Pyrethrins are a form or organic insecticide derived from certain species of chrysanthemums. In relatively high doses, the pyrethrins will quickly paralyze and kill many garden insect pests; if applied at low doses, however, insects are often able to recover. (Insects treated with the synthetic pyrethrins, called pyrethroids, are known to have rapidly evolved resistance to repeated applications.) Using naturally derived pyrethrins is considered safe for humans as it will break down within 48 hours, but it should not be used in areas where it may runoff into bodies of water (as it is toxic to aquatic animals). Pets should be kept out of the area.