Thermal sum drives abundance and distribution range shift of Panolis flammea in Finland

Climate change has assisted the northward range expansion of various forest insect pests, increasing the risk of forest damage in Northern Europe. The pine beauty moth, Panolis flammea, is common in European pine stands with cyclical outbreaks that have damaged thousands of hectares. In the 1970s, its Finnish northern distribution limit was reported to be 64 degrees N. We re-evaluated the northernmost distribution limit of P. flammea in Finland and studied the effect of temperature sum on its abundance. We used pheromone-based traps throughout Finland in 2019-2020. The number of individuals caught in each trap was coupled with previous year thermal sums from the same location, and the relationship between them was analysed using a generalised linear mixed model. P. flammea had spread into northern Finland to 68 degrees 51'N, at similar to 10.2 km/year, 50 years ahead of earlier predictions. We found a non-linear correlation between thermal sums and P. flammea abundance, increasing from cold to warm areas. Given the climate change scenarios for Fennoscandia, its range and population could continue to increase, allowing it to become another widespread coniferous-forest insect pest. Understanding the effects of temperature on its populations will allow us to model their probable future distribution and population dynamics.