Global environmental change exerts growing pressure on biodiversity. Anthropogenic climate and land use change are particularly important drivers of biodiversity loss. While their effects on biodiversity have been widely studied individually, interactions among them are poorly understood. Here, we investigate the effects of two common drivers of global change, increased temperature and nitrogen fertilization, on host-plant quality and herbivore performance in the butterfly Lycaena tityrus. We show that butterfly performance was positively affected by plants grown at increased temperatures and nitrogen fertilization, the latter being in line with the nitrogen limitation hypothesis. Effects were largely consistent across treatments; that is, nitrogen deposition and temperature did not interact strongly to affect herbivore performance. Overall, females suffered more strongly than males from poor host-plant quality. Our study demonstrates changes in host-plant quality caused by variation in temperature and nitrogen availability, which induced indirect effects in an herbivore. Such indirect effects of global environmental change are an important source of variation and should be considered in order to reduce uncertainties with regard to the effects of global change on biodiversity.

1. Anthropogenic climate change is a substantial threat to global biodiversity. It may affect insect herbivores directly and indirectly. Indirect effects are, among others, mediated by climate-change induced variation in host-plant quality. Although being potentially important, little is known on the significance of such indirect effects and on interactions among environmental stressors in plant-herbivore interactions.2. To simulate the potential impact of climate change, we investigated effects of host-plant temperature and soil moisture on herbivore performance in the tropical butterfly Bicyclus anynana under laboratory conditions.3. Maize grown at high temperatures or under wet conditions reduced herbivore performance, indicated by decreased body mass, storage reserves, phenoloxidase activity, and increased development time. Temperature and soil moisture acted largely independent of one another. Detrimental effects of the high plant temperature were restricted to males, indicating a higher vulnerability of this sex to environmental stress.4. In nature, B. anynana might be threatened by increasing temperatures during the wet season negatively affecting host-plant quality. Our study shows that herbivore performance can be substantially affected by indirect effects mediated through changes in host-plant quality, which deserves more attention in the current era of global climate change.

Anthropogenic climate change poses a substantial challenge to many organisms, to which they need to respond to avoid fitness reductions. Investigating responses to environmental change is particularly interesting in herbivores, as they are potentially affected by indirect effects mediated via variation in host‐plant quality. We here use the herbivorous insect Pieris napi to investigate geographic variation in the response to variation in food quality. We performed a common garden experiment using replicated populations from Germany and Italy, and manipulated host quality by growing host plants at different temperature and water regimes. We found that feeding on plants grown at a higher temperature generally diminished the performance of P. napi, evidenced by a prolonged development time and reduced larval growth rate, body mass, fat content, and phenoloxidase activity. Genotype by environment interactions (G × E) were present in several performance traits, indicating that Italian populations (1) respond more strongly to variation in host‐plant quality and (2) are more sensitive to poor food quality than German ones. This may reflect a cost of the rapid lifestyle found in Italian populations. Consequently, German populations may be more resilient against environmental perturbations and may perhaps even benefit from warmer temperatures, while Italian populations will likely suffer from the concomitantly reduced host‐plant quality. Our study thus exemplifies how investigating G × E may help to better understand the vulnerability of populations to climate change.

Biodiversity is globally under pressure, and the current decline in insect biomass and diversity is likely caused by human activities. Key drivers of biodiversity loss include agricultural intensification and anthropogenic climate change. Nevertheless, a thorough understanding of potential interactions between both factors and the mechanisms underlying insect declines, in general, is still lacking. Here, the authors investigate the combined effects of nitrogen fertilisation and drought, as applied to host plants, on the preference and performance of the butterfly Lycaena tityrus. Individuals performed best on plants having received medium nitrogen levels, while performance was reduced by either a lack of or strong fertilisation, the former potentially caused by nitrogen limitation and the latter by increased concentrations of toxic allelochemicals. Female oviposition preference though was positively related to nitrogen fertilisation, resulting in a mismatch between preference and offspring performance at high nitrogen levels. Plant drought stress additionally reduced herbivore performance, and females appeared to suffer more from low‐quality food than males. Our results indicate that increasing nitrogen fertilisation, as applied in intensive agriculture, may substantially reduce host‐plant quality for insect herbivores, which may be exaggerated in the course of climate change due to the more frequent occurrence of droughts. Our study thus contributes to a better understanding of the mechanisms underlying human‐driven insect declines in agricultural landscapes and beyond.