These insecticidal compounds occur naturally in the flowers of some species of chrysanthemum plants. The toxins penetrate the insects' nervous system, quickly causing paralysis. In high-enough doses or in combination with other pesticides, the insects die. The compound breaks down rapidly when exposed to sun and air, and becomes less effective if stored for longer than one year. Many commercial products contain pyrethrins.
Powerful synthetic compounds that imitate the natural chrysanthemum compounds are called pyrethroids. Pyrethroids are not approved for use in organic farms and gardens. Also avoid any pyrethrins that list piperonyl butoxoid on the label. This additive is not approved for organic use.
Although relatively harmless to humans, pyrethrins are highly toxic to fish and bees and moderately toxic to birds. They kill both beneficial and pest insects. To keep bees safe, spray pyrethrins in the evening after bees have returned to their hives for the night, and avoid spraying blooming plants.
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