Abstract: 【Aim】 Surveys and analyses of the overwintering geographic population size monitoring, genetic diversity and phylogeny of Chilo suppressalis in Guangdong Province, South China were conducted to provide theoretical foundations and references for formulating effective joint prevention and control strategies against this pest. 【Methods】 During 2017-2022, the five-point sampling method was adopted to collect C. suppressalis samples from paddy fields in various regions of Guangdong Province, and the occurrence level of C. suppressalis was determined based on the average number of overwintering generations. PCR was used to amplify mitochondrial genes COI and COII of nine overwintering geographic populations of C. suppressalis in Guangdong Province. The genetic diversity and genetic differentiation trends of the overwintering geographic populations of C. suppressalis were analyzed using MEGA 7 and DNA Sequence Polymorphism 5.10.01 software, and the phylogenetic tree and a haplotype network diagram were constructed.【Results】 From 2017 to 2022, the geographical population base size of the overwintering C. suppressalis in Guangdong Province was very high. Since 2018, the number of overwintering C. suppressalis populations has basically remained at 7 500 individuals/hm², and in the most severely affected areas, it could reach 18 408 individuals/hm². A total of 132 COI and COII sequences were analyzed from nine overwintering geographic populations of C. suppressalis in Guangdong Province. The COI fragment was 749 bp in length, with 30 haplotypes and 54 polymorphic sites. The COII fragment was 474 bp in length, with 22 haplotypes and 29 polymorphic sites. Phylogenetic trees based on COI and COII sequences showed that the geographic populations of C. suppressalis in Guangdong and Jiangxi clustered together. The Nanxiong geographic population in northern Guangdong was relatively independent compared to other geographic populations which clustered together. The haplotype diversity (H_d=0.800-0.909) and nucleotide diversity (P_i=0.0035-0.0102) of the geographic populations of C. suppressalis in the Pearl River Delta Region, Northern Region of Guangdong, Western Region of Guangdong, Eastern Region of Guangdong, and Northern Region of Jiangxi were at high levels. The genetic diversity of geographic population in the Western Region of Guangdong was the lowest. Neutrality test results showed that the Tajima’s D value for the total population was －1.043 based on COI sequence and －1.907 based on COII sequence, while the Fu’s Fs values for the total population based on COI and COII sequences were －5.023 and －10.468, respectively. The Tajima’s D value and Fu’s Fs value for the total population were －1.313 and －31.67, respectively, based on combined COI+COII sequences. The genetic differentiation index F_st values for the total population based on COI and COII sequences were 0.144 and 0.102, respectively, while the gene flow N_m values were 1.49 and 2.20, respectively. The genetic differentiation index F_st value and gene flow N_m value for the total population based on combined COI+COII sequences were 0.229 and 0.84, respectively. 【Conclusion】 In recent years, the overwintering geographic populations of C. suppressalis in Guangdong Province have shown a trend of being predominant or even occurring in large numbers, and these populations have high levels of haplotypic polymorphism and genetic diversity. The overwintering geographic populations in eastern Guangdong are relatively rich in polymorphism, while those in western Guangdong have the lowest polymorphism. There is a certain degree of genetic differentiation among the overwintering populations of C. suppressalis in Guangdong Province, and some geographic populations have already undergone relatively large differentiation, such as the Nanxiong geographic population. The overwintering geographic populations of C. suppressalis in Guangdong Province and Jiangxi Province have close genetic relationships. Apart from the geographic populations from Western Region of Guangdong, the genetic differentiation between other different geographic populations is not significant. The genetic differentiation between geographic populations is weakly correlated with their distribution, and frequent gene exchange occurs among populations. The populations have not experienced significant expansion but are in a stable growth trend.

Key words:  Chilo suppressalis, mitochondrial genes; COI; COII; COI+COII, genetic diversity, phylogeny