Threshold Density of the Rice Leaf Beetle, Oulema oryzae KUWAYAMA, for Insecticidal Control
II. Estimation of the Threshold Density
1979 Volume 23 Issue 1 Pages 1-10
Details
1979 Volume 23 Issue 1 Pages 1-10
In Niigata Prefecture, Oulema oryzae attacks rice plants from May to June. After the attack, the rice plant shows many symptoms in proportion to the intensity of attack. Retardation of growth and decrease in number of leaves and stalks per hill are most frequent. In this experiment, it could be demonstrated that the decrease in number of stalks per hill brought about a direct decrease of the rice yield. Effects on the ripening of grains were negligible. The following formula was developed: Y=-22.24+26.07logX1, which represents the relationship between the rate of yield loss (Y) and the number of 3-4 instar larvae (X1) attacking the rice plant in the peak of the season (middle June). From this formula, the minimum number of insects which caused actual loss of yield was calculated to be 7 per hill. If the lower value of 95% confidence interval for Y was adopted, the minimum number of insects (X1) became 3.5 per hill. Thus, such value can be used to express the threshold density of insects causing actual loss of yield. When the rate of injured leaves (X2) was used instead of the number of 3-4 instar larvae, the formula became Y=-20.22+0.39X2. In this case, the minimum rate and the threshold rate of injured leaves causing actual loss of yield were estimated to be 50 and 20%, respectively. These figures can be used as an index when deciding whether to carry out insecticidal control of the insect.
