The Role of Tyramine β-Hydroxylase in Density Dependent Immunityof Oriental Armyworm ( Mythmina separata ) Larva

High population density alters insect prophylactic immunity, with density-dependent prophylaxis (DDP) being reported in many polyphonic insects. However, the molecular mechanism for DDP remains unclear. In current study, the role of tyramine β-hydroxylase (Tβh) in the immune response of larvae that were subject to different rearing densities conditions was investigated. The tyramine β-hydroxylase activity of larvae from high density treatments (10 and 30 larvae per jar) was significantly higher than that of the larvae from low density treatments (one, two, and five larvae/jar). A (designated ) containing a 1779 bp open reading frame was identified. Multiple sequence alignment and phylogenetic analysis indicated that was orthologous to the that was found in other lepidopterans. Elevated expression was observed in larvae under high density (10 larvae per jar). Silencing expression by the injection of dsRNA in larvae from the high density treatment produced a 25.1% reduction in octopamine levels, while at the same time, there was a significant decrease in phenoloxidase (PO) and lysozyme activity, total haemocyte counts, and survival against infection 56.6%, 88.5%, 82.0%, and 55.8%, respectively, when compared to control larvae. Our findings provide the first insights into how mediates the octopamine level, which in turn modulates the immune response of larvae under different population densities.