Genetic diversity and population structure of the fall armyworm, Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) in the Philippines

The fall armyworm (FAW), Spodoptera frugiperda (J.E. Smith), is an invasive pest species that has incurred massive economic damages in areas cultivating economically important crops like corn, rice, cotton and many others. Known to be a native of the Western Hemisphere, FAW has since then established itself in many parts of Africa, Asia and Australia and Oceania. The Philippines had been among the Asian countries vulnerable to FAW invasion, which was validated in June 2019. This study aims to characterize the existing strains and haplotypes of FAW and describe the genetic structure and the signature of its recent demographic history nationwide. COI sequences reveal the predominance of rice-strain over the corn-strain FAW, while Tpi sequences are more accurate in defining the corn-strain found in corn and sugarcane. Discordant COI-Tpi configurations frequently occur the most in the entire FAW population at 65.29%. Nucleotide and haplotype diversity are low with an average of 0.005976 and 0.4019 respectively. In addition to non-significant values in neutrality tests suggesting neutral evolution, the low nucleotide and haplotype diversity indicate population contraction from founder effect. Variations are mostly found within regions, and genetic structure is moderate among regions (F_ST = 0.14375). These results can be attributed to low selective pressures accompanying its establishment, or more likely due to recent migration and/or human-assisted factors such as trade. Implications on pest management and prospective studies in the future are also discussed in this study.