From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-156716-460197
Question: I have several pyrethrum daisies. I know that pyrethrum is
an insecticide. How can I use the plants to control insect pests?
Answer: You are correct that pyrethrum is a botanical insecticide
derived from the flowers of pyrethrum daisies. It's possible to dry
the flowers, then grind them into a fine dust to use on flowers, or to
use alcohol to create a liquid extract. Since the active chemical is
toxic it is important to keep it away from children and pets. Also,
don't use your regular kitchen equipment to prepare the spray or dust.
Finally, it's impossible to give dilution rates since homemade
concoctions will vary widely in strength. Why not eliminate the
guesswork and use a commercially prepared, pyrethrum-based
insecticide?