Butterfly host plant use driven by microclimate not nutritional quality

The 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 European Moles (Talpa
europaea) 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.