Characterization and bioassay of indigenous isolates of the fall armyworm, Spodoptera frugiperda nucleopolyhedrovirus in India

Background Maize fall armyworm, Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae), is a highly migratory polyphagous insect pest that has posed itself as a very threatened insect invaded India since May 2018 and devastated the maize crop. This insect pest caused 62.5% damage on maize plants in Hassan district of Karnataka state, India. Surveys were undertaken in different parts of the country to assess the natural occurrences of all three categories of entomopathogens (bacteria, fungi, and viruses) associated with fall armyworm. Maximum occurrence of infection of entomopathogens (15.13%) was recorded from Chikkaballapura, Karnataka, followed by 12.23% from Hassan, Karnataka. Results Totally 13 isolates of S. frugiperda nucleopolyhedrovirus (SpfrNPV) were collected from 13 locations of the country. Electron microscopy studies clearly showed the tetrahedral shaped occlusion bodies of SpfrNPV with the size of 1.48–1.68 µm. The identity of highly virulent SpfrNPV NBAIR1 (Chikkaballapura isolate) was confirmed using conserved polyhedrin gene-specific primers and NCBI GenBank accession number was obtained (MT422725). Bioassay studies revealed that the SpfrNPV NBAIR1 Chikkaballapura isolate was highly virulent with the highest larval mortality (95.50%) and the lowest LC 50 value of 2.11 × 10 3 OBs/ml. SpfrNPV is exclusively host specific and did not infect any other insect species other than S. frugiperda tested in this present study. Conclusions Among 13 isolates of SpfrNPV, SpfrNPV NBAIR1 Chikkaballapura isolate was highly virulent with respect to the larval mortality. Hence SpfrNPV NBAIR1 Chikkaballapura isolate is having a great potential to play in the management of maize fall armyworm S. frugiperda .