Studies on phosphine fumigation of wheat bag-stacks under different storage conditions for controlling phosphine - resistant insects.

Effects of different phosphine application rates (2, 3 and 4 g/t) on control of phosphine-resistant and -susceptible strains of lesser grain borer (Rhyzopertha dominica) and rice weevil (Sitophilus oryzae) in stored wheat were investigated for up to 8 wk. Stacked bags of wheat (total wt. of stacks, 146-149 t) were stored in a covered outbuilding or outdoors on wooden plinths or on soil. Levels of phosphine gas emitted from wheat stacks were recorded at 24 h intervals. Average daily losses of phosphine gas from stacks in the outbuilding, outdoors (plinth) and outdoors (soil) were in the ranges 6-10, 13-17 and 21-29%, respectively. It was determined that effective control of susceptible and resistant insects could only be achieved in wheat stacks stored in the outbuilding (indoors); phosphine gas losses were too high in outdoor stacks. Recommendations are given for min. concn. of phosphine required for effective control of highly resistant strains of R. dominica and S. oryzae during long term storage of bagged wheat.