The western tent caterpillar, Malacosoma californicum pluviale (Dyar) was treated externally with farnesyl methyl ether and the crude ether extracts of two forest trees, Western Red Cedar (Thuja plicata) and Douglas-Fir (Pseudotsuga menziesii). Within 24 hr of the intervening molts, late fourth- and early fifth-instar larvae weighing 178 mg received 55 μg, and late fifth-instar larvae and new pupae weighing 450 mg received 140 μg in 0.6 μl of acetone. Controls received only acetone.Treated groups suffered increased mortality due to structural abnormalities, molting difficulties, and increased infection with polyhedral virus and microsporidia. Many surviving adults also were structurally abnormal, and the proportion of females depositing normal egg masses was reduced. The progeny were less viable than those of the controls. Reductions in the viability of progeny were only slightly affected by time of treatment, but egg-laying behavior was more affected by later than by earlier treatments.All materials produced examples of juvenility, but many of the abnormal individuals produced by cedar treatment died early in the pupal stage, whereas those induced by the other treatments lived longer. In most instances, samples were too small to indicate statistical superiority of any one material but, as received, the cedar extract seemed more potent than the fir. Farnesyl methyl ether had the greatest influence on reproductive capacity.