Comparative Biology and Susceptibility of Cabbage Looper (Lepidoptera: Noctuidae) Lines to a Nuclear Polyhedrosis Virus

The susceptibility of 12 lines of cabbage looper, Trichoplusia ni (Hubner), to the singly embedded nuclear polyhedrosis virus of T. ni (TnSNPV) was examined. Eight lines had been reared previously in the laboratory for several years and 4 lines were recently established from larvae collected from cabbage field plots or greenhouses. Susceptibility varied among lines and LD50 values varied 3.5-fold. The duration of the larval and pupal stages, sex ratio, and wing condition of adults surviving viral infection did not differ from controls. Pupal weight varied significantly with exposure to TnSNPV in 5 of 12 lines, but the effect was difficult to interpret because it was not related to dose in a simple way. Finally, control females from lines with high LD50s laid fewer eggs, suggesting that resistance to TnSNPV (i.e., high LD50s) may be reproductively costly.