Levels of phenolic glycosides and protein influence the quality of aspen leaves to herbivorous insects, and vary in relation to genetic and environmental factors. This research was conducted to assess the independent and interactive effects of phenolic glycosides and protein on the performance and detoxication enzyme activities of gypsy moths, Lymantria dispar (L.), and forest tent caterpillars, Malacosoma disstria Hübner. We fed fourth-stadium larvae aspen leaves supplemented with 0, 2, or 4% (wet weight) phenolic glycosides and 0 or 5% (wet weight) casein. We measured stadium duration, growth and consumption rates, and food conversion efficiencies. In addition, we measured the activities of three midgut enzymes likely involved in the metabolism of phenolic glycosides: β-glucosidase, esterase, and glutathione transferase. Phenolic glycosides reduced performance of both insect species in terms of increased developmental time and decreased growth rates. Casein supplementation increased growth rates of gypsy moth larvae but slightly reduced growth rates of forest tent caterpillars. Phenolic glycosides and protein did not interactively influence stadium duration or growth rates. β-glucosidase activities declined for both insect species when reared on diets with phenolic glycosides. Esterase activities were induced by phenolic glycosides only in gypsy moths, whereas glutathione transferase activities were induced by phenolic glycosides in both species. Casein supplementation had little influence on enzyme activities, and phenolic glycosides and protein interactively affected only forest tent caterpillar esterase activity.