Analysis of Yearly Fluctuations in the Occurrence of Migratory Rice Planthoppers, Nilaparvata lugens STAL and Sogatella furcifera HORVATH, Based on Light-Trap Data in Northern Kyushu.

Yearly changes in the occurrence of long-distance migratory planthoppers were analyzed from light trap data recorded at Chikugo in Fukuoka Prefecture for 40 years from 1951 to 1990.The number of catches at the immigrant generation (G0) and three successive generations (G1, 2, 3) were identified on the basis of effective heat units required for completing one generation.Annual catches of N. lugens showed a downward trend throughout the 40-year period, but annual catches of S. furcifera increased from the mid 1970s.The variances of G0 and G1-3 in N. lugens were the largest in 1980s.The variance of G0 in S. furcifera was also the largest in the 1980s, but that of G1-3 decreased consistently after the 1960s.Negative relationships were found between G0 and population growth rate (G1-3/G0) for both planthopper species.The rate of density-dependent reduction of the population growth rate in S. furcifera in the 1980s was greater than in the other decades.Population growth patterns from G0 to G3 were classified into three groups according to the 1st and 2nd component scores of principal component analysis for both species.Groups I and II differed in the population growth rate of successive generations, although the immigrant density was similar.The causes of the difference were the initial date of immigration and the August temperature for N. lugens, and September precipitation for S. furcifera.Group III was characterized by a high G0 and low population growth rate.