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

1. Zinc is a widespread pollutant released from industrial combustion, auto- mobile 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.

2. While zinc tolerance thresholds have been previously assessed in insect pollinators, most observations are limited to model organisms and pest species. We lack understand- ing of zinc tolerance in insects of conservation concern.

3. 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 varyin...