Host plant nitrogen enrichment has both positive and negative effects on the larval growth of a specialist butterfly

1. The nitrogen limitation hypothesis posits that phytophagous insects benefit from nitrogen enrichment of their host plants through a reduction of the concentration of toxic compounds and an increase of free amino acids and proteins. However, species' response to nitrogen enrichment varies substantially and high nitrogen levels are associated with population decline, suggesting there are major costs to feeding on nitrogen‐rich host plants. 2. To test the hypothesis that larval growth performance is maximal at intermediate nitrogen enrichment, nitrogen levels were measured in 18 populations of the host plant of Lycaena helle, a specialist butterfly inhabiting nutrient‐poor wet meadows. The nitrogen content of host plants was then modified to mirror average natural nitrogen levels (C), highest field‐recorded levels (T1), and levels higher than those observed across our study populations (T2). 3. Caterpillars fed with T1 leaves had a greater maximum body mass than caterpillars of the C group because of their improved food assimilation during the early stages of their development. Caterpillars of C and T2 groups had similar growth patterns but high nitrogen content had detrimental effects, as caterpillars fed with T2 leaves had a slower ingestion rate than C and T1 groups. 4. Quantifying the fitness consequences of these changes in growth performance is necessary to fully understand the implications of nitrogen enrichment for L. helle (rapid growth may result in fitness costs). However, conservation plans for this emblematic glacial relict species should also consider the preservation of its host plant quality to ensure its persistence.