Pyrethrum is made from the dried flowers of a little white daisy classified as “Tanacetum cinerariifolium.”
Photo Courtesy EverGreen
This article is part of our Organic Pest Control Series, which includes articles on attracting beneficial insects, controlling specific garden pests, and using organic pesticides.
What Is Pyrethrum?
One of the oldest pesticides known, pyrethrum is also the strongest insecticide allowed under National Organic Standards guidelines. Made from the dried flowers of a little white daisy now classified as Tanacetum cinerariifolium, pyrethrum insecticides are known for their fast knock-down of unwanted insects. Insects typically become paralyzed as soon as they come into contact with pyrethrum, so it’s often used in wasp sprays. Pyrethrum use in the garden should be undertaken with care and only after cultural methods that might manage a pest have been exhausted. Pyrethrum insecticides are highly toxic to bees, wasps and other beneficial insects, as well as to fish.
Which Pests Does Pyrethrum Control?
Aphids, armyworms, cucumber beetles, cutworms, squash bugs, whiteflies, leafhoppers, thrips and Colorado potato beetles are often brought under control with pyrethrum. Pests that cannot be reached with the spray — for example, corn earworms or leaf miners — should not be treated with pyrethrum products. Additionally, very challenging pests such as cucumber beetles and squash bugs are best managed by excluding them with row covers, with pyrethrum used as a late-season remedy should pests get out of control.
How to Use Pyrethrum
Pyrethrum degrades quickly in sunlight, but precautions still should be taken to protect beneficial insects from exposure. When using pyrethrum to control insects that take flight, such as cucumber beetles, apply pyrethrum early in the morning and then cover the treated plants with row cover or an old sheet to exclude bees and other beneficials for 24 hours. To put a damper on squash bug populations, spray plants as soon as the first nymphs are seen, and again one week later.
Do not use pyrethrum in situations where lady beetles, honeybees and other beneficials are active. Used carelessly, pyrethrum can wipe out these and other beneficial insects.
Home production of pyrethrum pesticides is practical for the resourceful homesteader. Native to current-day Yugoslavia, the Dalmation daisy is a cousin of feverfew, which it closely resembles. Hardy to Zone 6, the plants grow as short-lived perennials and often reseed in hospitable spots. If the dried flowers are soaked in warm water for three hours, the resulting spray is highly toxic to insects for about 12 hours. People who are allergic to other members of the Aster family may react badly to this daisy’s pollen.
Purchased pyrethrum products require less handling and therefore may be safer to use. Look for the OMRI label when choosing a pyrethrum insecticide, because non-listed products often contain piperonyl butoxide, which is considered a possible human carcinogen. Organic pyrethrum products often contain oils or soaps to enhance their effectiveness.
How to Store Pyrethrum
Mix only as much concentrate or infusion as you will need. If not used within one day, place the container in the sun for a few hours and dispose of unused solution by pouring it out in the sun. Sunlight rapidly degrades pyrethrum, and the half-life of pyrethrum in soil is only one to two hours. Store pyrethrum products in their original containers on a high shelf, out of the reach of children and pets, in a dark place at cool room temperatures. Under good storage conditions, the shelf life of pyrethrum is about 1 year. Dried pyrethrum daisies can be stored in the freezer in an airtight container for at least six months.
More information on pyrethrum is available from Cornell University.