A Study on the Nymphal Aggregation of the Cabbage Stink Bug, Eurydema rugosum MOTSCHULSKY(Heteroptera : Pentatomidae)

Abstract

Eurydema rugosum lays egges en masse normally composed of twelve eggs. Nymphs form aggregation throughout their nymphal life. The effects of group size on nymphal development, mortality, synchronism in moulting and the weight of ensuing adults were studied to assess the ecological meaning of aggregation. Nymphs were reared at 25°C with pods of rape at the densities of 1, 2, 6, 12, 24 and 48 insects per container. Among the groups employed, nymphs of the six insect group developed fast synchronizing with each other with a lower mortality and gave rise to heavier adults. The nymphal development, mortality and adult weight became respectively slower, higher either with increase or decrease in size of group. These results were compared with those obtained from the same kind of experiment conducted in Nezara viridula which lays a great egg mass, i. e. 80-100 eggs per mass (KIRITANI, 1964). It was concluded that there were two biological types of Pentatomidae : In the first type the fermales lay large egg masses and the ensuing nymphs form aggregation in early stages which are followed by a more solitary life. The second type is exemplified females which lay small egg masses and the nymphs tend to form aggregation throughout their nymphal life as is the case of E. rugosum.