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Abstract Abundance of insect herbivores often depends on host plant suitability for their specialised immature stages. Suitability can be strongly influenced by both microclimate and the nutritional quality of the plants themselves. Where soil nitrogen is high, host plants tend to have high nutritional quality, but vigorous growth of surrounding vegetation reduces microclimatic temperatures. Thus, thermophilous insects may face a choice between host plants with optimal microclimates and those with optimal nutritional quality. We investigated how microclimate and nitrogen content influence oviposition choices by the declining Small Copper butterfly, Lycaena phlaeas , on its host plant, Rumex acetosa . We predicted that warmer plants would have lower nitrogen content and that butterflies would choose cooler, high‐nitrogen plants during warmer ambient conditions. Although warmer R. acetosa plants had lower nitrogen content, L. phlaeas consistently chose to lay eggs on plants in warm microclimates, implying a trade‐off between temperature and the nutritional quality of host plants. Patches of bare ground created by Talpa europaea (European Mole) near R. acetosa plants increased microclimatic temperatures and decoupled the negative correlation between nutritional quality and thermal suitability. Our results have implications for the conservation of thermophilous insect herbivores, especially close to their range margins and in the context of climate change. Rather than maximising host plant abundance or nutritional quality, management that creates suitable microclimatic conditions is likely to be critical. Our findings also suggest that, while nitrogen pollution may increase host plant nutritional quality, its negative impacts on microclimate will likely further reduce breeding habitat for L. phlaeas and other insects in grassland habitats.