A fine sprinkling will suffice, but the more dust you use, the better the effect. Renew after a heavy rain or irragation. To protect plants from ants, sprinkle around the base of plants in an area as wide as the widest leaves. 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. 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.