Tree susceptibility and suitability for herbivorous insects depends upon a wide array of chemical compounds including potential toxins such as monoterpenes. Silvicultural techniques such as thinning and fertilization may change the concentration of these compounds within tree tissue foliage. The relative toxicities to Douglas-fir tussock moth larvae of five monoterpenes commonly present in host foliage were determined in laboratory assays. Of the five monoterpenes tested, limonene and γ-terpinene were significantly more toxic to the larvae then β-pinene, 3-carene or α-pinene. We also examined the effect of three fertilization treatments applied to previously thinned stands on the concentration of foliar monoterpenes one year following stand thinning. No significant differences were detected among treatments in the concentration of individual or total monoterpenes present in foliage. The effect of stand thinning may have overwhelmed any short-term fertilization impacts.