Characterisation of Response to Phosphine in Adults of Representative Strains of Sitophilus Zeamais (motsch.), Sitophilus Oryzae (l.) and Rhyzopertha Dominica (f.) from China

The effects of exposure period and phosphine concentration were quantified in adults of representative strains of Sitophilus zeamais (Motsch.), Sitophilus oryzae (L.) and Rhyzopertha dominica (F.) from China. All three species are major pests of stored grain in China. Although phosphine resistance is common in Chinese S. oryzae and R. dominica, with some very resistant populations, it is not present in Chinese S. zeamais. Therefore, the study was undertaken on a susceptible strain of S. zeamais, a strong resistant strain of S. oryzae, and a strong resistant strain of R. dominica. Adults were fumigated at a range of concentrations for periods of 20-144 h at 25oC and 70% r.h. The data fitted the probit model well except for data from the Chinese strains of S. oryzae and R. dominica at 20 h, where the LC99.9 values were much greater than the highest test concentrations. The LC50 and LC99.9 tended to decrease with increasing exposure period, and the effects of concentration (C) and time (T) could be described for all strains by the equation CT = K. Time and concentration were equally important dosage factors for the susceptible strain of S. zeamais (n ≅ 1), but time was a more important dosage factor than concentration for the resistant strains of S. oryzae and R. dominica (n < 1).