Bioassay of nucleopolyhedrosis virus against larval instars of Malacosoma neustria

The relative susceptibility of third- and fourth-instar Malacosoma neustria larvae to a nucleopolyhedrosis virus was studied by bioassay in the laboratory. Larval mortality increased with increased dosage, whereas the dosage-incubation relationship was reversed. Larval age inversely affected mortality and incubation. The computed LD₅₀'s for third- and fourth-instar larvae were 1,405 and 12,320 polyhedral inclusion bodies (PIB's)/larva. The median lethal doses calculated as number of PIB's/mg body weight showed that third-instar larvae were only twice more susceptible to virus than fourths. The LT₅₀ values for 3 × 10⁵, 3 × 10⁴, and 3 × 10³ PIB's/larva were 5.9, 6.58, and 8.15 days, respectively, in third-instar assay; the corresponding figures for the two highest concentrations were 9.3 and 10.7 days in the older larvae. Lethally infected individuals died after one or, exceptionally, two molts. No correlation was found between pupal weight or adult emergence of survivors and the virus dose administered to the larvae.