Pyrethrin is a natural, biodegradable insecticide derived from the chrysanthemum flower. Unlike pyrethroid, its synthetically made cousin, pyrethrin is not toxic to pets; in fact, it's the active ingredient in many flea medications. Pyrethrin will kill and repel mosquitoes, but it is most effective when combined with other ingredients.
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Pyrethrin has been used in the United States for more than a century, and of all insecticides is considered the safest -- an important consideration if you're concerned about the environment. Because pyrethrin breaks down quickly in sunlight and air, it has the advantage of only killing insects present when it's applied, which helps spare beneficial insects. Pyrethrin is used in aerosol space sprays, roll-ons and sprays for personal use, and metered aerosol dispensers for outside use. It is extremely toxic to aquatic life, however, and should not be used near aquariums or fish ponds.
Pyrethrin works on mosquitoes by paralyzing their nervous systems, causing them to fall from the air, a phenomenon known as knockdown. Knockdown, however, is not always lethal. A certain number of sprayed mosquitoes will be able to detoxify the pyrethrin and recover.
Pyrethrin and PBO
To increase the lethality of pyrethrin, a substance called piperonyl butoxide, or PBO, is often added. PBO is made from sassafrass oil, and is also considered safe. Because PBO blocks a different chemical pathway than pyrethrin, the combination of these two insecticides is extremely effective. In a 1976 El Salvadoran study, mosquitoes sprayed with a mixture of pyrethrins and PBO. were knocked down at a rate of between 81 and 100 percent within an hour. The result was a 42 percent decrease in malaria in a nearby village for the next year. The control village, which was unsprayed, suffered a 75 percent increase (see Reference 1).
Pyrethrin by itself will kill and repel some mosquitos, but for maximum effect, look for products containing both pyrethrin and PBO.
Precautions
Although pyrethrin and PBO are both less toxic than many other insecticides, read labels and follow directions carefully, keep the product away from eyes, and wash repellent off with soap and water after coming inside.
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