Abstract <p>The nature of changes in the state of heat supply was studied based on empirical and statistical analysis of meteorological observation data. In the Arkhangelsk and Leningrad regions and the Republic of Karelia (northwest of the European part of Russia—NWER), the dynamics of average temperatures during the growing season have similar trends of a steady increase in the average temperature during the growing season in 1960–2021. This trend has been especially pronounced since the late 1990s. The indicators are stably correlated with each other for all regions. This allowed using the averaged model to analyze the relationship between temperature and biological processes in the region under consideration. The analysis included the construction of simple second-order deterministic models of temperature dynamics and the method of decomposition into empirical modes, which made it possible to identify low-frequency temperature fluctuations along the averaged curve based on local regression using the weighted linear least squares method and a second-degree polynomial model. Despite the increase in heat supply, a sharp decrease in the frequency of outbreaks of pine sawflies was established in 1998–2003, and then their complete absence in all subjects of NWER included Arkhangelsk, Leningrad, Novgorod and Pskov regions, the republics of Karelia and Komi. The dynamics of outbreaks of pine sawflies is not associated with changes in heat supply. The possible absence of outbreaks in the 2000s of sawflies may be due to changes in the volume and structure of the food supply and subjective factors.</p>