Monarchs don’t much care for heavy metal!

Heavy metals are found naturally in the Earth’s crust. However, human activity stirs up an important amount of these metals into the environment. Most organisms are now exposed to much higher concentrations than those encountered in their evolutionary history. But what’s the story with the monarch? Does it risk being exposed to these heavy metals, and above all, it is sensitive to those pollutants?

Where are heavy metals found?

Originating from different sources (mining sites, areas of heavy traffic, runoff of pesticides used in agriculture…), heavy metals are well-known toxic contaminants in aquatic and terrestrial ecosystems. Studies have shown that certain heavy metals like cadmium, nickel and zinc can accumulate in the soil and be absorbed by plants. Those metals are then transported to their tissues and their nectar.¹

When we think of the monarch butterfly, there’s reason for worry. On the one hand, the monarch eats milkweed leaves in the caterpillar stage and feeds on nectar in the adult stage. It can therefore be exposed to these pollutants throughout its development. In addition, roadsides and the borders of fields, generally contaminated by heavy metals, are also environments targeted by conservation efforts designed to help the monarch.²

What are the dangers for the monarch?

Most studies focus essentially on model organisms or agricultural pests, never on species targeted by conservation efforts, like the monarch. Researchers in the United States therefore carried out a laboratory study on the monarch’s tolerance to zinc, a heavy metal common along roads where the butterfly is found.¹ The results showed that caterpillars’ survival rate for this butterfly drop when they’re exposed to concentrations of zinc greater than 344 mg/kg. The researchers conclude that the monarch is indeed sensitive to high zinc concentrations, but that it’s unlikely that the presence of zinc along the roads has a negative impact on the survival of the species all by itself, since the concentrations recorded in that type of habitat are not that high.³

Good news for the monarch, but we still have to be cautious. The experiment took place in a controlled environment where the monarchs were isolated from other stresses normally encountered in a natural setting (other pollutants, food stress, climatic stress, and so on). It cannot therefore be ruled out that zinc contaminations paired with other forms of stress may have an adverse effect on the monarch’s development.

Finding the right sites to protect in order to help the monarch butterfly is no simple matter. That’s why we need you so much. Continue sharing your observations on Mission Monarch, because it’s in working together that we’ll succeed in helping the monarch!

References:

1. Shephard, A.M., Mitchell, T.S., Henry, S.B., Oberhauser, K.S., Kobiela, M.E. & Snell‐Rood, E.C. (2020). Assessing zinc tolerance in two butterfly species: consequences for conservation in polluted environments. Insect Conservation and Diversity, 13, 201–210.
2. Thogmartin, W.E., López-Hoffman, L., Rohweder, J., Diffendorfer, J., Drum, R., Semmens, D., et al. (2017). Restoring monarch butterfly habitat in the Midwestern US: ‘all hands on deck.’ Environmental Research Letters, 12, 074005.
3. Mitchell, T.S., Agnew, L., Meyer, R., Sikkink, K.L., Oberhauser, K.S., Borer, E.T., et al. (2020). Traffic influences nutritional quality of roadside plants for monarch caterpillars. Science of The Total Environment, 724, 138045.