Abstract
Larval and egg populations of P.rapae crucivora were affected by mechanical detachment of eggs from host plant leaves, drowning of larvae by rainfall and by such natural enemies as spiders, Polistis wasps, Hyla arborea japonica, Apanteles glomeratus and Pteromalus puparum. In April, a remarkable reduction in the numbers of eggs and 1st instar larvae was mainly induced by abiotic factors while that of 5th instar larvae was mainly caused by biotic factores. In June and July, a remarkable reduction in the numbers of eggs, 1st and 2nd instar larvae was observed mainly due to the predation of micryphantids. Then, the suriving mature larvae were preyed on mainly by Polistis wasps. Therefore the surviorship curves were concave in June and July. In October, only eggs and 1st instar larvae were affected by abiotic factors before the survivorship curve levelled off.
