High nitrogen loading impacts the temperature‐size rule and heat tolerance in a nettle‐feeding butterfly

Abstract 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.