Entomopathogenic potential of isolated baculovirus from natural populations of true armyworm (Mythimna unipuncta Haworth, 1809) in Northern Spain

Baculoviruses are a family of entomopathogenic double-stranded DNA viruses enclosed within a protein matrix (occlusion bodies, OBs), widely employed in biological pest control due to their high host-specificity. This family comprises four genera, of which two selectively infect Lepidoptera: Nucleopolyhedrovirus (NPV) and Granulovirus (GV).

Mythimna unipuncta, a phytophagous noctuid moth native from North America, established itself in Europe in the 19^th century and its attacks have become more prominent in recent years in Spain. Its larvae nourish on Gramineae, causing significant losses in maize and forage grasses used for mowing and cattle grazing. Four baculovirus were isolated out of field samples from dead larvae with symptoms compatible with baculovirus infection, two Nucleopolyhedrovirus (MyunNPV-SP1 and MyunNPV-SP2) with similar REN profiles, and two Granulovirus (MyunGV-SP1 and MyunGV-SP2), with significant differences. This study explores the entomopathogenic potential of these viruses in pest control against M. unipuncta.

Phylogenetic analysis of concatenated sequences from the highly preserved regions lef8, lef9, and polyhedrin/granulin confirmed the NPVs are closely related to M. unipuncta’s Kentucky NPV isolate, while MyunGV-SP2 groups with its Hawaiian GV isolate. MyunGV-SP1, however, clusters with GVs from Xestia c-nigrum and Helicoverpa armigera, although its distance with these suggest it may represent a novel GV species.

Bioessays on ten noctuid species revealed NPVs’ host range limits to M. unipuncta, while GVs infect both Trichoplusia ni and Chrysoideixis chalcites, and MyunGV-SP1 C. includens and Spodoptera exigua additionally. Median lethal concentration (CL50) on second instar larva reached values over 10⁴OBs/ml for the NPVs and around 10⁸OBs/ml and 10⁷OBs/ml for GVs in M. unipuncta. Simultaneous infection (coinfection), a natural phenomenon, showed non-antagonistic interaction between the viruses, although NPV production prevails over GVs. Further studies are underway to assess the mean time to mortality, OB production per larva, and the effects of coinfection.