Posted: 03/05/2013 at 18:06
Pyrethrum IS safe in that it is not persistent in the way neonicotinoids seem to be. And as you say, spraying once the bees and hover lfies have gone to bed is the best option (IMHO).
Pyrethrum is actually made from the roots of the plant of the same name, Pyrethrum which is a white daisy.
Pyrethrin insecticide effects on bees and beneficial agricultural insects & Fish & Cats
Pyrethrins are “highly toxic” to bees; 0.02 micrograms is sufficient to kill a bee.50 Toxicity of commercial pyrethrin products to bees was demonstrated by an entomologist at Auburn University who showed that a commercial pyrethrin insecticide caused 100 percent “knockdown,” the inability of the bee to walk or fly. Some of the inert ingredients used in pyrethrin products appear to increase knockdown potency.51
It is not surprising that pyrethrins, because they are insecticides, are toxic to agriculturally useful insects and spiders. The International Organization for Biological Control found that a commercial pyrethrin product killed over 99 percent of two parasitoid wasps and a predatory fly. (Parasitoids are insects which develop in and kill the eggs or larvae of another species.) This study also found pyrethrins caused 80 percent mortality of two other parasitoid species, a fly and a wasp.52 Cornell University Agricultural Experiment Station researchers found that pyrethrins killed four common species of wasp that are parasitoids of house and stable flies in dairies.53 Another study found that Both the pyrethrin products tested led to 100% mortality in the adult parasitic wasps and ladybird larvae on glass plates and plants.
I cannot really help with advice on really safe pesticides as we do not use them. The only spray of that kind which I do use is a Horticultural soft soap one specially made for fruit trees and then that is sprayed on in February when there tend be very few insects around. It kills the eggs of Apple pests on the trees.
It is not easy! Today I found a whole frame full of Alliums of various ornamental types, all with aphids on them. It took me ages to wipe then off with a damp cloth.