The massive migration of monarch butterflies is amazing—the insects go from grazing on milkweed plants as caterpillars in the midwest to spending winters in Mexico. But Monarch populations have been on the decline for some time, with a variety of factors being considered: lost habitat in Mexico, damage from pesticides, or climate change.

Conservation strategy for a species that traverses thousands of miles is complicated business, so a team of scientists from the University of Guelph decided to sort out which factors were the most responsible for the monarch's population declines—changes at the breeding grounds, the wintering sites, or climate changes.

Their conclusions suggest that we can't blame deforestation in Mexico for this environmental problem. The monarchs are suffering from a lack of milkweed, the only plant the caterpillars eat. In fact, a model built by the researchers suggested that monarch populations were four times more sensitive to the loss of milkweed on their breeding grounds than the loss of the forested habitat in which they spend the winters.

The spatial model was built using population dynamics data, which incorporated locations and life stages, known survival rates at different stages, and standard reproductive success. It used this data to predict how various changes in the system, from climate to habitat, would affect the insects' complex lives.

Using their model, the scientists found a 21 percent decline in milkweed abundance between 1995 and 2013. The largest declines, in the midwest, line up with the largest declines in butterfly population.

The monarchs depend on milkweed—it's the plants' chemical defenses that give the butterflies their infamous unpalatability. The adults only lay their eggs on milkweed to give the larvae a strong start in life, so the researchers say the plants' decreasing abundance has implications across the life cycle.

Milkweed is disappearing, they write, because of the increasingly intensive land use of agriculture; although the study didn't do primary research on this connection, it has been demonstrated by others. Milkweed is still common in nature preserves, gardens, and along roadways, but for farmers, it's a weed. In the corn belt, agricultural land is being used more intensely, which means fewer buffers and borders of natural plants between the fields, and more powerful herbicides to reduce the number of weeds. The invention of herbicide-tolerant corn and soybeans has made growing more efficient, since it allows farmers to spray and kill off everything else, but it's bad news for milkweed and monarchs.

This doesn't come as a surprise to everyone—Chip Taylor, of Monarch Watch and the University of Kansas, talks about how shifts in the agricultural practices in the midwest have reduced milkweed and, therefore, caterpillars. But many thought that habitat protections in Mexico would solve the problem.

Efforts to protect the wintering habitat in Mexico are important too, the scientists write, because the forest cover protects the insects as they huddle together in million-monarch masses to survive the cold. But those protections aren't enough.

Taylor offers a simple suggestion, and the Guelph scientists back him up: if we want monarchs, we need to plant more milkweed, perhaps in gardens and along roadsides. But to stop the decline, the scientists say we also need to preserve undeveloped lands in the corn belt, like parks and prairies, where milkweed can feed caterpillars that grow into beautiful butterflies.

Journal of Animal Ecology, 2014. DOI not yet available.