Correction for differences in volatility among olfactory stimuli and effect on EAG responses of Dioryctria abietivorella to plant volatiles

We describe a method using paraffin oil solutions and gas chromatography to measure and correct for differences in volatility among test compounds app lied to filter paper and to address the problem of minimizing solvent contr ibutions to EAG responses. To examine the effect of the volatility bias, we evaluated the EAG responses of female fir coneworm, Dioryctria abietivorel la, to five volatile plant compounds, using a new method to normalize EAG r esponses to account for the loss of antennal sensitivity that occurs over t ime. Stimuli were generated either from equimolar (uncorrected) solutions o r from corrected solutions that were adjusted to yield equimolar airborne c oncentrations in the air puffed over antennae. When uncorrected solutions w ere tested, the two most volatile compounds, (E)-2-hexenal and (E)-3-hexeny l acetate, elicited significantly larger EAG responses than three terpenes. When corrected concentrations were tested, the ranking of these responses changed: (E)-2-hexenal elicited significantly smaller EAGs than (-)-alpha-p inene, (-)-limonene, and (E)-3-hexenyl acetate. On the other hand, there wa s no effect on the ranking of EAG responses to the two monoterpenes and a s esquiterpene, (-)-trans-caryophyllene, relative to each other. Normalizatio n of EAG data did not affect the overall results (i.e., stimulus rankings) but did reduce their variance within preparations. The results show that wh en compounds with widely different volatilities are compared in olfactory b ioassays, the concentrations of test solutions should be adjusted to produc e emissions with equimolar airborne concentrations.