JUNE
2008

 

By
Tom Turpin
 
Professor of
Entomology
Purdue University

 

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06-12-08

Download the audio of On Six Legs: MP3, WMV.

Repellent or Not a Repellent, That is the Question!

For eons we humans have struggled with insects bent on taking a bite out of our possessions or us.  One approach to this dilemma has been to try to kill the offending insects.  For that purpose we use devices such as fly swatters and bug lights.   We also use chemicals called insecticides that are designed to kill insects.

Another approach to dealing with damaging insects is to keep the culprits away from intended targets.  Window screens and mosquito nets are physical barriers that function in this way.  Chemical barriers can also be used for this purpose. These are known as repellents.

Repellents are widely used to keep blood-sucking arthropods such as mosquitoes and ticks from biting humans.  These chemicals function by causing the arthropod to leave without biting, which is what we humans want to happen.

Blood sucking arthropods depend on a complex series of chemical and physical cues to hone in on a meal.  Each of us has unique body chemistry.   That means that certain individuals are more attractive to some insects, like a biting mosquito for example, than other people.  In addition humans vary considerably in symptoms expressed as a result of an insect bite.  Some people will hardly notice a mosquito bite while in other folks the bite will leave a welt and a big, red itchy spot for several days.

Sensitivity to bites is not the most important thing relative to insects and ticks.  The fact is that these arthropods can transmit serious human diseases.  That means that all of us should minimize providing blood meals to arthropods.  We can do that by avoiding areas harboring the undesirable arthropods or by using repellents to discourage biting.

Insect repellents can be categorized into one of two types, plant derived or synthetic chemical.  Some plants have chemicals in tissues that insects don’t like.  Such chemicals help plants avoid becoming a meal for a hungry insect.  The well-known citronella is such a chemical. 

Modern chemistry techniques have resulted in the creation of thousands of chemicals.  Over the years 40,000 of these synthetic chemicals have been tested for ability to repel insects.  A few, like DEET, do.

So how do scientists go about determining if a chemical has insect-repellent properties?  By running a test with the target insects, that’s how.  For instance the potential for a chemical to be a mosquito repellent would be determined by an arm-in-cage study.  The study is just as it sounds.  The potential repellent is applied to a bare arm.  The arm is then extended into a cage infested with hungry female mosquitoes.  Scientists then note whether or not mosquitoes land and feed on the research subject’s arm. 

If the chemical repels the mosquitoes at first exposure a time study is implemented.  The treated arm is reintroduced to the cage of mosquitoes at regular time intervals until the mosquitoes will begin to land and take a blood meal.  That way it is possible to estimate the effective time a repellent would provide protection.  Of course it is important that the mosquitoes used in such a study are not carrying disease organisms!

Of all the synthetic chemicals tested over the years only two, DEET and Picaridin, have been shown to be effective enough to be included in commercial products.  DEET has been the primary ingredient in mosquito repellents for over 40 years.

Several plant-derived chemicals such as citronella have been used as mosquito repellents.  While these natural products do repel mosquitoes they generally need to be applied more frequently than something like DEET.

Urban myths abound about things that work to repel mosquitoes.  In spite of passionate testimonials about stuff that works research has shown that the following do not work.  Eating garlic or onions. Citronella candles.  Taking certain vitamins.  Ultrasonic devices.  Wristbands impregnated with repellents.  Skin moisturizers. Mosquito plants on the deck.  And any product based on essential plant oils that “ puts out a high vibration to keep insects away long after you stop smelling it” and also sooths dry skin, hot spots, and eczema.

When it comes to mosquito repellents, there is a sucker born every second. And it is not a female mosquito in search of a blood meal!

 

Writer: Tom Turpin
Editor: Olivia Maddox