Bioassay experiments for investigating the potential of recombinant nucleopolyhedroviruses expressing a scorpion toxin (AaIT), and a juvenile hormone esterase from Manduca sexta (Linnaeus) (AcMsJHE) in killing first instar larvae of Agrotis ipsilon (Hufnagel) were tested in comparison with the wild type virus (AcMNPV). Protein expression and viral replication in insect were determined using SDS-PAGE and agarose gel electrophoresis. Results of lethal concentrations indicate that AaIT was the most efficient virus compared with AcMsJHE or AcMNPV. Median lethal concentration of AaIT was 4.0 × 105 PIBs/ml compared with 7.0 × 105 and 7.6 × 105 PIBs/ml for AcMsJHE and AcMNPV, respectively. AaIT killed the treated first instar larvae of A. ipsilon markedly faster than AcMsJHE or AcMNPV, and significantly affected third instar larval weight after 96 hours post-infection (h.p.i.), which the LT50s of AaIT were 94 and 89 h.p.i. when 1 × 106 and 1 × 108 PIBs/ml were used, respectively. SDS-PAGE for protein separation ...