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The larvae of ermine moths from the Yponomeutidae family (Lepidoptera) feed on a range of species and varieties of fruit and ornamental trees. Some species of this family pose a serious threat to the environment, mainly because of the significant defoliation they cause but also due to the widespread use of insecticides used to control them. This study was designed to assess the sensitivity of <i>Yponomeuta padella</i> and <i>Yponomeuta cagnagella</i> larvae and pupae to a native strain of <i>Steinernema feltiae</i> ZAG15 nematodes under laboratory conditions and to test the biological activity of these nematodes against the larvae and pupae of these species in field studies. The following doses were used in the laboratory tests: 50 IJs/insect (Petri dish tests) and 100 IJs/insect (container tests). Petri dish and container tests were performed at 20 °C and 60% humidity. Mortality of two stages (larvae and pupae) was determined 3 days after treatment. In the field trials, the nematodes were applied at the following doses: 4000 IJs/web for the caterpillars of <i>Y. padella</i> and <i>Y. cagnagella</i> and 1000 IJs/web for the pupae of <i>Y. padella</i> and <i>Y. cagnagella</i> (this corresponded to approximately 200 IJs/insect). Nematodes were applied using a 1 L hand sprayer and a lance. The efficacy of the application was assessed after seven days. The results of our study showed that the larvae (81.7%) and pupae (88.3%) of <i>Y. padella</i> had a greater susceptibility to entomopathogenic nematodes (EPNs) than those of <i>Y. cagnagella</i> (50% and 33.3%, respectively). However, our promising laboratory results did not translate into results in field trials, where the application of EPNs proved to be ineffective.