Assessing zinc tolerance in two butterfly species: consequences for conservation in polluted environments

Zinc is a widespread pollutant released from industrial combustion, automobile residue, and mining. Zinc accumulates in soils and mobilises into plant tissue where it may be consumed to potentially toxic levels by leaf feeding insects, including developing pollinators. While zinc tolerance thresholds have been previously assessed in insect pollinators, most observations are limited to model organisms and pest species. We lack understanding of zinc tolerance in insects of conservation concern. We assess dietary zinc tolerance of developing caterpillars from wild populations of the monarch butterfly (Danaus plexippus), a species of conservation concern, whose caterpillars are commonly exposed to elevated dietary zinc exposure in milkweed plants along roadsides. We contrast monarch zinc tolerance with that of cabbage white butterfly caterpillars (Pieris rapae), a non-native pest species. Tolerance was assessed by rearing caterpillars on artificial diets containing varying levels of zinc. Zinc reduced monarch survival at levels as low as 344 mg kg⁻¹ but positively impacted cabbage white survival at 227 and 738 mg kg⁻¹. Cabbage whites also displayed prolonged development time, smaller adult body size, and slower growth rate, consistent with the possibility that zinc tolerance had fitness costs. Our results support previous observations that heavy metal tolerance varies between species and highlight the importance of broadening our understanding of tolerance to insect species of conservation concern before drawing general conclusions about insect susceptibility to heavy metal pollution. Nonetheless, our results suggest that zinc pollution alone is unlikely a risk in developing roadside habitat for monarch butterflies.