High population density (crowding) becomes a stress factor in insects. The oriental armyworm, Mythimna separata (Walker), displays gregarious and solitary phases at high and low population densities, respectively. In this study, we compared life history, disease resistance, and induction of antioxidant enzymes and heat shock protein (HSPs) in two phases of M. separata larvae. Results showed that gregarious larvae had a faster growth rate and lower pupal weight compared to solitary larvae. Furthermore, gregarious individuals exhibited higher survival rates than solitary individuals after Beauveria bassiana infection. The gregarious larvae had higher malondialdehyde content compared to solitary ones, but no differences in total antioxidant capacity were observed between the two larval phases before or after infection. Superoxide dismutase and glutathione peroxidase activities were significantly lower in gregarious M. separata larvae than solitary individuals before infection, but no difference was detected in two phases after infection. However, peroxidase and catalase activities in the two phases showed no difference either before or after infection. Hsp19.8 and Hsp90 expression in gregarious larvae were up-regulated when compared to solitary individuals before or after infection. CuZnSOD expression was not different between the two phases before infection, but it was up-regulated in gregarious ones compared to solitary ones after infection. However, expression of other stress-related genes in gregarious larvae was either repressed or unchanged when compared to solitary individuals before or after infection. Thus, larval crowding changed life history, improved disease resistance of M. separata larvae, and induced variable response of antioxidant enzymes and HSPs to fungal infection.

中文翻译：

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幼虫拥挤提高发育率，提高抗病性，并诱导丝虫抗氧化酶和热休克蛋白的表达（鳞翅目：夜蛾科）

高种群密度（拥挤）成为昆虫的压力因素。东方粘虫 Mythimna separata (Walker) 在人口密度高和人口密度低时分别表现出群居和独居阶段。在这项研究中，我们比较了 M. separata 幼虫两个阶段的生活史、抗病性以及抗氧化酶和热休克蛋白 (HSP) 的诱导。结果表明，与独居幼虫相比，群居幼虫的生长速度更快，蛹重量更轻。此外，白僵菌感染后群居个体的存活率高于独居个体。与群居幼虫相比，群居幼虫的丙二醛含量更高，但感染前后两个幼虫阶段的总抗氧化能力没有差异。感染前群居的M. separata 幼虫的超氧化物歧化酶和谷胱甘肽过氧化物酶活性显着低于单独个体，但在感染后的两个阶段没有检测到差异。然而，两个阶段的过氧化物酶和过氧化氢酶活性在感染前后均无差异。与感染前后的单独个体相比，群居幼虫中的 Hsp19.8 和 Hsp90 表达上调。CuZnSOD 表达在感染前的两个阶段之间没有差异，但与感染后的单独阶段相比，它在群居阶段上调。然而，与感染前后的独居个体相比，群居幼虫中其他应激相关基因的表达要么受到抑制，要么没有变化。因此，幼虫的聚集改变了生活史，