Knowledge of the factors affecting host selection by herbivorous insects is essential to predictions of their distribution and abundance over landscapes. In the laboratory, we studied the oviposition preferences of two eruptive loopers (Lepidoptera: Geometridae) native to western Canada (western hemlock looper,
Lambdina fiscellaria lugubrosa(Hulst), and phantom hemlock looper, Nepytia phantasmaria(Strecker)) for different species and condition of hosts. When offered a choice, phantom hemlock loopers laid nearly three times as many eggs on western hemlock, Tsuga heterophylla(Raf.) Sarg. (Pinaceae) as on either western redcedar, Thuja plicataDonn ex D. Don (Cupressaceae) or western white pine, Pinus monticolaDouglas ex D. Don (Pinaceae). Western hemlock loopers were less specific, laying equal numbers of eggs on western hemlock and western redcedar, though fewer eggs were deposited on western white pine when compared with western hemlock. When offered western hemlock trees grown under different nutrient and shading regimes, phantom hemlock loopers preferred to oviposit on high nutrient hosts, irrespective of shading; western hemlock loopers exhibited a preference for high-nutrient hosts only if they were grown without shading. These patterns of host preference can be combined with information regarding forest composition to help quantify the conditional probability of western and phantom hemlock looper distibution and abundance.