Characterization of baculovirus isolates from Trichoplusia ni populations from vegetable greenhouses

A survey of cabbage looper, Trichoplusia ni, populations in greenhouse vegetable crops in the Fraser Valley (FV) of British Columbia, Canada led to the isolation of a large number of Nucleopolyhedrovirus (NPV) single-infected-larva isolates. These NPVs were identified from cadavers by phase-contrast light microscopy and characterized as either T. ni SNPV (TnSNPV) or Autographa californica MNPV (AcMNPV) by mulitplex-PCR. Among the 57 NPV isolates collected in 2000, 54 were TnSNPV and three, all from one greenhouse, were AcMNPV. In 2001 over 100 single-infected-cadaver NPV isolates were characterized by PCR and all were TnSNPV. Restriction endonuclease (REN) analysis confirmed the PCR identification of individual isolates. In addition, REN analysis showed that all TnSNPV isolates had identical REN profiles that were similar to but distinct from the reference strain TnSNPV-RJ suggesting that TnSNPV-FV isolates constitute a single unique strain of the virus. In contrast, only a few AcMNPV were isolated and these constitute two strains based on REN profiles that were distinct from other AcMNPV strains. Dose–response bioassays with 2nd and 4th instar T. ni indicated there was no significant difference in infectivity of TnSNPV and AcMNPV isolates. However, in 5th instar T. ni AcMNPV was as much as 10-fold more infectious than TnSNPV. In addition, AcMNPV appeared to be more virulent as infected 4th instar larvae died approximately 18 h sooner than TnSNPV infected larvae. TnSNPV produced approximately five times more occlusion bodies per cadaver than AcMNPV. Both AcMNPV and TnSNPV appear to have good potential as candidates for biological control agents of T. ni.