The values for concentrations in air of the vapours of approximately one hundred different chemical substances required to produce 50% mortality in samples of the grain weevil Calandra granaria, have been determined. Relationships between chemical constitution of these substances and their toxicity to the grain weevil are discussed, using as a measure of toxicity the relative saturation of the vapour concentration producing 50 % kill. The relative saturation is equal to the thermodynamic activity of the vapour at the concentration in question.

It is found that the substances used can be divided into two broad classes: (a) one class in which the toxic thermodynamic activities lie between o-1 and 1 -o and (b) another in which the activities are much lower—down to 0–0004.

The first class exhibits certain linear relationships between the logarithms of toxic concentration and saturated vapour pressure, which are not shown by the second class. The toxicity of the first class is ascribed to a physical mechanism, and is believed to be such that phase equilibrium relations play a predominant part in determining the value of the toxic concentration measured externally to the insect (Ferguson, 1939).

The toxicities of members of the first class, as measured by their toxic thermodynamic activities, show the following relations with chemical constitution:

  • 1
    Toxicity in homologous series decreases steadily from the first member as the chain length increases, finally disappearing.
  • 2
    Halogen substitution in hydrocarbon skeletons has only a small effect on the ‘physical’ toxicity, increasing it slightly. There is practically no difference in the effect of different halogens.
  • 3
    Hydroxyl substitution markedly decreases ‘physical’ toxicity.
  • 4
    The substitution of—CO or —CHO groups markedly increases ‘physical’ toxicity. The aliphatic amines and the alkyl formates examined do not appear to act by a physical mechanism, nor do many of the alkyl bromides and iodides.