Insecticidal activity of pyrethrum extract and its four insecticidal constituents against house flies. II.—Synergistic activity of piperonyl butoxide with the four constituents

Abstract The toxicity to female house flies, 5‐6 days old, of the four insecticidal constituents of pyrethrum, combined with piperonyl butoxide at six ratios (from 10 : 1 to 1 : 20 w/w), was determined by a measured drop technique. The order and magnitude of the toxicities of the synergised constituents (relative to synergised pyrethrum extract) vary with the amount of synergist present. At 1 : 8 (pyrethroid/piperonyl butoxide) the relative toxicities of the four constituents were: pyrethrum extract, 1.0 (1.0); pyrethrin I, 1.31 (0.85); cinerin I, 1.28 (0.53); pyrethrin II, 0.90 (1.37); cinerin II, 0.58 (0.49). (The relative toxicities of the constituents alone are given in brackets.) At this ratio, the synergistic factors of the four constituents were: pyrethrin I, 18; cinerin I, 27; pyrethrin II, 6; cinerin II, 14; pyrethrum extract, 10. The possible explanations for differences of the extent to which the constituents are synergised are discussed.