Insecticide resistance and mechanisms of resistance to selected strains of Helicoverpa armigera (Lepidoptera: Noctuidae) in the south of France

In the south of France, the cotton bollworm Helicoverpa armigera has recently developed resistance to pyrethroids. The resistance factors and resistance mechanisms were investigated by comparing two selected strains deltamethrin-resistant strain (Rd strain) and methomyl-resistant strain(Rm strain), with a strain maintained under insecticide-free conditions called field strain. The three strains were collected in 2000 and 2001 in the same processing tomato crop plot. The investigations were performed on third-instar larvae treated by topical application with and without synergists (PBO, DEF, DEM) and by analysis the biochemical characteristics of three enzymatic systems (mixed function oxydases, esterases and glutathione-S-transferases (GSTs)). The deltamethrin-resistance factors were 32.4-fold for Rd strain and 11.5-fold for Rm strain. However, the LD50 of the Field strain was higher than those reported in the literature. The three strains showed synergism factors of deltamethrin with the PBO range from 30- to 80-fold. The synergistic factors of deltamethrin with DEF was 10.6- and 6.2-fold, respectively, for the Rd and Rm strains. The enzymatic activities for MFO and EST of the Rd and Rm strains were higher than the Field strain. For GST levels, only the Rm strain was significantly different from the Field strain. Deltamethrin or methomyl synergism with the DEM was not observed. Taken together with the role played by MFOs in increasing resistance to deltamethrin, these results suggest the presence of several resistance mechanisms. The species' migratory movements, long demonstrated in the Pyrenean mountains, could explain the spread of resistance, recently reported in Spain, to southern France. (c) 2005 Elsevier Ltd. All rights reserved.