Mosquito and Insect Repellents

- Repellents are still the best defense -

Summary: Repellents containing the chemicals DEET or picaridin are still the best defense against mosquitoes, ticks and biting flies and the diseases they can spread. Repellents can also be combined with an insecticide applied to clothing and gear for maximum protection.

Jack DeAngelis, PhD

OSU Ext. Entomologist (ret.)

Insect repellents are still your best defense against biting pests like mosquitoes, black flies, biting midges, chiggers, ticks and the diseases they may carry. However, there is a lot of confusion about which repellents are most effective, safest and the best way to use them.

There are now two different synthetic chemical ingredients used in formulating insect repellents that are roughly equivalent in effectiveness, namely DEET and picaridin. There are also a number of natural products that claim repellency but are generally less effective. Insect repellents are compared on the basis of how long they remain effective after application and how much they reduce the frequency of bites. There's generally a direct relationship between the concentration of active ingredient and how long the repellent remains effective.

common misspelling: mosquito repelant; repellant

Yellow fever mosquito Line drawing of adult mosquitoes.

DEET

Since the 1950's most insect repellents were made from the chemical DEET (N,N'-diethyl-3-methylbenzamide) which had been developed by the military. Maximum strength repellents contain 100% DEET but some use concentrations as low as 5%. DEET is an oily liquid that can be applied to skin or clothing but it dissolves certain plastics and synthetic fabrics such as eyeglass frames and tent fabrics.

Picaridin

After 50+ years of DEET being the only really effective insect repellent a new contender has entered the arena. Picaridin, also called icaridin, is a colorless and odorless liquid that equals the effectiveness of DEET but is less irritating and does not dissolve plastics or damage synthetic fabrics.

What repellent should you use, and when?

Gardeners: should use a DEET-based or picaridin-based repellent when working in areas where chiggers or mosquitoes may be a problem. A low concentration (less than 10%) product will provide several hours of protection.

Hikers, hunters and campers: should take special precautions against ticks since they are more likely to pass through tick habitat. Treat clothing and camping gear with permethrin and exposed skin with a DEET or picaridin-based repellent. These combination treatments will protect against mosquitoes, black flies, biting midges and ticks.

Horse owners: (yes, horses need a biting fly repellent too!) treat horses with fly repellents whenever mosquito or biting fly pressure is high. Even if horses have been vaccinated against West Nile virus these repellents will reduce annoyance from mosquito bites and protect against biting flies that cause sweet itch. See What is Sweet Itch in Horses?

Using repellents safely

Like any insecticide or medication you should use the least amount and lowest concentration of insect repellent that is still effective in order to avoid potential side effects. Products are available from 5% - 100% active ingredient. For most insect repellents exceeding concentrations of 50% offers very little added benefit and may increase health risks especially for prolonged use. Products used on children should generally not exceed 10-20%.

I prefer products in non-aerosol sprays or lotions and in concentrations less than 50%. Non-aerosol spray pumps allow for more careful, targeted application and produce a larger droplet size that is less likely to be inhaled. See this Centers for Disease Control Factsheet About Mosquito Repellents for additional suggestions and precautions regarding the use of insect repellents.

Treat clothing and camping gear with permethrin

Permethrin is an insecticide that can be safely applied to clothing and gear, but not skin, to protect against mosquitoes, black flies, chiggers, ticks, and other biting pests. Permethrin has been used by the military for many years to treat fabrics and is now available to the general public. It can be used by hunters, hikers, campers and others that venture into areas where mosquitoes or black flies are a problem. The combination of permethrin-treated clothing and a DEET or picaridin-based repellent applied to skin is highly effective. Treated cloth remains effective for several weeks even when washed.

Sawyer markets a permethrin spray that can be applied to clothing, tents, backpacks and so forth.

What about citronella oil and oil of lemon eucalyptus?

Citronella is a plant oil and natural insecticide. It is widely used in "non-DEET" based repellents and in mosquito repellent candles. Citronella oil has not been tested as extensively as DEET or the new picaridin-based repellents. However, in tests that have been done citronella oil only provided protection when applied to the skin and the protection was short term compared to DEET and picaridin. Oil of lemon eucalyptus is another natural plant oil that is about as effective as low concentration DEET according to the Centers for Disease Control.

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