none It is shown that concordance degree of seasonal cycles of some parasites of Diprion pini and of the host itself depends on the population density of the latter. This dependence shows the regulatory role of the seasonal population cyclicity of the host and parasites in their numbers' dynamics. To demonstrate this dependence the quantitative criterion of the concordance degree mentioned above is proposed. This criterion is used for the first time to the insects which have simple seasonal cycles as well as complex ones, when the parasite has several reproductive waves while the host has several waves of the sensibility to the infestation during the season. During the period of mass reproduction the seasonal cycle of D. pini reconstructs itself from the mostly single-generation to the mostly two-generation one. As the result the index of concordance grows in parasite Agrothereutes adustus and declines in parasits Chrysonotomyia ruforum, Exenterus oriolus and Pleolophus basizonus. Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Ecology; Population dynamics; Land and freshwater zones; Palaearctic region; USSR Tachinidae; Hymenopteran hosts; Diprion pini; Host parasite seasonal population cycle relationships; Host parasite relationships; Diprion pini (Hymenoptera), seasonal cycle coordination; Seasonal abundance; Cycle coordination with hymenopteran hosts; Russian SFSR; Rostov region; Seasonal cycles Diprion pini (Diprionidae); Dipteran parasites; Tachinidae; Hymenopteran parasites; Braconidae & Ichneumonidae; Host parasite seasonal population cycle relationships; Host parasite relationships; Diptera & Hymenoptera, seasonal cycles coordination; Population density; Dipteran & hymenopteran parasite phenology, effect on; Russian SFSR; Rostov region; Seasonal cycles, dipteran & hymenopteran parasites Braconidae; Ichneumonidae; Hymenopteran hosts; Host parasite seasonal population cycle relationships; Host parasite relationships; Diprion pini, seasonal cycle coordination; Seasonal abundance; Cycle coordination with Diprion pini; Russian SFSR; Rostov region; Seasonal cycles in Diprion pini