Effectiveness of short exposures of propylene oxide alone and in combination with low pressure or carbon dioxide against Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae)

Toxicity of the fumigant propylene oxide (PPO) alone and in combination with low pressure (100 mm Hg) or 92% CO 2 to all life stages of Tribolium castaneum using short exposure times (4 and 8 h) at 30°C was studied. Results indicated that PPO was moderately toxic with Ct products ranging from 120 to 608 mg h/l required to obtain complete mortality of the different life stages. A marked difference in susceptibility between life stages was recorded. Eggs were the most sensitive with a LD 99 value of 30.1 mg/l for 4 h, whereas pupae were the most tolerant with a LD 99 value of 146.5 mg/l. It was shown that an increase in exposure time from 4 to 8 h resulted in 23%, 42%, 48% and 47% reductions of LD 99 values for eggs, larvae, pupae and adults, respectively. There was no or very limited mortality of all stages except the egg (53% to 62%), when exposed to either 100 mm Hg or 92% CO 2 for 4 h. However, when 100 mm Hg or 92% CO 2 were combined with PPO, the LD 50 and LD 99 values for PPO in all stages except the egg were significantly reduced. Combinations of PPO with 100 mm Hg or 92% CO 2 produced equal reductions in the LD 99 value from 146.5 to about 22 mg/l for the most tolerant pupal stage. Both combinations also produced significant reductions in the LD 99 values for larvae and adults (6.3- to 6.6-fold) compared with those exposed to PPO alone. These results indicated that 100 mm Hg and 92% CO 2 each had a synergistic effect on the toxicity of PPO to T. castaneum. The combination of PPO with vacuum or CO 2 can thus provide a potential alternative to methyl bromide.