The preparation of a pyrethrum extract in highly refined heavy mineral oil, suitable for use as a basis of reference in the biological evaluation of commercial pyrethrum-heavy oil preparations, is described. The solution was standardized with respect to colour, resin content and equal proportions of the pyrethrins. A standard solution containing pyrocatechol remained stable, when judged by the chemical determination of the pyrethrins, over a period of months.
When applied in a heavy mineral oil medium, pyrethrin II was shown to possess a toxicity to Tribolium castaneum Hbst. approaching, if not equal to, that of pyrethrin 1. When applied in an aqueous medium the toxicity of pyrethrin II was very much lower than that of pyrethrin I.