RELATIONSHIPS BETWEEN THE CHEMICAL CONSTITUTION AND FUMIGANT TOXICITY OF THE ALKYL IODIDES

Nineteen alkyl iodides have been tested as fumigants against the grain weevil, Calandragranaria , using the technique previously described. Relative toxicities, and also the toxicities of mixtures of selected pairs of iodides, were determined. Secondary and tertiary isomers were in general less toxic than the primary, irrespective of whether the toxicity was measured by molar LD 50' or the corresponding relative saturation (Pt/pB) values. Despite differences in LD 50', isomeric primary iodides often acted at about the samep,/p, value. It is suggested that these observations provide evidence that, although chemical reactivity appears to be the principal factor determining the values of LD so's in the iodide series, such physical factors as phase distribution also play their part, and their effect becomes noticeable for the primary isomers, for which the chemical reactivities may be nearly the same. The deaths of individual grain weevils which had been exposed to iodides were more or less delayed. The change of the kill with time was, under standard conditions, the same when all except methyl and the tertiary iodides were used, which suggests that with these possible exceptions, all the iodides have the same type of chemical action on the organisms. The toxicities of mixtures of selected iodides tended to confirm this, for they varied from about 1.0 to 1.25 times the value predicted from the toxicities of the single iodides using the similar action equation. Despite the apparent similarity in mode of action the LD 50's and pt / p8 values varied widely, as did the ratios of the LD 50's of successive homologues in different isomeric series. This suggests that neither the magnitude of the pt / p8 value, nor the way in which it changes from member to member in a series, gives always a useful indication of the type of action shown by a compound or group of compounds.