Response of nuclear polyhedrosis virus-resistant Spodoptera frugiperda larvae to other pathogens and to chemical insecticides

Selection in the laboratory for Spodoptera frugiperda (Sf) resistant to nuclear polyhedrosis virus (NPV) affected the susceptibility of the insect to certain other mortality agents, including a chemical insecticide. Median lethal concentrations (LC 50s) and associated statistics were compared for several mortality agents between colonies of NPV-resistant and -susceptible (control) insects. Compared to the susceptible insects, the NPV-resistant insects were cross-resistant to the S. frugiperda granulosis virus and to the Autographa californica NPV based on nonoverlap of 95% fiducial limits of the LC 50s. The NPV-resistant insects were significantly more susceptible to methyl parathion than the control insects. The two colonies of S. frugiperda did not differ significantly in their response to Bacillus thuringiensis, Varimorpha necatrix, or carbaryl. The cross-resistance experiments were based on per os exposure of the insects to the pathogens and insecticides; the susceptibility of the resistant and control insects did not differ significantly when the Sf NPV was injected into the hemocoel or when methyl parathion was applied topically.