Testing insecticide resistance management strategies: mosaic versus rotations.

BACKGROUND Developing scientifically valid, economically acceptable insecticide resistance management (IRM) programs is critical for sustainable insect management. The diamondback moth, Plutella xylostella (L.), has demonstrated an ability to develop resistance to many different classes of insecticides, including proteins produced by the bacterium Bacillus thuringiensis Berliner (Bt). Recently it has developed resistance to the novel compounds spinosad and indoxacarb. In greenhouse cage experiments, a laboratory-selected population of P. xylostella resistant to spinosad, indoxacarb and Bt was used to compare population growth and resistance evolution if these three insecticides were rotated or used in a mosaic fashion. RESULTS The average population density through nine generations was lowest in the treatment in which the insecticide was rotated every generation (R-1) (x = 20.7 ± 3.20) compared with the treatment in which the insecticide was rotated every third generation (R-3) (x = 41.4 ± 17.6) or where the insecticides were applied as a mosaic (M) (x = 41.8 ± 6.53). After nine generations, the survival of resistant individuals increased for each insecticide (7.2-73.5%) compared with the population without selection (CK) (0.73-3.1%). Survival on spinosad was significantly lower (23.7%) in the single-generation rotation than for the other two treatments, both of which exceeded 72%. The calculated survival on all three insecticides treated simultaneously, according to the survival on each insecticide, was 0.26, 0.81 and 1.6% for R-1, R-3 and M treatments respectively. CONCLUSION Results of both population density and resistance development indicated that insecticide rotation every generation was better for IRM than if the insecticide was rotated every third generation or if the three insecticides were applied as a mosaic.