幼虫密度对斜纹夜蛾抗病能力的影响

为了阐明斜纹夜蛾Spodoptera litura Fabricius幼虫密度对其抗病能力的影响,在室内条件下(温度23℃±1℃,相对湿度75%)对不同幼虫密度(1、2、5、10、15头/皿(直径为12cm))饲养的斜纹夜蛾幼虫抵抗斜纹夜蛾核型多角体病毒侵染的能力及其免疫指标进行了研究。结果表明:幼虫密度对斜纹夜蛾幼虫接种核型多角体病毒后的存活率、存活时间及血淋巴酚氧化酶活性影响显著。随着幼虫密度的增加,接种核型多角体病毒后幼虫的存活率降低,存活时间缩短。当幼虫密度达到15头/皿时,幼虫存活率显著低于其它幼虫密度。不同幼虫密度幼虫的存活时间以1头/皿的最高,15头/皿的最低,且二者之间差异显著。幼虫血淋巴中酚氧化酶活性随幼虫密度的增加而明显降低,当幼虫密度达到5头/皿时,幼虫酚氧化酶活性显著低于1头/皿的。另外,幼虫溶菌酶活性和血细胞总数受幼虫密度影响不显著。不同密度幼虫抗病性的变化与其血淋巴中酚氧化酶活性的变化趋势较为一致。所以斜纹夜蛾幼虫抗病能力的降低可能与幼虫酚氧化酶活性的下降有关。因此,幼虫密度是影响斜纹夜蛾幼虫抗病性变化的重要因子。

Effects of larval density on disease resistance in the common cutworm, Spodoptera litura

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 larvae infected 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 rate of larvae at a density of 15 larvae per dish was significantly lower than that of larvae at a density of 1 larva per dish. The survival time of larvae was highest at a density of 1 larva per dish, and this was obviously higher than that at a density of 15 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 the haemolymph. The trend in disease resistance with increasing larval density was consistent with the trend in phenoloxidase activity. Therefore, decreased resistance to disease may be due to reduced phenoloxidase activity. Thus, larval density is an important factor with regard to disease resistance in S. litura larvae.