Threatening a keystone species: how climate and host phenology interact to trigger eruptive outbreaks of the invasive psyllid Acizzia uncatoides in a biodiversity hotspot

Abstract The invasive psyllid Acizzia uncatoides poses a critical threat to the biodiversity of La Réunion Island by colonizing Acacia heterophylla , a strictly endemic keystone tree. To unravel the drivers of this invasion, we analyzed a five-year time series (2017–2022) of psyllid abundance across an elevational gradient using Seasonal Autoregressive Integrated Moving Average (SARIMA/SARIMAX) modeling. The psyllid population displayed extreme seasonality and high-amplitude outbreaks (peaking at > 35,000 individuals per minute of vacuum sampling), consistent with a lack of effective top-down regulation in their introduced range, as confirmed by the absence of natural enemies documented at these sites (Baujeu et al. 2025). Spatially, infestation severity supported the Resource Concentration Hypothesis: monoculture-like stands at high altitudes sustained massive outbreaks, whereas structurally complex forests conferred significant associational resistance. Modeling identified host phenology, specifically the proportion of trees exhibiting green flush (young growing tissues), as the primary bottom-up driver, exhibiting a strict trophic synchronization. Conversely, abiotic factors played a secondary role; temperature imposed delayed negative feedback (2–3 month lag), likely by accelerating leaf sclerification rather than through direct thermal stress. These findings confirmed the high susceptibility of this insular plant host of Acizzia uncatoides and provided a crucial baseline for the implementation of classical biological control to mitigate the decline of this cloud forest ecosystem. While SARIMA models provided robust predictive tools for outbreak forecasting, SARIMAX models elucidated proximate ecological mechanisms. These findings underscore the urgent need for integrated management, including biological control, to protect this endemic keystone tree species.