Dirk Maes

Host‐plant quality and ambient temperature are key environmental drivers of herbivorous insect performance, affecting growth, development, and survival. While temperature accelerates physiological processes in ectothermic insects, nutrient limitation in host plants can constrain insect herbivore fitness. Yet, few studies have examined how these two factors interact to shape insect herbivore performance. We experimentally quantified the combined effects of host‐plant stoichiometry and rearing temperature on the larval growth rate, development time, and adult heat tolerance of the Map butterfly ( Araschnia levana L. 1758). Caterpillars were reared on stinging nettle ( Urtica dioica L.) cultivated under three nutrient addition regimes differing in N:P ratios—to alter the nutrient stoichiometry of their host plants—and under two diurnally fluctuating temperature regimes simulating ambient (day–night: 21°C/12°C) and elevated (day–night: 24°C/16°C) conditions. Both temperature and host plant stoichiometry affected butterfly performance. Warming and a balanced stoichiometry enhanced growth and development; under these favourable conditions, caterpillars exhibited rapid growth, faster development, higher survival, and increased adult heat tolerance. A trade‐off between size and age at maturity—individuals either matured rapidly at smaller sizes or more slowly at larger sizes—was observed only under conditions that were favourable for growth, suggesting that developmental time was the main limiting factor under these conditions for the final butterfly body size. Our results thus demonstrate that host plant nutrient imbalances can mediate how insect herbivores prioritize between size and age at maturity under warming conditions, influencing thermal tolerance too. This suggests that eutrophication and climatic warming may jointly reshape the allocation strategies, performance, and resilience of insect herbivores in Anthropocene ecosystems. Read the free Plain Language Summary for this article on the Journal blog.

The conservation community sorely lacks a global indicator of change in insect populations. Given widespread insect declines, addressing this gap is key for conservation and policy targets. We suggest that butterfly monitoring programs can serve as the foundation for an effective global network of insect monitoring. To assess this potential, we bring together an international consortium and calculate a “Global Butterfly Index” using the Living Planet Index approach. Based on 10,386 population trends of 213 univoltine species, we found that overall declines in butterfly populations are predictable based on species traits. Our effort should pave the way for the development of a global network of butterfly population monitoring schemes. Since butterflies are the best monitored insects and have strong emotional value for the public, a global infrastructure for butterfly monitoring can be a flagship for insect conservation, informing policymaking and spurring societal transitions towards sustainable futures.

Aim: To examine how butterfly population trends respond to climate change and urbanisation at a continental scale, and whether responses differ between urban and rural environments. Location: 869 sites across 12 European countries, spanning six bioclimatic zones. Time Period: 1976–2021. Major Taxa Studied: Butterflies (Lepidoptera). Methods: We analysed long‐term monitoring data from > 8400 populations of 145 species representing a wide range of ecological and life‐history traits. Population trends were modelled in relation to climate variables (temperature, precipitation and aridity), urbanisation (built‐up surface), and their interactions with urban context (urban vs. rural) and species traits (trophic specialisation, body size, reproductive rate and thermal adaptation). Results: Climate warming and aridification were consistently linked to population declines in both rural and urban contexts, while precipitation effects varied by location and species. Urbanisation alone did not predict trends, but the urban–rural context strongly modulated species' responses to warming, indicating potential synergies between climate change and urbanisation. The stronger impact of warming in urban populations likely reflects elevated baseline temperatures and reduced habitat suitability and connectivity in urban landscapes, limiting thermal buffering. Species with colder thermal niches and lower reproductive rates were most vulnerable to warming, as warming exceeds the thermal optima of cold‐adapted species and lower reproductive rates limit their capacity to buffer climate‐driven population declines. Under aridification, which can reduce host‐plant availability, trophic specialists declined more in urban areas, whereas generalists unexpectedly declined more in rural sites, suggesting context‐dependent constraints under increasing water limitation. Main Conclusions: Our findings highlight the complex interplay between climate change, urban context, and species traits in driving population dynamics. Importantly, our results suggest that urbanisation generally amplifies the negative impact of climate change on insect population trends.

Invasive species can significantly alter ecosystems, often through competitive dominance and indirect ecological effects. The harlequin ladybird ( Harmonia axyridis ) has become a widespread invasive species that, after a period without natural enemies, became parasitized by different organisms in the invaded range, including the microfungus Hesperomyces harmoniae . These parasite–host interactions remain understudied. Here, we investigated how temperature and humidity influence parasitism of He. harmoniae on Ha. axyridis using controlled laboratory experiments. We tested different environmental conditions: low versus high relative humidity and low versus high temperature. We assessed parasite prevalence (proportion of infected ladybirds), rate of development (time to maturity of fungal thalli) and parasite load (average number of thalli per host). High humidity increased both parasite prevalence and rate of development. High temperature had a slight negative effect on both measures, whereas low humidity and low temperature had a strong negative effect. Hesperomyces harmoniae thrives best under moderately warm and humid conditions and its prevalence may be constrained in drier or cooler environments. Understanding the environmental factors influencing He. harmoniae dynamics provides insights into the role of abiotic conditions in shaping parasite–host interactions. Our findings have implications for the population ecology of Ha. axyridis in different climatic regions.

Species populations naturally fluctuate, yet long-term trend analysis can reveal patterns of success, decline, or stability under global change pressures. While responses to climate change are well-documented, its synergy with another major global driver, urbanization, remains understudied. Here, we analyzed long-term monitoring data from over 8,400 populations of 145 butterfly species across Europe, representing a high diversity of species traits, to assess population trends in response to climate change and urbanization. We examined how population responses vary between urban and rural contexts, providing insights into the influence of site-specific conditions. Climate warming was associated with population declines, which were more pronounced in urban areas. The effect of precipitation varied between environments: increases in precipitation generally benefited populations in rural areas but had detrimental effects in urban ones. Aridity consistently drove population declines across environments, with slightly stronger effects in urban areas. Species with colder climatic niches declined the most in response to warming, increased aridity, and reduced precipitation, while trophic specialists were particularly vulnerable to aridity and precipitation changes in urban environments. Although increasing urbanization did not explain overall population trends, its effects became evident when considering species traits, with certain traits being more vulnerable to urbanization. Specifically, species with narrow climatic niches declined the most in response to urbanization in rural areas, while those and larger body sizes decline the most in urban environments. Our findings highlight the complex interplay between environmental change, landscape context, and species traits in shaping biodiversity outcomes. Importantly, our results suggest that urbanization generally amplifies the impact of climate change on insect population trends.