Climate change is driving significant shifts in the geographical ranges of numerous species, including crop pests and diseases. Over the past six decades, a poleward movement of these organisms has been observed, largely attributed to rising temperatures and global trade. The response of different species, however, varies considerably depending on their geographic origins. Tropical species, often operating near their thermal limits, are predicted to experience habitat reduction under warming conditions. In contrast, temperate species, with greater climate tolerance, are expected to benefit from climate change and expand their ranges. This study focuses on Nearctic leafhoppers (Hemiptera: Cicadellidae), a diverse group of herbivorous insects that serve as vectors for phytoplasmas, bacterial pathogens responsible for a wide array of plant diseases. While considerable research exists on the impact of climate change on Lepidoptera and Diptera, the effects on Hemiptera remain understudied, despite their significant role in plant disease transmission. Nearctic leafhoppers are particularly relevant because of their high diversity and adaptation to diverse climatic conditions across North America. Their adaptability and the potential impact on phytoplasma diseases warrant a focused investigation into how climate change may alter their distribution and abundance. This research aims to address this knowledge gap by employing ecological niche modeling (ENM) techniques to predict the potential impact of climate change on leafhopper species richness and environmental niche similarity.

Existing research highlights the significant impact of climate change on species distribution and abundance. Studies have demonstrated the poleward shift of crop pests and pathogens, with an average rate of approximately 2.7 km per year since 1960. This trend is mainly driven by rising temperatures and increased global trade. The response of insects to climate change is complex and influenced by factors such as their geographical origin and thermal tolerance. Tropical species, generally characterized by narrower thermal tolerances, are expected to experience habitat contraction, while temperate species may show range expansion. Species distribution modeling (SDM) or ecological niche modeling (ENM) has emerged as a powerful tool for assessing the impacts of climate change on species distribution. While SDM has been widely applied in entomology, studies have been predominantly focused on Lepidoptera and Diptera, neglecting other important insect orders like Hemiptera. Leafhoppers, a significant component of Hemiptera, are important plant disease vectors, transmitting phytoplasmas responsible for over 600 plant diseases worldwide. Nearctic leafhoppers, specifically, exhibit high endemism and diversity, adapted to a wide range of climatic conditions across North America. Increased temperatures are predicted to favor their population growth by shortening their life cycles and facilitating dispersal through continental winds. However, a considerable research gap exists concerning the specific effects of climate change on the distribution of Nearctic leafhoppers and the subsequent impact on phytoplasma disease transmission.

This study employed ENM-based methods to assess the potential impact of climate change on the distribution and species richness of 14 Nearctic leafhopper species associated with phytoplasmas in North America. The MaxEnt species distribution algorithm was used to model species distributions under current and future climate scenarios. Fourteen leafhopper species were selected based on their confirmed, potential, or suspected status as phytoplasma vectors. Occurrence records for these species were obtained from the Global Biodiversity Information Facility (GBIF) and other sources. Records were cleaned to remove duplicates and spatially thinned to reduce bias. Nineteen WorldClim bioclimatic variables (v. 2.1) covering the period 1970-2000 were used for the current climate model. To account for multicollinearity, a Pearson correlation analysis was performed, resulting in the selection of five variables: annual mean temperature, mean temperature diurnal range, temperature annual range, annual precipitation, and precipitation seasonality. Future climate scenarios were obtained from the GISS-E2-1-G global circulation model under four Shared Socioeconomic Pathways (SSPs): SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5 for the period 2041-2060. MaxEnt models were developed using 10,000 background points, employing a 'clamping' function to prevent extrapolation beyond the environmental range. Model performance was evaluated using the Area Under the Receiver Operating Characteristic Curve (AUC) and the True Skill Statistic (TSS). The best models were selected based on AUC and TSS values. Species richness under current and future scenarios was determined by overlaying binary distribution maps for each species and calculating the sum of unique species per pixel. Environmental niche similarity was assessed using principal component analysis (PCA) and similarity tests, comparing the environmental conditions where each pair of species occurs. A network analysis was conducted to visualize and quantify the environmental similarity among species under various scenarios. The wallace shiny app, SDMtoolbox, ArcGIS Pro, and R packages were used for the analyses.

The MaxEnt models exhibited high predictive performance, with AUC values ranging from 0.883 to 0.980 and TSS values from 0.629 to 0.864. Temperature-related variables were the most important predictors for most species, although precipitation also played a role for some species. Current species richness projections suggest the highest richness in eastern North America, particularly in the region encompassing southern Ontario and Quebec, with lower richness in drier and colder regions. Future projections across all four SSP scenarios (SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5) indicate an overall increase in suitable habitat and species richness, especially in northern Canada. The eastern region of North America is still projected as the region with highest potential species richness under all climate scenarios, although a gradual eastward expansion is observed. Network analysis revealed high environmental niche similarity among most leafhopper species under current conditions (density = 0.467). This similarity tended to increase across all four future scenarios, indicating a greater degree of interconnectedness between species and potentially more overlap in their habitats. Specifically, *Colladonus geminatus*, *Edwardsiana rosae*, and *Exitianus exitiosus* showed lower environmental similarity compared to other species, with *Exitianus exitiosus* remaining less similar even under future climate scenarios. The change in suitable area for leafhopper species ranged from a 2% decrease to a 27% increase depending on species and scenario.

The study's findings demonstrate the potential for both persistence and northward range expansion of Nearctic leafhoppers under future climate scenarios. The significant influence of temperature on leafhopper distribution aligns with previous observations of large-scale dispersal events triggered by warm periods. The high environmental niche similarity among most species suggests a degree of ecological redundancy and potential for increased species overlap in their habitats. The less similar species may have more specialized habitat requirements. While this study focuses on abiotic factors, biotic interactions (e.g., natural enemies, host availability) could significantly influence the realized niche of each leafhopper species. The observed increase in environmental niche similarity under future scenarios could be interpreted as increased competition, however, the study does not allow to draw such conclusions. The northward range expansion poses challenges for pest management, necessitating the development of new strategies to adapt to changing distributions and potential increases in pest abundance and disease transmission.

This study provides valuable insights into the potential impacts of climate change on Nearctic leafhoppers and their role in phytoplasma disease transmission. The findings highlight the need for proactive pest management strategies to address the potential for increased pest pressure in northern regions. Future research should incorporate biotic factors, investigate insecticide resistance in these species, and explore new approaches such as RNAi technology and biocontrol to mitigate the impact of climate change on leafhopper populations and the diseases they transmit. Standardized sampling methodologies across different regions are needed to better understand changes in species composition and overlap.

This study primarily focuses on abiotic factors and does not fully account for biotic interactions that might influence leafhopper distribution and abundance. The use of presence-only data and limitations in the availability of historical records for some species may have influenced the model's accuracy. The lack of standardized sampling methods across previous surveys limits the comparison of results with prior studies. Distinguishing between taxa within species complexes presents a challenge, potentially impacting the accuracy of absence records. Finally, the actual distribution of many Nearctic leafhoppers remains understudied, which might lead to underestimation or overestimation of current and future richness.