Independent and joint biological activity of isomeric forms of synthetic pyrethroids

Various isomeric mixtures of pyrethroids were examined in topical application tests against houseflies, Musca domestica. On the basis of the activities of the separate isomers of 5‐benzyl‐3‐furylmethyl (±)‐cis,trans‐chrysanthemate, it was shown that when combined in pairs to give the (±)‐trans or (±)‐cis or (+)‐cis,trans mixtures the observed mortalities did not differ from those expected by simple additive action calculated by the harmonic mean. In contrast the (±)‐cis,trans mixture showed considerable antagonism with a mortality only 60% of that expected. Similar evaluations using the separate and combined isomers of bioallethrin [(R,S)‐3‐allyl‐2‐methyl‐4‐oxocyclopent‐2‐enyl (allethronyl) ( + )‐trans‐[(1R,3R)‐chrysanthemate] and the corresponding (+)‐cis‐(1R,3S)‐chrysanthemate indicate antagonism calculated to be correlated with the content of the (R)‐isomer of the alcoholic moiety. Hence the activity of the most active isomer of the “allethrin” series, (S)‐3‐allyl‐2‐methyl‐4‐oxocyclopent‐2‐enyl ( + )‐trans‐(1R,3R)‐chrysanthemate, (S)‐bioallethrin, is not fully realised unless it is present in pure form and a substantial part of the value of bioresmethrin (5‐benzyl‐3‐furylmethyl ( + )‐trans‐chrysanthemate] as a killing agent is lost when the racemic form is used. In racemic mixtures there is mutual antagonism between pairs of isomers so that considerable masking of activity occurs.