Factors Responsible for the Difference in Densities between Southwestern and Northern Japanese Populations of the Green Rice Leafhopper, Nephotettix cincticeps(UHLER)(Hemiptera:Deltocephalidae), in Their Later Generations : I. Effects of the Heading Time of Rice Varieties on the Oviposition and Propagation of the Second Generation

Seasonal population trend and peak density of Nephotettix cincticeps differ considerably between southwestern and northern Japan. In southwestern Japan, the peak density is relatively low and very stable, whereas in northern Japan it sometimes reaches a higher level and causes economic damage to the rice yield. To elucidate causal factors responsible for this difference, several rice varieties differing in their heading data were tested for the oviposition of N. cincticeps in the second generation. In the field, the population density in the third generation in each varietal plot was correlated with the heading date of respective variety : the earlier the heading data of a variety, the higher was the density in it. When each of the potted rice plants was caged with a certain number of adults, the number of eggs laid on the early varieties after heading was much larger than it was on the late varieties before heading. Early varieties and late ones are prevalent in northern and southwestern Japan, respectively. Thus, the difference in the heading data associated with dominant varieties is considered to be one of the major factors responsible for the difference in the peak density of N. cincticeps between the two regions.