I found this article on the stuff,
thought that you might like it, and I can fill in more info as I use it on my fruit trees and vegggies in my garden:
"pyrethrum does degrade in sunlight and under artificial light, this has turned out to be a blessing in disguise. Pyrethrum is considered biodegradable and is sought for sensitive applications like the post-harvest treatment of fruits and vegetables. Natural pyrethrum is so safe that the U.S. Government approves its use on such insect-prone foods as tomatoes, even while they are on their way to the supermarket or processing plant. And in 1946 the city of Amsterdam added pyrethrum to the municipal water supply to kill a population of insects that were threatening 'to choke the system. The insects were destroyed, while humans continued to drink, wash and cook with the treated water without suffering any harm."
pyrethrum is an extract from the seeds of the chrysanthemum and is all natural. You have probably seen the stuff at work if you have seen citranella candles, or smoke coils that burn like incense.
I have used it for many years as that is what my Gramps used, and I have never heard of any harmful effects or plant stunting from it. Good Luck
smoke in peace
KingKahuuna
