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2012年49 No.6
Effects of larval density on disease resistance in thecommon cutworm，Spodoptera litura
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Key Words：Spodoptera litura，nuclear polyhedrosis virus，larval density，disease resistance，immune index
Abstract：To investigate the effects of larval density on disease resistance in the common cutworm，Spodoptera litura
Fabricius，resistance to S． litura nuclear polyhedrosis virus ( SLNPV) and immune parameters of larvae at densities of 1，2，5，10 and 15 larvae per dish were tested in a laboratory． The results show that survival rate and survival time of larvaeinfected by SLNPV，and the activity of phenoloxidase in larval haemolymph，were significantly affected by larval density．The survival rate and survival time of larvae infected by SLNPV declined with increasing larval density． The survival rateof larvae at a density of 15 larvae per dish was significantly lower than that of larvae at a density of 1 larva per dish． Thesurvival time of larvae was highest at a density of 1 larva per dish，and this was obviously higher than that at a density of15 larvae per dish． Phenoloxidase activity in larval haemolymph significantly declined with increasing larval density．Phenoloxidase activity was significantly lower at larval densities of 5 larvae per dish，and higher at a density of 1 larva per
dish． In addition，larval density had no significant effect on lysozyme activity and total haemocyte counts in thehaemolymph． The trend in disease resistance with increasing larval density was consistent with the trend in phenoloxidaseactivity． Therefore，decreased resistance to disease may be due to reduced phenoloxidase activity． Thus，larval density isan important factor with regard to disease resistance in S． litura larvae．