THE LOSS OF WEIGHT OF PYRETHRIN‐TREATED FLOUR BEETLES, TRIBOLIUM CASTANEUM (HERBST), AND ITS APPLICATION TO BIOASSAY

Abstract

Pyrethrin‐treated flour beetles, Tribolium castaneum (Herbst), lose weight rapidly. The weight lost by batches of beetles, a readily determined graded response, has been investigated with a view to its use in bioassay. Strong evidence was obtained that (1) in pyrethrum extracts only the pyrethrins cause the loss, and that (2) the other extractives do not modify the loss caused by the pyrethrins. Allethrin also caused a rapid weight loss; piperonyl butoxide, by itself producing little loss, greatly increased the loss caused by pyrethrum solutions and by allethrin, paralleling its effects on toxicity. The pyrethrin content of solutions appeared to account quantitatively for their weight‐reducing activities; irradiation of pyrethrum solutions that destroyed their toxicity also destroyed their weight‐reducing properties; and neither pyrethrum oleoresin heat‐treated to destroy the pyrethrins, nor irradiated solutions, modified the weight loss due to allethrin.Factors influencing weight loss were investigated. Beetles were exposed, mostly for 3 hr. at 25°C., on hard filter‐papers treated with solutions of pyrethrins in a non‐volatile oil (Shell Risella oil 17, formerly called P31). At fixed deposit the loss rose sigmoidally with increasing pyrethrin concentration from about 0.7 % for zero concentration to an upper limit of about 6.5 % of initial beetle weight. A log‐logit transformation made the sigmoid curve linear. The carrier oil and the length and temperature of the exposure to the pyrethrins influenced the weight loss. At 25°C, the rate of air movement and the relative humidity between 40 and 90% did not. Pyrethrins in Risella oil 17 could be applied satisfactorily to the filter‐papers by means of a spraying tower, or by pipette if the solutions were first diluted with a volatile solvent, which was allowed to evaporate subsequently. Deposits of given pyrethrin content were about 1.3 times as active in producing a weight loss if prepared by spraying than if prepared by solvent evaporation.The technique, design, and analysis of results, for bioassays based on weight loss are described. The most elaborate method of analysis ever likely to be needed is given, with a worked example, but a considerably simpler method, only slightly longer than the probit analysis for quantal‐response assays, would probably suffice normally.For a given number of beetles used, the weight loss method of assay has about the same inherent precision as the film method of Parkin & Green (1943), but is less precise than the direct spray method of Hewlett (1947). However, larger numbers of beetles can be used in the new method, the assessment of the insecticidal effect is more objective, and results are obtained more quickly y‐BHC and DDT cause T. castaneum to lose weight less rapidly than do pyrethrins.