Biological Assay of Insecticide Residues

Abstract

Recently, Gunther & Blinn (SS ) have described the characteristics of insecticide residues and have given a definition which differentiates them from insecticide deposits. They used the word for insecticide initially laid on a surface by treatment, and the word for insecti­ cide, regardless of its locale on or within a substratum, which has in any way aged. If there is some interval between the time of the application and the time the insecticide reaches the insect, so that weathering, meta­ bolic degradation, and other processes take place, the deposit becomes a resi­ due. Thus, all kinds of stomach poisons are used as a form of residue. With systemic insecticides also, there is some lapse of time before the active ingredients reach the insects through the plant tissues. Contact insecticides which are used so as to come in contact with insects after having been sprayed on the substratum, not directly on the insects, must also be con­ sidered residues. Therefore, in a broad sense, we can say that insecticides used in forms other than residue are rather scarce and that the description of the method of biological assay of insecticide residues is tantamount to that of the biological assay of all insecticides. It is consequently very diffi­ cult to discuss it within limited pages. Fortunately, Sun ( 1 1 9) has recently reviewed in detail the same problem. Therefore, the present writer wishes to review in this paper the laboratory methods for the assay of insecti­ cides which, after spraying, remain in a very minute amount as the result of the various environmental factors. Accordingly, the review of the results of practical field experiments will be omitted. With some chemicals used as insecticides, such as pyrethrum, derris, cube, the active ingredients decompose very rapidly; on the other hand, 'for such chemicals as lead arsenate, which is very slow in its decomposition, appropriate physical or chemical methods for determination of minute amounts of residues have been developed. As long as insecticides were limited to these kinds, the problem of residues was not important. But the appearance of many effective organic insecticides, which followed the syn­ thesis of DDT, led to difficulty in the detection of residues, for the several limited methods available were unsuitable. At the same time, the adoption of various auxiliary substances which prolong toxicity and, in addition, the prevalence of organic phosphorus insecticides which are highly toxic to men and cattle have made even a minute amount of residue an important subject of discussion. There is no doubt that the importance of the problem of residues has