The monarch butterfly is an iconic species because of its remarkable natural history. The eastern North American population of monarchs undergoes an incredible migratory journey over multiple generations. Adults overwinter at a few isolated mountains in one area in Mexico, where they gather in the millions, completely covering the trees. As spring approaches, the butterflies fly north to Texas and the Gulf Coast states, breed, lay their eggs, and die. The caterpillars that hatch from these eggs eventually develop into new adult butterflies, which fly farther north to breed, lay their eggs, and then die. The next generation then matures and flies even farther north, where the process repeats. As many as six generations can breed within a single summer. The butterflies track the spring emergence and growth of milkweed plants, the only food source for monarch caterpillars. Milkweeds are perennials, so their roots remain alive year-round, but they grow new stems, leaves, and flowers each summer.

Monarchs have developed a close relationship with milkweeds that bypasses the plants’ defense mechanisms against herbivores. Milkweeds produce toxic steroids called cardenolides in their sap that, when ingested, bind with cardiac muscle and stop insect hearts from beating, killing them. Monarchs, however, evolved the ability to mainly prevent these cardenolides from binding to their cardiac muscle, allowing them to consume milkweed plants without harm. They accumulate toxins in their tissues, effectively stealing the chemical defense of their food plant to protect themselves from predators.

Milkweeds engage in a fascinating yet one-sided relationship with Monarch butterflies. While the vibrant flowers attract these striking insects, offering them a source of nectar, the milkweeds themselves gain no tangible benefit from the interaction. Monarchs rely on milkweed not only for nourishment but also as a critical habitat for laying their eggs; yet, the plants receive no reward in return for their role in this ecological partnership. Despite this imbalance, the presence of Monarch butterflies contributes to the rich biodiversity of the environment, highlighting the intricate connections within ecosystems. Still, they are too big to be pollinated by bees, so milkweed pollination has to be accomplished by beetles.

As summer wanes, the last generation of butterflies begins to fly south. Despite being several generations removed from their ancestors that overwintered in Mexico, and having never made the trip before, these butterflies manage to fly back to those few isolated mountains in Mexico for the winter. They won’t breed until the following spring, when they fly North again to Texas, and the process starts all over again.

Since 1993, there has been a loss of many hundreds of millions of monarchs. There is no easy explanation for this, as multiple factors are likely at play, including climate change and habitat loss in overwintering areas caused by illegal logging. One additional hypothesized cause for monarch declines is a long-term decline in the abundance of their primary food plant, the common milkweed. Flowers are not needed for the nectar — Monarch butterflies can feed on nectar from any number of flowers, but milkweeds are the only plant that the monarch caterpillars can eat and develop on.

There are two leading causes for the decline in common milkweed. Like many of our native plants, common milkweed is killed by herbicides like Roundup. The use of Roundup has increased dramatically over the past three decades as new crop cultivars have been developed that are resistant to the herbicide (during the same time frame as the monarch population decline). Native habitat has also been lost as more acreage is put into agricultural production or developed for other commercial purposes. Overall, common milkweed has declined by 50-90%, with estimates varying with geographic area, time frame, habitat, and method of study.

In addition to other conservation efforts, such as educating users about more conservative herbicide use and establishing no-mow zones along roadways, conservation agencies like the National Wildlife Federation are growing, collecting, and distributing milkweed seeds to restore common milkweed populations. While this is commendable, one potential problem with this approach is that common milkweed populations may have adapted to local environmental conditions. This phenomenon is termed “local adaptation.” Because of local adaptation, it may be essential to use locally sourced seeds in common milkweed restoration efforts. Plants grown from non-local sources of seeds may not be adapted to the environment they’ve been introduced to, thereby hampering milkweed restoration efforts and monarch butterfly conservation.

The phenomenon of local adaptation is relatively common in plants, but its extent varies widely. The main goals of this research are to determine whether local adaptation occurs in common milkweed, how strong it is if it does, and which environmental variables drive it. This will better inform efforts to restore milkweed populations.