Comparative Studies on Population Dynamics of the Important Lepidopterous Pests on Cabbage

In the vicinity of the North Eastern end of Honshu, three crops of cabbages are produced each year: early summer, late summer and fall crops. To investigate the seasonal trends in population dynamics of the caterpillars in cabbage fields, egg, larvae and pupae of Pieris rapae crucivora BOISD., Plusia nigrisigna WALKER and Mamestra brassicae L. on cabbage plants were counted in fields of respective crops, during May to September, 1959. The seasonal changes in the average number of eggs, each instar larvae and pupae of the three species per plant in respective fields are given in Figs. 1, 2 and 3. Among these species, Mamestra brassicae was the most abundant during June to early July, and Pieris rapae crucivora during the late July, August and September. Plusia nigrisigna did not attain a significant proportion of the total population at all. As to the total number of the insects observed throughout the overall season, Pieris rapae crucivora was the most numerous, Mamestra brassicae came next, and Plusia nigrisigna showed no significant proportion. The caterpillar population reached its highest level in early September. In general, 50 per cent or more of the initial population in each generation of respective species was destroyed during the egg and the first larval instar. Moreover, 80 per cent or more of the survivors failed to pupate. The rate of reduction in number of early summer population of Pieris rapae crucivora was particularly high when the population density of Mamestra brassicae was the highest.