In this study, the effects of various secondary metabolites in the economically important plants on the pupal weight, the pupal total protein and the pupal total lipid contents of Hyphantria cunea larvae infected by Bacillus thuringiensis subsp. kurstaki were investigated. In order to find out their effects on the larvae, the phenolic compounds present in Morus alba (mulberry), Malus pumila (apple), Prunus dometica (plum), and Juglans regia (walnut) leaves samples, which are the most preferred by H. cunea and have economic importance, were determined by phytochemical methods. The changes observed in the pupae’s weights, total protein and lipid contents were related to the plant leaves’ chemical contents. Among the non-infected groups, the mulberry-fed group had the highest both pupal weight and the pupal protein content. The minimum amount of gallotannin, as well as the catechin and rutin contents, was present in the mulberry leaves. The minimum pupal weight and the pupal protein contents were obtained in the larvae fed by the apple leaves with the highest rosmarinic acid and protocatechuic acid. The pupal weights and the pupal total protein contents were decreased by the bacterial infection, while the pupal total lipid contents increased by the bacterial infection. As a result of this study, the effects of both plant secondary metabolites experimentally used and B. thuringiensis infection on the pupal parameters of H. cunea were shown to be statistically significant.