Insect fecundity is a quantitative phenotype strongly affected by genotypes and the environment. However, the interaction between genotypes and the environment factors in modulating the insect fecundity remains largely unknown. In this study, we investigated the density-dependent population dynamics in the brown planthopper (BPH), Nilaparvata lugens , by using two populations respectively carrying homozygous high-fecundity (HFG) and low-fecundity (LFG) genotypes. Under low population densities, the fecundity and population growth rate of the two genotypes showed increasing trends across generations, while the trends between HFG and LFG under high population densities were opposite. RNA-seq was applied to unveil the key factors for these density-dependent progenitive phenotypes. Combined with the temporal analysis and weighted gene co-expression network analysis, we identified two gene modules that simultaneously enriched in four pathways related to the neural system. Among these pathways, the gene Nlpale encoding a tyrosine hydroxylase was identified as the key gene. The RNA interference of this gene and manipulation of its downstream product dopamine could significantly impact the basic and density-dependent progenitive phenotypes of BPH. This study shows that the dopamine biosynthesis is the key regulatory factor for the determination of fecundity in response to density changes in different genotypes of BPH , which gives an insight into the interaction of a typical environmental factor and insect genotype during the process of population regulation.