Abstract
Mortality factors of eggs of Nilaparvata lugens STAL, Sogatella furcifera HORVATH and Laodelphax striatellus FALLEN on the winter-spring rice crop were studied in the Red River Delta in early April, 1993. Mymarid parasitism was responsible for almost all mortality of the three planthoppers on the winter crop. The mean parasitism rates were 20.6%, 32.8% and 53.2% for N. Lugens, S. furcifera and L. striatellus, respectively. On the other hand, both parasitism (12.2-45.5%) and physiological death caused by rice plant reaction (4.5 - 57.1 %) were important mortality factors of S. furcifera eggs on three varieties of early-spring and spring crops. Physiological egg mortality of S. furcifera colonies originating from populations obtained in the Red River Delta and Chikugo, Japan were high on two japonica varieties (Reiho and Niigatawase) and low on and indica variety (TN 1) and a japonica X indica variety (Saikai 184). The difference in the egg mortality rate on the 4 varieties between the two colonies was insignificant. This gives support for the view that the winter-spring crop in the Red River Delta is a major source of air-born rice planthoppers invading southern China where they multiply and produce ocean-crossing migrants into Japan in the Baiu season.
