Two host-plant strains in the fall armyworm

Abstract

The fall armyworm (Spodoptera frugiperda) is one of the major pest insects damaging diverse crops including cotton, corn, rice, and sorghum. Fall armyworms have been identified as two morphologically indistinguishable strains, the corn strain, and the rice strain, named after their preferred host-plants. Although initially recognized as host-plant strains, there has been an ongoing debate regarding whether the corn and rice strains should be considered as such. In this article, we present arguments based on recent population genomics studies supporting that these two strains should be considered to be host-plant strains. Furthermore, host-plant adaptation appears to be a driving evolutionary force responsible for incipient speciation in the fall armyworm.

Keywords: Spodoptera frugiperda; fall armyworm; host‐plant adaptation; incipient speciation.

Fig. 1

Testing the power to detect small differences in fitness. We considered two groups in which a quantitative trait related to fitness differs by only 1%. The trait follows a normal distribution, and the standard deviation is 10% of the mean. With varying sample sizes for each group (x‐axis), we tested whether a one‐tailed t‐test can detect the existing differences with a 5% significance level with 10,000 replications. We then calculated the proportion of false positives among the replications (y‐axis).

Fig. 2

Suggested evolutionary scenario of the FAW. The ancestral fall armyworms with mtB‐type mitochondrial sequences underwent a divergence that led to the generation of corn and rice strains with differentiated host‐plants. Subsequently, the corn strain underwent additional nuclear differentiation, resulting in the generation of two substrains. One of these substrains experienced mitochondrial introgression from an extinct species with mtA‐type mitochondrial sequences. As a result, within the corn strain, two substrains had highly differentiated mitochondrial genomic sequences (mtA and mtB). Invasive populations originated from corn strains with the involvement of genetic admixture between mtA and mtB (Yainna et al., 2022).