Here's what I found:
How to Make: If you grow your own pyrethrum daisies, youâll 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. Youâll 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, but instead only repels them, 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 wherever you expect the bugs will be, 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, because they are deadly to fish (although they are harmless to mammals).