Abstract: Ascoviruses are double-strand DNA viruses with potential biocontrol for the insect pests. In this study, Spodoptera frugiperda ascovirus isolate 84-36 was used to inoculate the first to the fifth instar of Spodoptera exigua (Hübner) larvae by both per os (1.09×10¹⁰ copies per larva for the first to second instar larvae; 9.77×10⁹ copies per larva for the third to fifth instar larvae) and intrahaemocoelic injection (1.56×10⁹ copies per larva) with the titer of 9.77×10¹² genomecopies/mL hemolymph. Mortality, growth and development of the inoculated larvae were observed and recorded. While the corrected mortalities of the first to the third instars inoculated per os were 18.9%±1.11%, 4.4%±1.11%, 2.2%±1.11%, respectively, the treatment showed no effects on the fourth and fifth instars. The first instars were significantly easier to be infected by the virus than the second and the third instars. Corrected mortalities of the larvae inoculated by injection in both the third and fourth instars were 100.00%, and that in the fifth instars was 80.00%±0.00%. The survival time of the diseased larvae was apparently extended. Meanwhile, food intake of the larvae inoculated in the third and the fourth instars was reduced remarkably at day 2, and the body weight remained fairly constant from day 3-postinoculation. These results reveal that the Spodoptera frugiperda ascovirus isolate 84-36 has the potential for biological control.

Key words: Spodoptera exigua, growth and development, ascoviruses, pathogenicity, mortality rate