Using a stochastic phenology model to assess the potential northern range limit of Conotrachelus nenuphar in Europe under current and future climate

Technical Summary: Assessing the Potential Northern Range Limit of Conotrachelus nenuphar in Europe

Problem Statement
Conotrachelus nenuphar (plum curculio), a major pest of stone and pome fruits in North America, is classified as a priority quarantine pest under European Union (EU) plant health legislation. While broad assessments suggest the pest could establish wherever suitable hosts exist in Europe, these evaluations often rely on coarse climatic classifications (e.g., Köppen–Geiger) that fail to capture fine-scale thermal constraints. Specifically, the northern strain of C. nenuphar (adapted to colder North American climates) has an obligate winter diapause and must complete its entire life cycle within a single growing season. In northern Europe, where summers are cooler than in the pest's native northern range, there is uncertainty regarding whether thermal conditions are sufficient for life cycle completion. Current EU regulations require strict surveillance for priority pests unless it can be unequivocally demonstrated that establishment is impossible due to unsuitable ecoclimatic conditions. A more precise assessment is needed to determine where official surveillance is warranted and where resources might be allocated more efficiently.

Methodology
The study developed a stochastic phenology model to simulate the temperature-dependent development of the northern strain of C. nenuphar on sour cherry (Prunus cerasus), selected as a host due to its wide distribution and the availability of specific thermal development data. Unlike traditional Growing Degree Day (GDD) models that use mean values, this model incorporates intraspecific variation by representing thermal requirements as probability distributions derived from empirical literature.

The model simulates four sequential developmental steps:

1. Spring Emergence and Immigration: Simulates the accumulation of GDD (threshold 6.1°C) required for overwintered adults to emerge and migrate to host trees, followed by the timing of petal fall (GDD4) and the subsequent thermal requirement for oviposition readiness (GDD10).
2. Larval Development: Simulates egg hatching and larval development within the fruit using GDD (thresholds 11.1°C to 35°C), accounting for a non-linear development function at high temperatures.
3. Pupation: Simulates soil pupation and adult emergence using GDD (threshold 8.7°C).
4. Overwintering Readiness: Simulates the post-emergence feeding period required to accumulate energy reserves for diapause. Due to a lack of specific data for C. nenuphar, this step was modeled using a proxy species (Anthonomus grandis) and tested under three scenarios ("fast," "intermediate," and "slow") representing different durations of days with mean temperatures ≥13°C.

The model was implemented in R with 500 stochastic iterations per location to capture population variability. It was first validated against the pest's native range in North America (1940–1959) using ERA5 climate data. Subsequently, it was applied to central and northern Europe under:

• Current Climate: ERA5 data for 2006–2025.
• Future Climate: A multi-model ensemble of eight CMIP6 models under the SSP2-4.5 scenario for 2041–2060.

Performance was evaluated using two metrics: the mean proportion of iterations completing the life cycle and the proportion of years (or model-years) in which at least one iteration succeeded.

Key Results

• Model Validation: In the native range, the model successfully reproduced the historical distribution, showing high life cycle completion proportions inside the range and a sharp decline outside it. Sensitivity analysis indicated that while overwintering readiness scenarios influenced results near the range limit, the model robustly distinguished suitable from unsuitable areas.
• Current Climate (2006–2025): The model indicates that central Europe is thermally suitable for the northern strain. However, significant portions of northern Europe are unsuitable. Specifically, Finland, most of Ireland and Sweden, and parts of the Baltic region showed low mean life cycle completion proportions and multiple years with zero successful completions, suggesting a very low likelihood of establishment. A transition zone exists in the southern UK, northern Germany, Poland, and southern Scandinavia where suitability is marginal and highly dependent on the overwintering readiness scenario.
• Future Climate (2041–2060): Projections under SSP2-4.5 show a northward shift in the transition zone, extending suitability into southern Finland, southern Sweden, the Baltic states, and parts of Ireland. However, even under these future conditions, the northernmost EU regions (e.g., most of Finland) remain thermally unsuitable for reliable establishment, with a large proportion of individuals failing to complete the life cycle and several model-years showing no successful completion.

Significance and Claims
The paper claims to provide the first detailed assessment of the establishment potential of C. nenuphar in Europe under future climate conditions using a stochastic framework that accounts for intraspecific variation. The authors argue that their results challenge the broad conclusion of previous EU-level assessments (e.g., EFSA 2018a, 2019, 2025) which suggested climatic conditions would not limit establishment. By demonstrating that specific northern regions are thermally insufficient for the completion of the life cycle, the study supports the legal provision in EU plant health legislation that allows Member States to exempt themselves from certain surveillance obligations if establishment is proven impossible due to ecoclimatic constraints.

The authors position the study as a tool to support risk managers in optimizing the allocation of official plant health resources. They explicitly state that while the model suggests surveillance may be less critical in the identified unsuitable northern regions, these findings should be interpreted with caution regarding uncertainties, particularly the lack of empirical data on overwintering readiness and the exclusion of other environmental factors like soil moisture and photoperiod. The study does not claim to definitively prove the pest cannot establish, but rather provides a probabilistic assessment indicating that establishment is unlikely in specific northern areas under both current and mid-century projected climates.