Seasonal climatic variables drive the dynamics of the pine processionary moth in Mediterranean pine forests

The pine processionary moth (PPM, Thaumetopoea pityocampa ) is a notable defoliator of conifers in the Mediterranean region. However, long-term and large-scale evidence is still needed to describe its temporal cycles, identify consistent spatial patterns, and quantify how seasonal climate constrains its dynamics. To address these aspects, we used harmonised plot-level data from three forest-damage monitoring networks in Spain and analysed 17,473 plot–year observations covering the period 2005–2022. We quantified occurrence as plot-level presence/absence and abundance as the within-plot proportion of infested trees. We then modelled infestation rate using generalized linear mixed models that included dominant stand species and seasonal climate predictors linked to key phases of the PPM life cycle, with random intercepts for plot and year. PPM was recorded in 30.3% of observations, showing strong interannual variability and a significant rise since 2016. Warmer winters and higher mean July temperatures increased infestation probability, whereas extreme late-summer heat (days above 34°C in September) and higher precipitation in March reduced infestation probability. Host species also influenced susceptibility: Pinus nigra showed the highest infestation rate, while P. halepensis and P. pinaster showed the lowest. Residual diagnostics indicated remaining spatial structure and plot-specific temporal dependence, which we captured best with an AR(1) correlation structure by plot. Overall, our results point to a seasonal balance between winter, spring, and summer constraints and highlight species-specific vulnerability patterns that could guide risk-based monitoring and adaptive management under climate change.