Growth-blocking peptide (GBP) is a 25 amino acid insect cytokine found in lepidopteran insects that has diverse biological activities such as larval growth regulation, paralysis induction, cell proliferation and stimulation of immune cells. Density-dependent phase polyphenism is a phenomenon of phenotypic plasticity in which the expression of a variety of traits can be affected by local population density. In the present study, the armyworm Mythimna separata larvae with four rearing densities (1 larva/vial, 2 larvae/vial, 4 larvae/vial and 6 larvae/vial) were tested for cuticular melanization and body weight throughout the third-fifth instar, and the functional role of GBP in regulating the changes was investigated. The results indicated that when reared at high densities, the larvae exhibited less body weight and more degree of cuticular melanization than larvae reared at low densities. The gene expression of GBP in armyworm larvae showed an initial rise and then decline trend with increased rearing densities in the third to fifth instar. Compared with control, more degree of cuticular melanization was observed in GBP-injected larvae (500 ng/larva in volume 50 μL) than that in Ringer’s solution-injected counterparts. Furthermore, the gene expression level of dopa decarboxylase and prophenoloxidase increased significantly in GBP-injected fifth instar larvae from 6 h to 12 h after injection, suggesting the role of GBP in modulating density-dependent phase trait of armyworm cuticular melanization.

生长阻断肽（GBP）是鳞翅目昆虫中发现的25个氨基酸的昆虫细胞因子，具有多种生物学活性，例如幼虫生长调节，麻痹诱导，细胞增殖和免疫细胞刺激。依赖于密度的相位多态性是一种表型可塑性现象，其中各种性状的表达会受到当地种群密度的影响。在本研究中，粘虫Mythimna separata在第三个五分龄幼虫中测试了具有四个饲养密度（1个幼虫/小瓶，2个幼虫/小瓶，4个幼虫/小瓶和6个幼虫/小瓶）的幼虫的表皮黑化和体重，以及GBP在调节幼虫中的功能作用变化进行了调查。结果表明，与在低密度饲养的幼虫相比，在高密度饲养的幼虫体重更轻，表皮黑化程度更高。夜蛾幼虫中GBP的基因表达呈现出先上升然后下降的趋势，在第三至第五龄中的饲养密度增加。与对照组相比，在GBP注射的幼虫（500 ng /幼虫，体积为50μL）中观察到的表皮黑色素化程度高于在Ringer溶液注射的幼虫中。此外，