Dominant strain shift in the invasive fall armyworm (Lepidoptera: Noctuidae) populations in Thailand as inferred from mitochondrial COI and nuclear Tpi genes.

The fall armyworm, Spodoptera frugiperda (J. E. Smith, 1797) (Lepidoptera: Noctuidae), is a significant global pest, that exhibits 2 discernible strains, corn strain (CS) and rice strain (RS). After initial detection in the eastern hemisphere in 2016, the dominant strain was identified as RS based only on cytochrome C oxidase subunit I (COI) mitochondrial gene from limited samples from various countries, including Thailand. This study aimed to assess strain and haplotype variation in the S. frugiperda populations in Thailand using both mitochondrial COI and nuclear triosephosphate isomerase (Tpi) genes. Analyses of COI sequences (n = 105) revealed 2 predominant haplotypes, COICSh4 (82.86%) and COIRSh1 (17.14%), and the analyses of Tpi sequences (n = 99) revealed 6 haplotypes, with TpiCa1a (53.53%) being the most prevalent. Of the 98 caterpillar samples, the majority exhibited true CS (83.67%) for both genes. Meanwhile, interstrain hybrids, indicated by gene discordance, accounted for the minority (16.33%). Interestingly, despite the initial dominance of RS during the 2018 outbreak, the current study identified CS as the prevalent strain across all localities in Thailand. These findings suggested a shift in S. frugiperda dynamics in Thailand that was possibly influenced by factors, such as competitive exclusion principle, pesticide usage in rice cultivation, and preferences for corn over rice. Our study suggests a need to reexamine the previous reports of rice-strain dominance in various countries in the eastern hemisphere after the initial invasion.