摘  要  【目的】 相多态性是昆虫个体的一种表观形态可塑性，其中多种表型性状的表达直接受到昆虫种群密度的调控。本研究通过饲养高（低）密度种群来开展基于种群密度依赖效应的草地贪夜蛾Spodoptera frugiperta相多态性研究。【方法】 通过比较群居和散居饲养条件下草地贪夜蛾幼虫个体之间的表皮黑化、发育历期、蛹重和同类相残行为开展研究。【结果】 群居饲养的草地贪夜蛾幼虫比散居饲养的幼虫表现出更小的体型和更深的表皮黑色化，且幼虫在群居饲养时蛹重减小，表明草地贪夜蛾幼虫可通过基于密度效应的适应性变化，以增强个体生存能力，并最大限度地减少种内竞争。此外，研究还发现，散居饲养的幼虫亦有较强的同类相残性。【结论】 草地贪夜蛾存在基于种群密度依赖效应的相多态性。而通过研究基于种群密度依赖效应的草地贪夜蛾相多态性，掌握其种群动态变化和暴发为害特征，个体间自相残杀的生态后果，以及对病原体抗性和成虫迁飞行为的密度依赖性影响等有助于更好地开展该种重大迁飞害虫的综合防控。

Abstract  [Objectives] To investigate phase polyphenism, an apparent morphological plasticity of insects in which the expression of multiple phenotypic traits is directly regulated by population density, in the fall armyworm, Spodoptera frugiperta. [Methods] Fall armyworms were reared at either high, or low, densities and cuticular melanin, developmental duration, pupal weight and cannibalistic behavior, of individuals reared under different densities were compared. [Results] Larvae reared at high densities had smaller body size, more cuticular melanin and lighter pupal weights, than those reared at low densities. There was a higher incidence of cannibalism in larvae reared at low densities. [Conclusion] These results suggest that S. frugiperda larvae undergo density-based adaptations to minimize intraspecific competition, thereby enhancing individual survival. These findings further understanding of the population dynamics and outbreak characteristics of the fall armyworm, and the ecological consequences of cannibalism in this species.