Since the 1950s, a significant decrease in the population numbers of Salix lapponum L. has been observed in Poland, particularly in the Polesie Lubelskie region. One of the most important threats comes from the biotic environment including interactions at the trophic level. An unidentified issue was the occurrence and importance of phytophagous insects inhabiting S. lapponum in terms of real threats to growth and development as well as specific plant reactions to herbivores. The aim of the study was to recognize the occurrence of phytophagous insects inhabiting S. lapponum, i.e. an endangered species, and estimation of physiological changes caused by the herbivorous species feeding. The field study was conducted in a locality of S. lapponum population in the peat bog near Bikcze Lake (Polesie Lubelskie Region) in 2014–2015. In each growing season from the end of April to September, 100 stems of S. lapponum shoots with similar length were examined once a month. Selected parameters of chlorophyll a fluorescence were measured and the accumulation of H2O2 was visually detected in leaves collected from the control plants and from those attacked by herbivores using the 3',3-diaminobenzidine (DAB) method. We found the presence of 8 phytophagous insect species from 4 orders (Coleoptera, Diptera, Hemiptera and Lepidoptera) on S. lapponum. Most of the shoots were inhabited by Lochmea caprea, Aphrophora salicina and Aphis farinosa. On the basis of the measurements of chlorophyll a fluorescence and H2O2 accumulation under phytophagous insects feeding, we have demonstrated that S. lapponum was able to maintain photosynthesis under insect attack and that the accumulation of H2O2 in leaf blades indicates local rather than systemic stress response to herbivore feeding. Phytophagous insects feeding caused relatively weak disruptions to host plant physiology and S. lapponum is able to maintain photosynthetic efficiency under insect attack. Variation in plant traits influence host plant selection and performance of herbivorous insects. Comparative studies are needed to assess the species composition and population numbers of insects occurring on plants from other S. lapponum populations. Detailed knowledge of the biotic factors affecting the occurrence of S. lapponum would facilitate establishment of the form of its conservation.