During the increase in population density as well as during its decrease in the time of the development of outbreaks of some insect species (ermine moths and their parasites, the braconid wasp Macrocentrus grandii Goid. and its parasites) a definite level of their density in primary foci does not occur simultaneously on the whole territory. In the course of the last two decades the formation, and subsequently the disappearance too, of primary foci of mass reproduction of the species investigated in the Lithuanian SSR (USSR) proceeded from south-west and west to north-east and east. After the movement of a definite level of population density of the host in primary mass reproduction foci the movement and redistribution of a definite level of population density of its parasites will follow too. During the outbreak of some insect species its population density in any point is determined not only by the parameters of the population itself, but also by the influence of the dynamics of population density of the surrounding species, which are rather closely interrelated. There is a direct dependance between population density of the investigated insects in primary foci of their mass reproduction and rate of the movement of its definite level across the territory. A maximum movement rate for Yponomeuta evonymella and its prasites reached up to 160 km per year.