Pyrethrins are a type of insecticide derived from the Chrysanthemum plant. They kill insects by attacking their nervous system. According to insecticide labels, Pyrethrins are safe for use on plants in the food supply even up to the day of harvest. They are also used to treat lice on humans and fleas on cats and dogs. Pyrethrins are very harmful to fish. Their effect on mammals is unnoticeable in low doses, but they can cause respiratory problems or central nervous system disorders in extremely high doses.