1. Any of several plants of the genus Chrysanthemum, especially C. cinerariifolium, grown for its flowers and seed cases, which contain natural insecticidal compounds (pyrethrins). Various synthetic insecticides—the pyrethroids—are chemically similar to pyrethrins. Examples include permethrin and cypermethrin. The pyrethrins penetrate the insect’s cuticle and are fast-acting, nontoxic to many animals (although fish are highly susceptible) and to plants, and readily biodegradable.

2. An insecticidal preparation containing natural pyrethrins.