Among the factors contributing to the initiation of outbreaks of the Siberian moth, a dangerous pest of the coniferous forests of Northern Eurasia, it is considered important to reduce the regulatory impact of entomophages. One of the most effective regulators of the pest abundance is the egg parasitoid wasp Telenomus tetratomus (Thomson, 1861). There is an established opinion that T. tetratomus is less cold-resistant than caterpillars of the Siberian moth, and outbreaks of the pest are provoked by cold winters, during which conditions (low temperatures and low snow cover depth) are tolerated by the moth caterpillars but lead to death of the parasitoid. However, the lethal temperature for T. tetratomus was determined in an insufficiently controlled experiment more than 60 years ago. We evaluated one of the cold hardiness characteristics of T. tetratomus, the supercooling point (SCP), which had not been measured previously. Both the host and the parasitoid overwinter in a supercooled state, and freezing is lethal to both. The mean SCP of T. tetratomus (-21.2±0.2 °C) was found to be 6.3 °C lower than the SCP of Siberian moth caterpillars. Comparison of SCP distributions and mortality rates at several temperatures allows us to tentatively estimate 50% mortality temperature of the wasp – about -16 °C. The obtained result, analysis of temperatures in the litter of various types of coniferous forests, and published data indicate that the asynchronous mortality of the host and its main parasitoid during wintering, due to differences in cold hardiness, can be considered as the cause of outbreaks of the Siberian moth mass reproduction only with caution.