Experimental Studies on the Movement Behavior of the Final Instar Noctuid Larvae of the Cabbage Armyworm and the Tobacco Cutworm

Abstract

Movement behavior of the final instar larvae of M. brassicae and S. litura from a food plant to another was experimentally investigated. In both solitary and crowded (5 or 10 larvae) conditions, M. brassicae larvae hardly emigrated from a leaf of Chinese cabbage or Chinese mustard, but tended to emigrate from that of sweet potato. On the other hand, S. litura larvae scarcely emigrated from any leaves when tested under the solitary condition. The order of feeding preference of M. brassicae larvae was Chinese cabbage>Chinese mustard>sweet potato, and that of S. litura larvae was sweet potato≈Chinese mustard>Chinese cabbage. Thus the emigration of M. brassicae larvae seems to occur depending less on the larval density during the experiment, more on difference of the species of food plant. However, the emigration of S. litura larvae may be more dependent on the larval density during the experiment. When a larva of one species was mixed with four or nine of the other, the former scarcely emigrated. This suggests the existence of some species-specificity in both the density-dependent stimulation and the larval susceptibility to such stimulation. When 20 larvae were released on a central plant of 25 Chinese mustard plants in a mimic field (in a wooden pallet, 1m×1m), M. brassicae larvae showed concentric consumption on the central several plants, but S. litura larvae showed a dispersed infestation almost all over the plants after a few days.