The beet armyworm, Spodoptera exigua (Lepidiptera: Noctuidae), is an important agricultural pest in China, causing serious economic losses in the main welsh onion-producing areas of North China. In order to effectively monitor and manage this pest, it is necessary to investigate its genetic variation and intraspecific phylogeographic structure in North China. In this study, we used a DNA fragment of cytochrome oxidase subunit I (COI) consisting of 518 bps to investigate its genetic diversity. A total of 500 individuals of this species were collected from 20 geographic locations, and 8 haplotypes were identified. Low levels of genetic diversity (h = 0.256 AE 0.025, p = 0.00058 AE 0.00008) for the total populations in these study regions were obtained. Phylogenetic and median-joining network analyses indicated that there was no distinct geographic distribution pattern among haplotypes. The results also indicated that the mean pairwise sequence difference between the populations was 0.1%, ranged from zero to 0.2%. The genetic differentiation (F st ) between the populations varied from À0.198 to 0.656, and there was a significant differentiation between the population of Kunshan (KUNS), Shenyang (SY), Lucheng (LUC), Binzhou (BZ) and the remaining populations, respectively. The analysis of molecular variance (AMOVA) showed that the percentage of variation within a population (69.05%) was greater than that between the populations (30.95%), and the level of population differentiation (F ST = 0.310, P < 0.001) was highly significant after 1000 random permutations. Finally, unimodal mismatch distribution combined with negative, Tajima's D (D = À1.956, P < 0.05) and Fu's Fs (Fs = À5.875, P > 0.05) indicated that S. exigua experienced recent population expansion in North China.

The beet armyworm, Spodoptera exigua (Lepidoptera: Noctuidae), is an economically important pest that causes major losses in some main crop-producing areas of China. To control this pest effectively, it is necessary to investigate its population genetic diversity and genetic structure around the Bohai Gulf area of China. In this study, we used two mitochondrial genes, COI (578 bp) and Cytb (724 bp), to investigate its genetic diversity. We obtained 622 COI sequences and 462 Cytb sequences from 23 populations, and 28 and 73 haplotypes, respectively, were identified. Low to moderate levels of genetic diversity (COI: Hd = 0.267 ± 0.023, Pi = 0.00082 ± 0.00010; Cytb: Hd = 0.689 ± 0.018, Pi = 0.00255 ± 0.00029) for the total populations were observed. Phylogenetic and median-joining network analyses indicated no distinct geographical distribution pattern among the haplotypes. Overall, this study revealed that there was significant differentiation among the populations (COI: F = 0.158, P < 0.001; Cytb: F = 0.148, P < 0.001). F values for Shenyang, Baoding, and Funing were significantly different to those for most of the other populations. Finally, unimodal mismatch distribution analysis, combined with negative neutrality test results, showed a recent population expansion of the beet armyworm around the Bohai Gulf area of China.