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

To clarify certain phases of fluctuation in the size of population of Mamestra brassicae, daily counts of eggs and larvae of the second generation were made in a cabbage field at Hirosaki from the middle of August till early October in 1959. Seven egg-batches were found in six plants among eighteen sampled plants and five of them hatched but two were lost immediately after hatching. The larvae from each egg-batch formed their own population on the plant on which they hatched. The first instar larvae congregated tightly on the leaf where they hatched. After the second instar the larvae migrated gradually to other leaves of the same plant, and all the final instar larvae entered into the head of the plant. The fourth and fifth instar larvae showed a strong tendency to migrate prior to the moulting stage. All the larvae always occupied the ventral side of the leaves of the host plant. The survival rate of each larval population from respective egg-batch gradually reduced with the lapse of time and 20 percent or less of the initial member was able to reach the final instar.