RESUMO
Granuloviruses (GVs) Betabaculovirus associated with the fall armyworm (FAW), Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae), especially those of the type I, have scarcely been studied. These GVs might be an effective alternative for the biocontrol of this insect. In this study, the native GVs SfGV-CH13 and SfGV-CH28 were isolated from FAW larvae and 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 GVs had ovoid shape and a length of 0.4 µm. They had the same DNA restriction profiles and their genome sizes were about 126 kb. The symptomatic infection with the 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 GVs, which are known as monoorganotropic or slow-killing GVs. The median lethal dose (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 days. LT50 values decreased as the viral dose was increased. The elapsed time from symptomatic infection until pupation 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.
RESUMO
A multiple nucleopolyhedrovirus native isolate (SfCH32) of Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) was encapsulated by spray-drying in a matrix based on oxidized corn starch without and with a fluorescent brightener. The microcapsules were exposed to UV radiation (365 nm) for 0, 2, 4, and 8 h at 25 °C or temperatures of 35, 40, and 45 °C for 8 h. The data obtained with temperatures 35, 40, and 45 °C were contrasted with those obtained at 25 °C. The microcapsules were evaluated for size, shape, and insecticidal capacity against third instar S. frugiperda larvae under laboratory conditions. The 82-84.2% of the encapsulating matrix, in a dry-weight basis, was recovered as NPV microcapsules of heterogeneous shape and size. The exposure to UV radiation and temperatures reduced significantly the insecticidal capacity of tested viruses; however, such capacity was higher for microencapsulated than for non-microencapsulated viruses. The non-encapsulated virus that had been exposed to 45 °C or maintained at UV radiation for 8 h showed the lowest insecticidal activity at 5th day post-inoculation, with a larvae mortality of 25.3 and 16%, respectively. The fluorescent brightener increased significantly the insecticidal capacity of encapsulated and non-encapsulated viruses, causing a mortality of 100% at that time point, and decreased the median lethal time independently of the incubation temperature and exposure time to radiation. The findings suggested that an encapsulating matrix based on oxidized corn starch might protect the insecticidal capacity of NPV under field conditions.
