Time,concentration and temperature relationships for phosphine activity in tests on diapausing larvae of Ephestia elutella (Hübner) (Lepidoptera: Pyralidae)

The activity of phosphine against diapausing larvae of Ephestia elutella was shown to depend on several different interactions of duration of exposure, temperature and gas concentration. Using the model proposed by Knight, (C? C₀) × (t? t₀) = k, a constant for mortality, the minimum effective concentration C₀ was raised and the minimum effective exposure t₀ extended as temperatures were increased. The effect of temperature on these thresholds was reduced at higher mortality levels. A range of concentration levels, influenced by temperature, elicited an increase in tolerance at the LD₉₉ level over that evident at higher or lower concentrations. These concentration ranges were 0.9–2-1 mg litre-¹at 25°C,0.7–2.3 mg litre-¹ at 20°C and 0.1–1.5 mg litre-¹ at 15°C. A mathematical model was fitted to relate concentration and time thresholds, mortality and temperature. At the LD₅₀ level, values for t₀ ranged from 2.5 h at 15°C to 5.0 h at 25°C. C₀ values increased from about 0.6 to 7.0 μg litre-¹ over this temperature range. At the LD₉₉ level, t₀ values ranged from 21 to 28 h and C₀ values from 11 to 19 μg litre-¹. Phosphine appeared most effective at each temperature at concentrations from about 40 to 60–100 μg litre-¹.