Toxicity of propylene oxide alone and in combination with low pressure or carbon dioxide against life stages of Ephestia cautella (Walker) (Lepidoptera: Pyralidae) under laboratory conditions

A series of tests was carried out in the laboratory for comparison of the insecticidal effect of propylene oxide (PPO) alone and in combination with low pressure and/or elevated levels of carbon dioxide against eggs, larvae, pupae and adults of Ephestia cautella (Walker) (Lepidoptera: Pyralidae). All life stages of E. cautella were exposed to PPO alone and in combination with either a low pressure of 100 mm Hg or 92% CO2 for 4 h at 26 ± 1 °C and 65 ± 5% relative humidity (r.h.). The results indicated that PPO alone was moderately effective with concentration x time products (Ct products) required to obtain 99% mortality of the different life stages of E. cautella ranging between 66.1 and 536.2 mg h/L. The life stages differed in their susceptibility to PPO. Eggs were the most susceptible with a LC99 value of 16.5 mg/L, whereas pupae were the most tolerant with a LC99 value of 134.1 mg/L. Mortality of all life stages was extremely low (1.3-18.8%), when exposed to either the low pressure or 92% CO2 alone for 4 h. However, when low pressure or 92% CO2 were combined with PPO, the LC50 and LC99 values for all stages of E. cautella were notably reduced. Combination of PPO with low pressure or 92% CO2 caused reductions in the LC99 values for all life stages, compared with those exposed to PPO alone. These results indicated that low pressure (100 mm Hg, equals to 13,332 Pa or 0.13 atm) and 92% CO2 clearly increased the efficacy of PPO to E. cautella. By considering the overall insecticidal efficacy data, the combination of PPO with low pressure was, in all cases, more effective than the combination of PPO with CO2.