Warmer temperatures make milkweed toxic for monarchs


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A monarch caterpillar scoots across a common milkweed leaf. (USFWSmidwest/flickr)

Jenna Ladd | April 6, 2018

A recent study published in the journal Ecology uncovered an unexpected consequence of climate change for monarch butterflies.

Researchers from Louisiana State University and University of Michigan set out to understand how warmer temperatures affect the relationship between monarch butterflies and milkweed plants. The insects, whose population has declined by more than 80 percent in the last decade, lay their eggs exclusively on milkweed plants. After the larvae hatch, they feed on certain species of milkweed, which provide protection to the butterflies. Milkweed plants produce chemicals called cardenolides in their leaf tissues, which are poisonous to most of the monarch’s predators. When monarchs consume the perfect amount of these chemicals as larvae, it sends a signal to larger predators to stay away from them.

However, scientists found that as regional temperatures rise, some species of milkweed plants produce more cardenolides. This poses a threat to the monarchs’ survival. One of the researchers, Dr. Bret Elderd, an associate professor at Louisiana State University, explained, “It’s a Goldilocks situation for monarch butterflies. Too few of these chemicals in the milkweed, and the plant won’t protect monarch caterpillars from being eating. But too high of a concentration of these chemicals can also hurt the monarchs, slowing caterpillar development and decreasing survival.”

One species of milkweed by the Latin name of A. curassavica has naturally high levels of cardenolides in its leaves and is especially sensitive to rising temperatures. Landscapers and environmentalists alike have been planting more of the nonnative plant to save the monarchs, but scientists warn that this plan may have backfired. They are working to spread the word that the native variety of milkweed, A. incarnata, has naturally lower levels of cardeolides and is much less likely to become toxic to monarchs as the climate warms.

The study reads, “It has become increasingly recognized that species interactions, especially interactions between tightly-linked species, need to be considered when trying to understand the full impacts of climate change on ecological dynamics.”

The full report can be found here.

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