Plummet in Monarch Butterfly Population is Blamed on Habitat Loss, Agriculture and Milkweed

Monarch butterflies, the flame-hued insects that journey hundreds of miles each year during their annual migration, are in trouble. Their populations are continuing to decline in the face of habitat destruction. Now, scientists have found that the main cause of these population declines are not due to habitat loss in wintering grounds in Mexico, but rather due to loss of breeding grounds in the United States.

Previously, scientists have thought that monarch butterflies are most vulnerable to disturbances on their wintering grounds in Mexico. Each year, the insects travel to specific locations where they congregate in high densities. They then wait out the winter months until temperatures warm once more. Yet it turns out that the wintering grounds aren't where the butterflies are being hit the hardest.

The researchers created a model to predict the effects of habitat loss on both breeding and wintering grounds and the effects of climate change. In the end, they found a link between the current population declines and an increase in genetically modified, herbicide-resistant crops in North America-something that's impacting milkweed availability.

"Our work provides the first evidence that monarch butterfly numbers in eastern North America are most sensitive to changes in the availability of milkweed on breeding grounds, particularly in the Corn Belt region of the United States," said Ryan Norris, one of the researchers, in a news release.

Milkweed is actually the only group of plants that monarch caterpillars feed on. This makes the plant crucial for the butterflies' success. Yet industrial farming has contributed to a 21 percent decline in milkweed plants between 1995 and 2013. In addition, about 70 percent of milkweed in eastern North America is located in agricultural-intensive landscapes.

"The rapid loss of milkweed projected for this region, attributable to land cover changes and shifts in agricultural practices, is a very large concern," said Tyler Flockhart, one of the researchers, in a news release. "Reducing the negative effects of milkweed loss in the breeding grounds should be the top conservation priority to slow or halt future population declines of the monarch in North America."

In the future, conservation efforts could focus on planting milkweed in south and central United States. This could have an almost immediate benefit for the butterfly populations. In addition, special attention should be paid to current farming practices which impact these butterflies.

The findings are published in the Journal of Animal Ecology.