Fitness cost of phosphine resistance determined by measurement of developmental rates of phosphine-resistant and susceptible populations of Rhyzopertha dominica and Tribolium castaneum

Strong phosphine resistance was reported in Oklahoma populations of Rhyzopertha dominica (F.) (Coleoptera: Bostrichidae) and Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae) in 2012. For continued effective use of phosphine, resistance management has to be implemented in the U.S. The goals of phosphine resistance management are to slow resistance development where it has not occurred and to mitigate resistance in populations where it occurs by infrequent use of phosphine and withholding use for extended periods of time, respectively. Knowledge of the fitness effects associated with phosphine resistance is important for the development of resistance management strategies. Therefore, the goal of our study was to determine if there are fitness effects associated with phosphine resistance in populations of R. dominica and T. castaneum from Oklahoma. We measured developmental rates of phosphine-resistant and –susceptible populations of these two species in a phosphine-free environment. Three resistant R. dominica populations exhibited lower developmental rates compared to the susceptible population, whereas the only resistant T. castaneum population tested exhibited a higher developmental rate compared to the susceptible population. Our data for R. dominica and T. castaneum indicate that there is a fitness cost and a fitness benefit, respectively, associated with phosphine resistance in these two species. This means phosphine resistance development in susceptible R. dominica populations can presumably be slowed by infrequent use of phosphine, whereas it can be mitigated by suspending phosphine use for extended periods of time in resistant populations. However, the same may not be true for T. castaneum.

KEY WORDS stored product, lesser grain borer, red flour beetle, fumigant efficacy, phosphine resistance management