Invertebrate Iridescent Viruses (Iridoviridae) from the Fall Armyworm, Spodoptera frugiperda

Invertebrate iridescent viruses (IIVs, family <i>Iridoviridae</i>) are icosahedral double-stranded DNA viruses that infect a wide range of invertebrates, particularly in humid and aquatic environments. During field trials in Chiapas, southern Mexico, larvae of the fall armyworm, <i>Spodoptera frugiperda</i> (Lepidoptera: Noctuidae), displayed an unexpected lavender iridescence, leading to the discovery of novel IIV isolates from this major agricultural pest. Restriction endonuclease analysis revealed evident diversity among isolates from individual infected larvae. Although one field experiment yielded inconclusive results, a second experiment revealed a positive association between IIV disease and SfMNPV infection, compared to a negative association with the prevalence of parasitoids, and no association with entomopathogenic nematodes (probably <i>Hexamermis</i> sp.). These findings require further investigation to determine the distinct ecological routes through which the virus may transmit across host species. IIV infection of <i>S. frugiperda</i> was also identified in Veracruz State, Mexico, and northern Argentina, revealing a previously unrecognized geographic and host range for these viruses. The genomic and evolutionary features of the three isolates from <i>S. frugiperda</i> were compared with those of two other lepidopteran isolates from <i>Helicoverpa zea</i> (IIV30C obtained from CSIRO) and <i>Anticarsia gemmatalis</i> (AgIIV). Genome sizes ranged between 196.1 and 205.4 kbp (~28% GC content), with several large inversions, and were rich in tandem repeats. The average amino acid identity of the complete genomes and phylogenetic analyses of 26 core gene sequences placed all five isolates within the genus <i>Chloriridovirus</i>, closely related to IIV22 and IV22a isolated from blackflies (Diptera) in Wales and a previously sequenced isolate of IIV30 from the USA. We conclude that these lepidopterans are all infected by closely related strains of the virus species <i>Chloriridovirus simulium1</i> across their native geographical range. These findings highlight the unexpected ecological breadth and evolutionary adaptability of IIVs.