The Douglas-fir Needle Midge (Contarinia pseudotsugae) is a serious pest of Douglas-fir Christmas trees in the Pacific Northwest. In this study, levels of monoterpenes were measured in needles of current (1999) and previous year’s (1998) growth from infested and uninfested branches of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) trees. Samples were taken at approximately bi-monthly intervals from May through October. Monoterpenes were identified and quantified using gas chromatography/mass spectroscopy. Nine monoterpenes occurred regularly in all trees. Beta-pinene was the most prevalent compound in all samples. Total terpenes were higher in 1998 age-class needles than the 1999 age-class needles in the early season. Camphene and bornyl acetate were higher in uninfested needles than in infested 1999 age-class needles. These two compounds are known to decrease budworm larval growth and may be important in influencing needle midge host choice.

Key words: Douglas-fir Needle Midge, Douglas-fir, gas chromatography, monoterpene