A film technique for the biological evaluation of pyrethrum-in-oil insecticides for use against stored product insects in warehouses



There is need for a technique whereby a given insecticidal preparation can be compared with a standard containing 0.8% (w/v) pyrethrin 1 in a heavy white oil and pronounced biologically equal or inferior to the standard within known and reasonable limits. A film method was investigated, Tribolium castaneum beetles being used as the test insects.

Numerous factors which may influence the kill achieved are discussed. The results of preliminary work, planned to help in the standardization of some of these factors, are summarized.

Several materials were tested to ascertain their suitability for reception of the insecticidal film and, as thin greaseproof paper showed promise, its possibilities were explored. Although 0.3 % pyrethrin I could be distinguished from 0.4%, 0.5% could not be distinguished from 0.8%, and the results throughout were very heterogeneous.

Woven Nylon proved more satisfactory, since the test just failed to establish a significant difference between 0.6 and 0.8% pyrethrin 1. The origin of a pink coloration of the substrate, when T. castaneum beetles were confined on Nylon sprayed with pyrethrum in oil, was investigated.

Whatman filter paper no. 544 proved the best substrate, and a comparative test on it will distinguish 0.65% pyrethrin 1 from the standard 0.8%; the limit could probably be raised to 0.7%.

The influence of several factors on the homogeneity of the results was investigated, and the importance was shown of allowing beetles time to recover from the mechanical shock inherent in the method of counting into batches.

Directions are given for carrying out the film technique for comparing preparations of relatively high pyrethrin content. The method does not measure the direct spray or fumigant action of the insecticide.

The technique described is compared with the Peet-Grady method for evaluating liquid household insecticides of low pyrethrin content. A simplified, but less accurate, design for the test is indicated.