Abstract
In the study of the threshold density for insecticidal control of Oulema oryzae, it is necessary to understand the population dynamics of the insect. Survivorship curves were therefore examined in 9 paddy fields. The survival rates of the 3rd instar larvae varied greatly among fields (8.7-64.9%), but 30.7% in the average. Mortality was especially high among 1st instar larva. The average survival rates from eggs of pupae and adults were 15.2% and 10.2%, respectively. The principal mortality factor for eggs was a parasitic wasp, but its effect was insignificant. The larvae, especially young larvae, are very sensitive to air humidity, and this fact was experimentaly demonstrated. Therefore, it is thought that in the field, larval mortality is largely due to low air humidity. From above examination, we concluded that survival of this insect is greatly dependent on weather conditions. Finally, we the feasilibity of using the survivorship curve for estimation of the threshold density for insecticidal control of this insect is discussed.
