Hey faithless,
Pyrethrum by itself is gone VERY quickly under intense light. It is actually very very safe.
The piperonyl butoxide is an activator that is why you have to wait so long.
If you can find a product that contains ONLY pyrethrins, you are much better off for use on a flowering plant.
Have you considered a bug bomb?
The one I use that is ONLY pyrethrum is made by Raid and you can get it at home depot. the box is blue and yellow I recall.
I would avoid using the product with the activator this close to harvest.