The dried, powdered flowers of the pyrethrum daisy, Tanacetum cinerarifolium, were used as early as 1880 to control mosquitoes. The popularity of pyrethrum insecticides waned when synthetic insecticides were introduced, but they are now enjoying a commercial comeback. Many new products formulated with natural pyrethrums are available.
Pyrethrins are the insecticidal chemicals extracted from the pyrethrum daisy. Do not confuse them with pyrethroids, the term for a new class of synthetic pesticides. Pyrethrums, which are mainly concentrated in the seeds of the flower head, are a contact insecticide, meaning the insect only has to touch the substance to be affected.
Pyrethrins have a quick knockdown effect on insects: Flying insects are paralyzed. pyrethrins can be applied up to one day before harvest because they are quickly destroyed by light and heat and are not persistent in the environment. Pyrthrins will kill lady beetes but do not appear to be harmful to bees. They are toxic to fish and to the aqautic insects and other small animals that fish eat. Pyrethrins do not seem to be toxic to birds or mammals.
Protection Offered: Pyrethrins are registered for flowers, fruits, and vegetables, including greenhouse crops. they are effective on many chewing and sucking insects, including most aphids, cabbage loopers, celery leaftiers, codling moth, Colarado potaotoe beetles, leafhoppers, Mexican bean beetles, spider mites, stink bugs, several species of thrips, tomato pinworms, and whiteflies. they are especially good against flies, gnats, mosquitoes, and stored products pests. Flea beetles are not affected, nor are imported cabbageworms, diamondback moths, pear psylla, and tarnished plant bugs.
How to Make: If you grow your own pyrethrum daisies, you’ll have the main ingredient for a make-it-yourself spray. The concentration of pyrethrums is at its peak when the flowers are in full bloom, from the time the first row of florets open on the central disk opens too the time all the florets are open. pick flowers in full bloom and hang them in a sheltered, dark spot to dry. Once the flowers have dried thoroughly, grind them to afine powder, using a mortar and pestle, old blender or small hammer mill. Mix with water and add a few drops of liquid soap. Store in a glass jar and keep the lid tightly closed, because the mixture looses activity if left open. You’ll have to experiment with the amount of water to add, because the concentration of pyrethins in the flowers is an unknown variable. If the spray you make does not seem to kill insects, use less water the next time you make the concentrated spray. Also keep in mind whole flower heads stay potent longer so do not grind until ready to use.
How to Use: Pyrethrins are more effective at lower temperatures, so for best results, apply in early evening when temperatures are lower. Spray both the upper and lower surfaces of the leaves, because spray must directly contact the insects such as thrips that hide in leaf sheaths and crevices. The first spray will excite them and bring them out of hiding, the second will kill them. Never use pyrethrin products around waterways and ponds.
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