Influence of Larval Age on the Lethal and Sublethal Effects of the Nucleopolyhedrovirus ofTrichoplusia niin the Cabbage Looper

The nucleopolyhedrovirus ofTrichoplusia ni(TnSNPV) shows promise as a bioinsecticide against cabbage loopers (T. ni). We examined the effects of larval age at time of treatment and of the dose of virus ingested on the lethal and sublethal effects of TnSNPV onT. ni.The survival and LD50of cabbage loopers increased with the age of larvae at the time of treatment. The sublethal effects of TnSNPV included prolonged development and reduced pupal weight, egg production, and hatching success of eggs. The sublethal effects of TnSNPV did not vary among the three doses of virus used but were a function of larval age. The sublethal effects of TnSNPV were greatest when caterpillars were treated with virus at 92 and 144 h of age (third and fourth instar, respectively). Individuals that were treated at 192 h (fifth instar) and survived to adulthood did not appear to be impaired. These findings suggest that TnSNPV should be applied when cabbage loopers are in the early stages of development to maximize the bioinsecticidal activity of the virus.