Pyrethrum extracts in non‐volatile oil carriers are effective insecticides in the field because they act as a direct spray killing the insect and also form a toxic film over which the insect crawls: it is necessary to study both these effects for a complete laboratory assessment of toxicity. Suitable laboratory techniques and methods for the assessment of results are described together with an account of experiments on the effect of various factors on the insecticidal efficiency of pyrethrum‐in‐oil preparations, both as direct sprays and as toxic films.The programme necessitated some duplication of work, but discussions that took place from time to time between the workers concerned at the two laboratories during the progress of the investigations reduced this to a minimum.
An apparatus for determining the relative toxicities of contact insecticides is described in detail. It is so arranged that successive batches of insects are sprayed under conditions as similar as possible, so that on using various substances at different concentrations, the results are directly comparable.The apparatus consists of a glass jar in the lid of which an atomiser is fixed. By means of compressed air at known pressure the atomiser throws a constant quantity of fine spray upon insects placed in a dish inside the jar. Photographs illustrating the details of the apparatus are given.The method used in practice for the spraying of aphides in this apparatus is described, and examples are given in the form of a table and a graph of the type of results obtained when different concentrations of nicotine are sprayed upon apterous agamic females of A. rumicis.
1. The relationship between chemical constitution and toxicity to wireworms of organic compounds is found to be of a twofold nature.2. The general effect of a group of compounds of the same type is directly determined by the chemical constitution of the type. The particular effects of individual members of the groups are limited by their physical properties such as volatility etc., which may be regarded as indirect consequences of their chemical constitution.3. The aromatic hydrocarbons and halides are on the whole more toxic than the aliphatic hydrocarbons and halides. The groups that influence toxicity most when introduced singly into the benzene ring are in order of importance the methylamido (most effective), dimethylamido, hydroxy, nitro, amido, iodine, bromine, chlorine, methyl groups (least effective). But this order is modified in presence of another group; thus when there is a CH3 already present in the ring the order becomes chlorine (side chain), amido, hydroxy, chlorine (ring), methyl. Chlorine and hydroxy groups together give rise to highly poisonous substances considerably more effective than where present separately. The association of chlorine and nitro groups in chloropicrin gives rise to one of the most toxic substances tested. Methyl groups substituted in the amido group of aniline increase toxicity more than if substituted in the ring.
Summary.
The effect of soil fertility upon the insecticidal value of the flowers has been studied in a series of pot experiments. On heavy soil the pyrethrin I content of the flowers was not increased by the application of fertilisers. The plants produced good yields of flowers, rich in pyre‐thrins, when grown in soil of low fertility.
Under conditions of normal growth and vitality, the extent of production of the pyrethrins in the flowers was characteristic of the individual plant and was dependent upon some factor which appeared to be genetical in character. A plant initially producing flowers of high or low percentage poison content continued to give flowers of the corresponding degree of richness in succeeding years, independently of the application of manures, or apparently of the number of heads produced. Plants derived from rooted shoots produced flowers corresponding in quality with those initially yielded by the parent plant. The insecticidal value of pyrethrum flowers may be improved by plant selection, followed by vegetative propagation.
In the fully open flowers, the complete disc florets contained 90 per cent. of the total pyrethrins present in the flowers, and of this the greater part was found to be located in the ovaries.
1. The acid method previously described has been used to evaluate samples of pyrethrum derived from both Swiss and Japanese seed, with equally successful results.2. A rapid method for the evaluation of pyrethrum by a determination of pyrethrin I is described.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.