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

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
prioritise 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.