POPULATION ECOLOGY Cabbage Looper Resistance to a Nucleopolyhedrovirus Confers Cross-Resistance to Two Granuloviruses

Previously,we showed that cabbage loopers (Trichoplusia niHübner) can evolve 20X resistance to the single (S) nucleocapsid nucleopolyhedrovirus (NPV) of Trichoplusia ni (TnSNPV). In this study, we investigate one potential cost that resistant cabbage loopers may incur, increased susceptibility to other mortality agents. Contrary to expectation, no such cost was observed with any of the six mortality agents tested. In fact, the LD50 of selected larvae was always greater than that of control caterpillars for each agent tested. However, the differences were never signiÞcant for permethrin or Bacillus thuringiensis subsp kurstaki (Berliner). The differences in the LD50 of control and selected T. ni for the wild-type multiple (M) nucleocapsid NPV of Autographa californica Speyer (AcMNPV, clone C6) and the recombinant AcMNPV (AcMNPV-AaIT) were small ( 2X) and signiÞcant in only one of three generations. Surprisingly, the highest level of cross-resistance was to the granuloviruses of Pieris rapae L. (4Ð5X; signiÞcant in two of three generations) and T. ni (20Ð30X; signiÞcant in three of three generations). This suggests that the infection pathway of TnSNPV may be more similar to TnGV than to that of AcMNPV.