13: Investigating the "sweet spot" in phosphine fumigations

In a series of bioassays, we used adults of different stored product beetle species with different susceptibility to phosphine to investigate the appearance of the so called "sweet spot". The tested insects were Oryzaephilus surinamensis, Tribolium castaneum, Sitophilus oryzae and Rhyzopertha dominica. The adults of most susceptible populations were instantly immobilized even in the shortest exposure period (15 minutes) when exposed to 3000 ppm, in contrast with adults of the resistant populations that were active even after 300 minutes at the same concentration. Moreover, the majority of adults of the susceptible populations were dead, whereas adults of the resistant populations recovered regardless of the species and the exposure time. The exposure of the adults to phosphine concentrations ranging between 500 and 3000 ppm, for exposures ranging between 1 and 40 hours, showed that the response was not linear, indicating the occurrence of a "sweet spot", i.e. the decrease of mortality with the increase of the concentration. For most of the tested species the "sweet spot" appeared in 1000 and 2000 ppm and at a 5-hour exposure interval. This observation is particularly important both in terms of the evaluation of resistance under short-exposure diagnostics, and potentials of surviving individuals at elevated concentrations and subsequent resistance development.