This is from Howard's Note in Jan/Feb 2009 DIRT
It's time that I make myself very clear on the subject of pyrethrum and PBO. Pyrethrum is a natural material made from the painted daisy (Chrysanthemum cinerariaefolium or Chrysanthemum coccineum). Pyrethrins are the six compounds in pyrethrum that have insecticidal power.
I've been concerned about the misuse of pyrethrum products for some time, and I have never recommended a pyrethrum product that contains other toxic materials. One of the common extra ingredients is PBO, which is a synthetic synergist that gives the basic insecticide more killing power.
As you can read in the Journal of Pesticide Reform (Vol. 22, No. 1), published by the Northwest Coalition for Alternatives to Pesticides, there are scary facets to pyrethrum. The journal says that in laboratory tests, insecticides made from pyrethrum have caused tumors in animals, increased the risk of leukemia, disrupted the normal function of *** horomones, and triggered allergic reactions including heart attack and asthma.
Because of a recent surge of interest in using pyrethrum for mosquito control, I feel that I need to make my warnings stronger. Pyrethrum and related products are neurotoxins. These days, they are being recommended for use as safe, natural insecticides. The concept of the backyard mosquito mist system in which they are being used is good, but cedar or other biological products should be used instead of neurotoxins. The alternatives work as well, but they don't kill beneficial insects and are considerably less toxic.
In addition, some consultants and quite a few stores and contractors are selling and using combination pyrethrum/diatomaceous earth/PBO products. This concerns me because they are promoting these products as organic, and the idea often is marketed as something that I approve. I do not approve of these products. In my opinion, these pest-control products are in the same unacceptable category as diazinon, Dursban, Sevin and Orthene.
PBO deserves specific comment as well. It shows up in a variety of pesticides, even some orange oil/d-limonene and neem products. These combinations are unacceptable in an organic program. You'll find information about PBO in the Journal of Pesticide Reform, too.
PBO has some of the same toxicity issues as pyrethrum, but it is exponentially worse when mixed with other toxic chemicals to make them more effective at killing bugs. Pyrethrum products are toxic to bees, fish and other aquatic life, but they are even more toxic when PBO is added. For me, pyrethrum no longer is an acceptable insect control in an organic program. Pyrethrum combined with PBO has never been acceptable.
Furthermore, synthetic pyrethroids that also contain PBO are even worse than the "natural" products. Synthetic pyrethroids are similar in chemistry and action to pyrethrum pesticides, but they are a bigger problem for people with allergies and asthma. One of those synthetic pyrethroid products is Scourge, which is being used by Dallas and other cities for mosquito control. Its active ingredients are a pyrethroid called resmethrin and PBO. Some consumer products containing synthetic pyrethroids also are available for the control of ticks, fleas, ants and other insects. I urge you to avoid them. http://www.dirtdoctor.com/organic/garde ... n/id/2536/