Pyrethroids are drawing increasing attention to the wood preservative formulators because: ( 3; 6) a) pyrethroids reveal good efficacy against a large variety of wood boring insects by contact. b) pyrethroids show good long term efficacy because of their high stability in timber. c) mammal toxicity is relatively low ( e.g. bats) (9). d) pyrethroids are almost odorless. It was found that different methods of pyrethroid synthesis result in different ratios of isomers (1). Special effort was put into the proof of the reciprocal relationship between cis-percentage of an isomeric pyrethroid mixture and its toxic value against wood boring insects. Permethrin (52645-53-1), an ester of the chlorine derivative of phenoxybenzylalcohol and vinyldimethylcyclopropancarboxylic acid is on the market as a (±) cis/trans-isomeric mixture between 60:40 and 25:75. The ratio 25:75 compromises between efficacy and toxicity.

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