Metals and metal isotopes in insect wings: Implications for diet, geolocation and pollution exposure

Anthropogenic activities are exposing insects to abnormal levels of toxic metals, with unknown implications for migratory insects. Simultaneously, metals and metal isotopes have become promising tools for the geolocation of migratory insects. Furthering our understanding of metal cycling in insect tissues is essential, both for the development of metals and metal isotopes as geolocation tools, and for assessing the toxicity risks of metals to insects. We conducted a diet-switching experiment on monarch butterflies ( Danaus plexippus ) with controlled larval and adult diets to evaluate the dietary and environmental sources of 23 metals and metalloids, strontium isotopes, and lead isotopes to insect wing tissues over a period of 8 weeks. Concentrations of Ca, Co, and Sb differed between the sexes. Ni and Zn bioaccumulated in the insect wing tissues over time, likely from the adult diet, while increases in Al, Cr, Cd, Cu, Fe, and Pb were likely from external sources (i.e., dust aerosols). Bioaccumulation of Pb in the monarch wings was confirmed by Pb isotopes to be from external anthropogenic sources, revealing the potential of Pb isotopes to become an indicator and tracer of metal pollution exposure along migratory paths. Concentrations of Ba, Cs, Mg, Na, Rb, Sr, Ti, Tl, and U appeared to be unaffected by dietary or environmental contamination and should be further developed for geolocation purposes. Strontium isotope ratios remained indicative of the larval diet, at least in males, supporting its potential as a geolocation tool. However, the difference in strontium isotope ratios between sexes, as well as the possibility of external contamination by wetting, requires further investigation. Our results demonstrate the complexity of metal cycling in insects and the need for further investigations, as well as the value of studying metals to develop new tools to quantify pollution exposure, metal toxicity and insect mobility.