Abstract
The effect of 5% Di-syston granules upon animal community was compared between the soil application and the top-dressing method in a soy-bean field. The insecticide was applied at the rate of 4kg per 10a at about 40 days after sowing, and fluctuations in the arthropod population was examined for 9 weeks after the application. Throughout the period of investigation, both methods resulted in the effective control of aphids. In the suppressive effect on total pest population, especially on the sucking-type, top-dressing appeared superior than soil application for 3 to 5 weeks after application. Direct lethal effect of the insecticide upon some species of predators and on the total of carnivores was recognized just after the application for 3 to 7 days in the case of soil application and about 1 to 2 weeks in the case of top-dressing. Total foliar arthropod population in the top-dressing was at first lower than that in the soil application, but about 4 weeks later it became higher than or similar to that in the latter plot. Total arthropod population on the ground above a few cm in depth was always lower in the soil application than in the top-dressing. During the first 4 weeks of the investigation, the ratio of total carnivores to total arthropods was lower, and the ratio of total soy-bean pests to total arthropods or to total carnivores was higher in the soil application than in the top-dressing. From these facts, it seemed that not only in the direct effect of pest control but also in the total ecological influence, top-dressing is more promising for pest control than soil application, at least during the first 4 weeks after the insecticide is applied. It was observed that there was a clear tendency for some of the predators, phytophagous spider mites, and other foliar arthropods to be more intensively affected by top-dressing than by soil application, and this might be the results of differences in the fumigative and systemic action between the two methods of insecticidal applications.
