Morphological, biological, and molecular characterization of Type I granuloviruses of Spodoptera frugiperda

Abstract

The granuloviruses or GVs (Betabaculovirus) associated with the fall armyworm (FAW), Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae), especially those of Type I, have scarcely been studied but they might represent an alternative for the biocontrol of this insect. In this study, the native granuloviruses SfGV-CH13 and SfGV-CH28 isolated from FAW larvae were characterized for morphology, molecular traits, and insecticidal activity. The elapsed time between symptomatic infection of larvae and stop feeding as well as the weight of larvae before death or prior to pupation were also evaluated. Both granuloviruses isolates showed ovoid shape with a length of 0.4 µm. They showed the same DNA restriction profiles and their genome sizes were about 126 kb. The symptomatic infection with tested GVs mainly caused flaccidity of larva body and discoloration of integument. The integument lysis was only observed in 8% of infected larvae. Infected larvae gradually stopped feeding. Overall, these symptoms are characteristic of infections caused by Type I granuloviruses, which are known as monoorganotropic or slow-killing. The median lethal doses (LD50) values for SfGV-CH13 and SfGV-CH28 isolates were 5.4 × 102 and 1.1 × 103 OBs/larva, respectively. The median lethal time (LT50) ranged from 17 to 24 d. LT50 values decreased as the viral dose was increased. The elapsed time since symptomatic infection until pupation (LD50) and body weight of larvae (third instar) were higher with SfGV-CH28 than SfGV-CH13. Both granulovirus isolates were able to kill the FAW larvae from the 12th day.