The chemical composition of the essential oils (EOs) of Thuja occidentalis L. (Cupressaceae) and Tanacetum vulgare L. (Asteraceae), obtained by steam distillation, and their insecticidal activity against larvae (10 and 20 days old) of the lesser mealworm, Alphitobius diaperinus Panzer (Coleoptera: Tenebrionidae), were studied. The activity of the oils was compared with the activity of their major components, aand b-thujone. Isolation, identification, and quantification of the volatile compounds were performed using a gas chromatograph (GC) coupled to a mass spectrometer (MS). The major components of T. occidentalis EO were the monoterpenoid ketones a-thujone (69.8%), b-thujone (9.5%), and fenchone (7.8%). The monoterpenoid alcohols terpinene-4-ol and p-cymene-8-ol, and the diterpenoid beyerene were only present in low amounts (<3.0%). All other compounds occurred in traces. In contrast to the thuja oil, the main compounds found in the EO of T. vulgare were b-thujone (61.0%), camphor (13.0%), and trans-chrysanthenyl acetate (6.5%). The insecticidal activity of both EOs and pure monoterpenes incorporated into a diet as acetone solutions at a concentration of 10 mg ml À1 depended on the age of A. diaperinus larvae. Young larvae displayed a higher sensitivity toward the tested chemicals than older ones. The survival of 10-day-old larvae, their body mass gain, and pupation rate were affected. The growth and development of the older larvae were similar to those of the control. In all tests, there were no significant differences between the activities of the EOs and their major components.