Polymorphism in Ontogenetic Duration in Dendrolimus (Lepidoptera: Lasiocampidae): Diapause, Developmental Variability, and the Role of Facultative Summer Diapause in the Siberian Moth

Representatives of the genus Dendrolimus are major conifer defoliators whose variable developmental duration complicates phenological forecasting, outbreak prediction, and pest-management timing. This review synthesizes evidence on polymorphism in ontogenetic duration in Dendrolimus and frames it at three levels: intrapopulation, interspecific, and spatial–geographical. We distinguish biological generation time, the interval from oviposition to adult emergence, from calendar span, the number of calendar years occupied by one generation; this distinction is essential for interpreting two- and three-calendar-year cycles. The evidence indicates that diapause is the main physiological mechanism generating alternative developmental trajectories, but its expression is shaped by interacting photoperiodic, thermal, trophic, density-dependent, and physiological factors. The strongest evidence for natural intrapopulation splitting is available for the Siberian moth in which fast and delayed trajectories appear associated with facultative summer diapause, or a diapause-like state. Key gaps remain concerning the induction cues, physiological status, molecular regulation, and field frequency of this state. We argue that models should incorporate delayed fractions, fast/delayed cohort ratios, and adult-emergence overlap to improve monitoring, forecasting, and control of Dendrolimus pests, especially under climate warming and during the early stages of outbreaks.