JUNE
1994

 

 

 

By
Tom Turpin
 
Professor of
Entomology
Purdue University

 

 

 

 

 

06-09-94

Herbal Insect Repellents: Plants Battle Insects

One of the ongoing battles in nature is a real life-and-death affair. It is the confrontation between plants and plant-eating animals. Many kinds of animals eat plants, including one-fourth of all known insect species.

Ending up in some animal's stomach is not one of the best things that can happen to a plant. Some plants try to avoid such a fate by producing toxic chemicals. Since many insects are attracted to plants by odors, some plants avoid becoming a meal by covering up their attractive odors with chemicals that are repellent to insects.

We humans have used this approach ourselves. For instance, in medieval times folks would throw herbs on the floor to repel all sorts of vermin, including insects. The herbs also acted to freshen the air between housecleanings.

Recently, we have seen a renewed interest in using naturally occurring chemicals to help avoid insect problems. Many of the currently used natural repellents are of ancient origin. For instance, the plant fleabane is so named because ancient people believed that the plant would ward off flea infestations. How exactly one should use the plant to eliminate fleas is somewhat in question. The book “A Modern Herbal” states that it should be burned, and the smoke will drive off fleas and other insects. However, other sources state that it is the juice of fleabane that is destructive to fleas.

Several species of the cedar tree have been shown to possess chemicals that are repellent to insects. For this reason, the wood of cedar trees was used to make cedar chests for storing clothing susceptible to destruction by clothes moths. The chemical odors produced by the wood provide some protection against these household pests.

Mosquitoes are frequently the targets of our efforts to repel insects. One common plant chemical used against mosquitoes comes from citronella, a fragrant grass of southern Asia. Oil of citronella is incorporated into candles that are burned in hopes of keeping mosquitoes from extracting some of our blood during our backyard parties. In general, burning citronella candles doesn't help with the mosquito problem, even though it does perfume the air. A better approach would be to rub some citronella oil on you. However, be prepared to replenish the supply often, as the oil evaporates rapidly.

Planting citronella-producing plants, sometimes called mosquito plants, is another ploy for avoiding mosquito bites. While some of these plants are nice decorator plants, their presence will not keep mosquitoes away. For the plants to repel mosquitoes, it is necessary to release the oil of citronella from the leaves. This happens when leaf tissue is damaged. Beating the leaves with a stick will cause citronella to be released, however it doesn't do much for the looks of the plant.

Such physical activity on the part of people does increase production of carbon dioxide, a chemical which attracts mosquitoes. So if mosquito plants provide some protection to your patio guests the reason just may be that you worked up enough of a sweat damaging the leaves of the plant, that the mosquitoes were attracted to you and not your guests!

 

Writer: Tom Turpin
Editor: Andrea McCann