Life Tables of the Tobacco Cutworm, Spodoptera Litura (Lepidoptera : Noctuidae), and an Evaluation of the Effectiveness of Natural Enemies

Abstract

Preliminary life tables of the tobacco cutworm, Spodoptera litura, were developed in taro fields in 1970 and 1971 in Ino, Kochi. Three kinds of plots were prepared to evaluate the role of natural enemies: (1) caging of host plants by nylon gauze to exclude all natural enemies; (2) enclosure by a plastic plate (30cm in height) which was coated by tangle-foot to prevent the invasion of the plot by natural enemies of ground fauna, and (3) the untreated plot. All the natural enemies in plot 1 and that of the ground fauna alone in plot 2 were eliminated one month before the experiment with insecticides (BHC and Mesomil) as well as by hand-picking. Six species of parasites and 13 species of predators were recorded, and they were responsible for 68-89 per cent loss of the caterpillars. Ground-living invertebrate predators attacked middle-stage caterpillars, whereas natural enemies of air-borne type did so at the early or late stages of the host. The survivorship curve was concave in early summer when the micryphantid was abundant. This spider greately affected the survival of hatchlings by disturbing the larval aggregation of the 1st instar. In midsummer when the density of micryphantids was low, the survivorship curve became diagonal. Nevertheless, predation of mature larvae by Polistes wasps became important and the survivorship curves in the plots from which natural enemies were excluded were convex throughout the seasons. The survival rate of the 1st instar larvae decreased linearly with increasing density of micryphantids. That of the 2nd instar larvae was lower in the rainy season than in the summer season and vice versa for the 4th and 5th instar larvae. These phenomena can be caused by seasonal variations in the activity of major natural enemies, i.e. ground-living invertebrate predators in the 2nd instar and the Polistes wasps in 4th and 5th instars. © 1972, JAPANESE SOCIETY OF APPLIED ENTOMOLOGY AND ZOOLOGY. All rights reserved.