Isolation of the Thosea asigna virus (TaV) from the epizootic Setothosea asigna larvae collected in South Sumatra and a study on its pathogenicity to Limacodidae larvae in Japan

Abstract

In 2005, natural epizootics were observed during an outbreak of Setothosea asigna (Lepidoptera: Limacodidae) larvae in an oil palm plantation in South Sumatra, Indonesia. The causative agent was filterable, which implied it was a virus. Since preliminary testing using reverse PCR gave a positive result for the Thosea asigna virus (TaV), we experimented with the purification of the viral agent from the diseased larvae. Electron microscopy revealed nonenveloped virus-like particles that were spherical in shape and about 40nm in diameter. cDNA cloning followed by sequencing demonstrated that RNA purified from the particles contained two large open-reading-frames (ORFs) with a partly shared sequence and extensive homology (>98% identity at the nucleotide level) with ORFs of TaV encoding an RNA-dependent RNA polymerase and the capsid protein, respectively. 3’ RACE suggested that there is, like in TaV genomic RNA, no poly(A) tract at the 3’-terminus of the RNA. The pathogenicity of the purified particles against Limacodidae larvae in Japan was demonstrated to be very strong for Monema flavescens and Austrapoda dentata. These results indicated that the agent causing the epizootic disease among S. asigna larvae in the oil palm plantations was TaV which also has potential as a biological control agent for Limacodidae pests in Japan.