The toxicity of phosphine to the immature stages of resistant and susceptible strains of some common stored product beetles, and implications for their control

All developmental stages of 13 strains of six species of stored product beetles, including 7 strains known to be resistant to phosphine as adults, were exposed to the gas at 15° and 25°C in fumigation chambers. The strains resistant to phosphine in the adult stage were found to have resistant immature stages, and in all the resistant strains the most tolerant stage in the life cycle was the pupa. All stages of susceptible strains of Oryzaephilus surinamensis, Tribolium castaneum, Cryptolestes ferrugineus and Rhyzopertha dominica could be controlled by 4 days exposure to phosphine at 15° and 25°C, although the dosage rate required for the latter two species at 15°C was 2 g m −3. In contrast 10–12 days exposure at this dose were required for the control of pupal Sitophilus granarius and S. oryzae at 15°C. Immature stages of resistant strains of each species showed higher levels of tolerance when compared with susceptible strains. For O. surinamensis and T. castaneum control was achieved by 4–6 days exposure, but the other species required 8–10 days exposure at 25°C, and single individuals of resistant R. dominica and S. oryzae strains survived a 14 day exposure at a dose of 2 g m −3 phosphine at 15°C. Results indicate that for resistant as well as susceptible strains, time of exposure to phosphine was a more critical factor of dosage than concentration. However, for control of the resistant insects it was also found necessary to achieve high dosage levels, which must be maintained for long periods. It is unlikely that either of these objectives could be achieved in a commercial treatment using current practices. Thus effective control of both susceptible and resistant insects will only be possible if structures used for storing grain are made more gas-tight, or if bulks of grain stored on the floor are effectively sealed using suitable impermeable sheeting secured to the floor.