Insecticidal effect of heat treatment in commercial flour and rice mills for the control of phosphine-resistant insect pests

Development of resistance to insecticides and fumigants in several stored product insect pests is of global concern as failure of routine treatment protocols is on the rise, causing serious economic implications. A series of trials in commercial flour and rice mills were conducted to evaluate the effectiveness of heat treatment against two major pest species, as an alternative to fumigant phosphine that has been under serious threat from resistance development in key pest species. Adults and immatures of both susceptible and phosphine-resistant strains of the red flour beetle, Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae) and the rice weevil, Sitophilus oryzae (L.) (Coleoptera: Curculionidae) were included in the study. The target temperature for the treatment was 50 °C and the treatment lasted for 16–36 h, depending on the building size. Vials containing test insect cohorts along with their preferred diets were placed at pre-determined locations within the trial sites. Wireless sensors were also placed next to test insect vials to record and monitor course of temperature change during the treatment. The adult mortality assessment of test insects was carried out immediately after the termination of each trial. Thereafter, the treated vials containing the diet and immature insect life-stages were shifted to a controlled environment cabinet and maintained at 25 °C and 65% relative humidity for 65 d when the progeny mortalities were assessed. Our results showed that heat treatment had caused very high adult (in most cases 100%) and suppressed progeny mortality in both T. castaneum and S. oryzae in all trials, irrespective of their susceptibility status to phosphine. The current study suggests that heat treatment in rice/flour mills holds potentials to be used as an alternative treatment to phosphine, especially to control phosphine resistant strains.