We examined essential oils (EOs) of Chamaecyparis lawsoniana and Thuja plicata (Cupressaceae), their antifeedant activity on Lymanthria dispar larvae and their antimicrobial activity. Studies of EOs showed that these two conifer species differed both in content of terpene classes and the major compounds. Total monoterpenes strongly dominated in T. plicata (96.4%), while C. lawsoniana was rich in both mono- and sesquiterpenes (40.8% and 30.3%, respectively) as well as in diterpenes (19.1%). The most dominant compounds of C. lawsoniana EO were: limonene (16.7%), oplopanonyl acetate (14.5%), beyerene (10.1%), and 13-epi-dolabradiene (6.7%). The dominant compound of T. plicata EOs was α-thujone (76.9%), followed by relatively small amounts of β- thujone (5.3%), sabinene (4.5%) and terpinene-4-ol (3.2%). The difference in EO compositions of the conifers was reflected on Lymantria dispar larvae performance. Larvae fed on the leaf discs treated by C. lawsoniana EO had a slight phagostimulatory effect at lower concentration shown by higher relative rate of food consumption and relative growth rate than the larvae in the control group. Contrastingly, leaf discs treated with EO of T. plicata EO had an antifeedant effect and lower relative consumption rate (RCR) and relative growth rate RGR than the larvae in the control group. Both tested EOs influenced substantially the colony growth of the subjected Phytophthora plurivora and P. quercina. Namely, 100% inhibitory effect was recorded at concentration of 0.1% in the case of C. lawsoniana EO, whereas of T. plicata colonies did not grow at 0.5% concentration. The implications of these findings and possibility of using the tested EOs in further experiments in vitro and in vivo are discussed.