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I wonder if diatomaceous earth should really be removed from the least of “least toxic” methods. I see Michelle posted that a reviewer rejected diatomaceous earth/silica gel because it is listed in the SF list as a Tier 2 product. The old SF product list only includes one product with silica gel (Drione), and this product also contains PBO. The product is labeled as a Tier 2 product, but I wonder if that the rating is based on the presence of PBO rather than diatomaceous earth. Perhaps diatomaceous earth alone would in fact qualify as a Tier 3 product. I reviewed diatomaceous earth in the PAN pesticides database and compared it against the criteria listed in Table 3 of the Guide to San Francisco’s Reduced Risk Pesticide List (http://www.sfenvironment.org/sites/default/files/fliers/files/sfe_th_guide_to_reduced_risk_pesticide_listposted.pdf). The criteria include cancer, reproductive toxicity, endocrine disruption, etc. The only flag that comes up is under the acute toxicity rating, which is rated as “slightly toxic.” However, according to Table 3 of the SF Guide, low product toxicity would be classified in the “low” concern category, i.e. the Tier 3 category. Based on this guidance, I would conclude that diatomaceous earth is suitable for Tier 3 classification. There is also an article by EPA that discusses the use of silica gel as a least toxic pesticide. It’s a little outdated, but it can be found here. http://www.epa.gov/oppsrrd1/REDs/factsheets/4081fact.pdf Has anyone received any additional feedback on this topic? If you have any other thoughts, I would appreciate it.

Following up on this post, I had reviewed the 3 products in Drione using the PAN database, and the ingredient PBO was the only one that raised a toxicity flag. Looking at this again, the issue is probably not PBO, but rather pyrethrin. There have been increasing concerns about impact of pyrethrin on water bodies. But that still leaves the question of whether silica gel and diatomaceous earth not tied up with pyrethrin would be allowed. Thoughts?