Vapor pressure and volatility of β-asarone,the main ingredient of an indigenous stored-product insecticide,Acorus calamus oil

The effect ofAcorus calamus products on insects is reviewed briefly. A sudden, dramatic drop occurs in the insecticidal, gaseous phase activity of calamus oil on stored-product pests, when the exposure temperature is lowered from 30° to 25°C. In an attempt to explain this finding, the vapor pressure of the main constituent of IndianAcorus calamus essential oil, β-asarone, was determined by three methods: (i) extrapolation with a gas-chromatographic method according to Jensen and Schall; (ii) calculation from boiling points with the Clausius-Clapeyron equation; and (iii) direct measurement with the Knudsen cell. The values of vapor pressure of β-asarone at 30°C according to these three methods were 3.3 x 10^-3, 3.7 x 10^-3 and 9.8 x 10^-4 Torr, respectively. The volatility of β-asarone under the conditions of the fumigant experiments mentioned above was assayed by determining the weight loss from 50 mg asarone allowed to evaporate at 30°C from a filter paper disk into a 400 ml hermetically closed glass vessel. The weight loss was approximately 0.75 mg/400 ml receptacle space (≈1.9 mg/l) for every 10 days from day 0 to day 40.