Knockdown in insects following application of an insecticide may be defined as the state of intoxication and partial paralysis which usually precedes death. Pyrethroid insecticides cause knockdown within a few minutes in contrast to the slower action of other groups of compounds. Users throughout the world have varying interpretations of knockdown and may apply different emphasis depending upon the species involved and local application techniques. To obtain valid assessments it is necessary to carry out tests under the most realistic conditions possible and to see that the effects of variations in procedure are taken into full account. Some factors which influence knockdown are: single rapid dosing or continuous pick-up during the observation period ; size and distribution of spray droplets; size of test chamber; interaction in mixtures of synergist and insecticides; use of organic solvents or water-based formulations. Examples of these are given.
A range of 5‐benzyl‐3‐furylmethyl cyclopropane carboxylates and other esters are evaluated against house‐flies, mustard beetles and two mosquito species. The results show the importance for activity of a gem‐dimethyl group on the cyclopropane ring and that substitutions at C3 give wide variations in insecticidal activity and marked species specificity. Some of the compounds had considerable knockdown activity against houseflies, but the structural requirements for this type of action differ markedly from those for kill.
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.
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