Combined control methods over resistant populations of grain pests: experimental evaluation of synergistic approaches on sample of rice weevil

The resistance to phosphine demonstrated by major grain pests is a critical challenge for grain protection. This paper includes the results of an experimental evaluation of combined control methods over a resistant field population of rice weevil Sitophilus oryzae in the Rostov Region (R≈2.5). Systematic investigation covered the following approaches as standard phosphine fumigation; phosphine with pyrethroid insecticide; cooling followed by fumigation; vacuum plus phosphine; complete combined treatment. By the obtained results, standard fumigation was an inefficient measure (40 % mortality) but combined approaches assured 92–99.5 % mortality with varying exposure time (24–96 hours). Vacuum plus phosphine showed optimal balance between speed (24 hours) and efficacy (95 % mortality). Practical recommendations developed for different farms included regular monitoring of phosphine sensitivity, rotation of treatment methods and application of specific combined schemes in case of resistant populations. The study confirmed the possibility of overcoming phenotypic resistance without increasing phosphine doses, thereby reducing selection pressure and extending the efficiency of this critical fumigant in Russia.