Understanding insect repellents

mosquito

 

Insect repellents are chemicals that protect us from blood-feeding arthropods:  primarily mosquitoes, ticks, black flies, and biting gnats. Using a repellent doesn’t make us invisible to pests. They will still be attracted to our warmth, movement, and exhaled CO2. Repellents work at the skin level to disrupt the feeding process just before they bite.

You can search for repellents on this EPA web page: http://cfpub.epa.gov/oppref/insect/#searchform. You pick the desired protection time and types of pests that will be encountered and get a list of registered repellents. Always read the label carefully before buying and using repellents. It will state precautions and any age restrictions. If no age limit is listed, then the EPA has not required a restriction.

Knowing something about the major types of repellents and how to get the most out of them can make time outdoors a lot more enjoyable and may reduce health risks. Here are some points to consider:

1) Likely pests. Some repellents are more effective against certain blood feeders than are others. Knowing what you will be facing will help in the selection process. For example, some repellents may be more effective against mosquitoes than ticks.

2) Potential disease threat. No one wants to be bitten but bites become much more important if you can be exposed to carriers of Rocky Mtn. Spotted fever; Lyme disease; or encephalitis, such as West Nile virus. Repellents are particularly important if there is a disease threat but there are other things that you can do, such as frequent self-examination for ticks.

3) Exposure period and pest intensity. Activity of biting pests varies with location and time of day. More frequent applications or use of products with greater concentrations of the repellent chemical are needed when you are outdoors for long periods of time or where biting arthropods are very troublesome. Look for pests and protection times on product labels.

4) Exposure conditions and your “meal appeal”.  Use of a repellent with a higher concentration, or more frequent applications will be necessary if you are active and perspiring freely or periodically getting wet.  People vary in attractiveness to biting arthropods, some are attacked ferociously while companions are not bothered. “Meal appeal” is related to the amount of attractants, like lactic acid. and other chemicals produced by individuals. More frequent repellent applications may be needed to protect people that attract a lot of biters.

5) Active ingredients in the product and personal preference – biopesticide or conventional. DEET, Picaridin, IR3535, and oil of lemon eucalyptus are recommended for use by the Centers for Disease Control and Prevention (CDC) based upon their endurance and effectiveness against biting arthropods.

Here is a quick review of some of the active ingredients in common repellents; they are divided into biopesticide repellents and conventional repellents.

Biopesticide repellents

These essential oils are concentrated volatile/aromatic compounds extracted from plants.  The most common materials are:

Oil of lemon eucalyptus (PMD) is a plant-derived repellent that is effective against mosquitoes, black flies, biting midges, gnats, and ticks.  It is as effective as DEET but provides shorter term protection. It is an eye irritant and should not be used on children less than 3 years old.

Citronella comes from lemongrass and related plants. It is available in candles, torches, and some topical repellents and provides limited protection against mosquitoes.  Citronella is not effective against ticks and other biting pests.

Other essential oils are from catnip, geranium, peppermint, and soybean. They provide limited protection against mosquitoes and are not effective against other biting flies, gnats, and ticks.

IR3535, based on a naturally occurring amino acid, is moderately effective against mosquitoes, other biting flies, and ticks.

Conventional repellents

DEET is the standard by which repellents are judged. It provides very effective protection against mosquitoes and works well against other biting flies, fleas, and ticks. DEET is available in a wide range of concentrations. In general, the higher concentrations provide longer protection. However, use of concentrations greater than 30% may cause skin irritation and does not provide increased protection. According to the American Academy of Pediatrics, DEET may be applied by an adult to children over the age of 2 months. DEET can dissolve some synthetic fabrics and plastics.

Picaridin is derived from pepper. It can be as effective as DEET against mosquitoes, biting gnats, fleas, and ticks. In contrast to DEET, it is relatively odorless, not oily, and does not damage synthetic fabrics and plastics.

Non-skin application repellents

Permethrin is an insecticide with repellent activity against mosquitoes, other biting flies, fleas, chiggers, and ticks. It not to be applied to the skin; products are available for application to boots/shoes, clothing, tents, mosquito netting, etc.

Metofluthrin, similar to permethrin, is available in impregnated repellent strips that can be hung around activity spots or in battery powered devices worn by people.  Metofluthrin vapors from the strips or devices repel mosquitoes.

Other ways to reduce bites – Cover exposed skin when outside and avoid being outside when mosquitoes are most active.

Repellent is a general term but individual products may work in very different ways. We don’t understand a lot about how repellents work but we can tell if they are effective or not.

Sources:

Insect repellents PM Fact Sheet 5105 http://umaine.edu/ipm/ipddl/publications/5108e/

Insect repellents: use and effectiveness http://cfpub.epa.gov/oppref/insect/#searchform

Updated information regarding insect repellents  http://www.cdc.gov/ncidod/dvbid/westnile/repellentupdates.htm

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