrin®) are synthetic pyrethroids commonly used in mosquito control programs to kill adult mosquitoes.

• Permethrin has been registered by the EPA since 1979. It is currently registered and sold in a number of products such as residential indoor and outdoor insect foggers and sprays, treated clothing, flea products for dogs, termite treatments, agricultural and livestock products, and mosquito abatement products. It is also regulated by the Food and Drug Administration as a treatment of head lice and scabies. Permethrin is the most widely used mosquito adulticide in the U.S. and is used to treat 9 to 10 million acres annually (out of 32-39 million acres treated with a mosquito adulticide). Permethrin’s widespread use can be attributed to its low cost, high effectiveness, low incidence of pest resistance, and broad labeling.

• Resmethrin has been registered by the EPA since 1967. It is currently registered for use only in public health and vector control programs to control adult mosquitos, biting and non-biting midges and blackflies. Because of its toxicity to fish, resmethrin is a Restricted Use Pesticide that is available for use only by certified pesticide applicators or persons under their direct supervision. The resmethrin registrants voluntarily canceled all of the resmethrin products. After December 31, 2015, resmethrin registrants will no longer be able to sell and distribute resmethrin products. However, user will be able to continue using any product they have purchased.

• D-phenothrin (Sumithrin®) has been registered by the EPA since 1976 for use to control adult mosquitos and other nuisance insects indoors and outdoors in residential yards and public recreational areas. Use sites include in and around residential/domestic dwellings, commercial and industrial buildings, transportation vehicles, recreation areas, animal quarters, direct animal treatment (dogs). While there are no direct applications to food crops, d-phenothrin labels allow for applications to control mosquitoes over agricultural as well as non-agricultural areas.

How are Synthetic Pyrethroids Used in Adult Mosquito Control?

Most pyrethroid mosquito control products can be applied only by public health officials and trained personnel of mosquito control districts. Mosquito control professionals apply pyrethroids as an ultra low volume (ULV) spray. ULV sprayers dispense very fine aerosol droplets that stay aloft and kill adult mosquitoes on contact. Pyrethroids used in mosquito control are typically mixed with a synergist compound, such as piperonyl butoxide, which enhances the effectiveness of the active ingredient. The product is often diluted in water or oil and applied at rates less than 1/100th of a pound of active ingredient or less than 4 fluid ounces of mixed formulation per acre.

Do Pyrethroids Pose Risks to Human Health?

We have conducted human health risk assessments for all labeled uses of pyrethroids. Based on the results of these assessments and any required label changes, pyrethroids can be used for public health mosquito control programs without posing unreasonable risks to human health when applied according to the label. At high exposure levels, such as those resulting from accidents or spills, pyrethroids can affect the nervous system.

Do Pyrethroids Pose Risks to Wildlife or the Environment?

When applied according to label directions, pyrethroids used in mosquito control programs do not pose unreasonable risks to wildlife or the environment. Pyrethroids are low in toxicity to mammals and are practically nontoxic to birds. However, pyrethroids are toxic to fish and to bees. Products for uses other than mosquito control may be subject to buffer zones to protect water bodies. There also is language on product labels to reduce risks to pollinators. Always read the product label and follow its directions carefully when using any pesticide.

What is The Current Regulatory Status of Pyrethroids?

We are currently reevaluating all pyrethrins, pyrethroids and synergists through registration review. Registration review is our program for systematically reviewing all registered pesticides every 15 years to make sure that every pesticide can still perform its intended function without unreasonable adverse effects on human health or the environment.

As a result of the Food Quality Protection Act, EPA must consider the cumulative risks of pesticides that, like the pyrethroids and pyrethrins, share a common mechanism of toxicity. In November 2011, we completed a cumulative risk assessment for the pyrethroids/pyrethrins and identified no cumulative risks of concern. This assessment is available from Regulations.gov, docket EPA-HQ-OPP-2011-0746.

For chemical-specific updates on registration review, see our pyrethrins and pyrethroids web page.