It has been developed a generalization of literature data, personal communications of scholars, and own findings that has resulted in a theory describing the phenomenon of stability of terrestrial ecosystems to plant pests - herbivores and phytopathogens. The stability is proceeded by a number of factors suppressing all the diversity plant pest species inhabiting of a given ecosystem. It has been shown stressors, which decrease ecosystem stability. The generalization is based on the Ch.Darwin's regularities of the evolution and the Ch.Darwin's ideas on population dynamics of organisms. It has been proposed practical implications to increase the stability against separate species of plant pests, and recommendations on maintenance of general stability of ecosystems against abiotic and biotic stressors. The text is presented as a list of generalizations - axioms and massif of data for substantiation of these axioms. The used approach has allowed developing a new system of knowledge- Natelogy, which is the first branch of science established in XXI century.