What’s killing monarch butterflies?
It’s not been easy to pin down. Their migration routes cover thousands of miles, journeys that span most of a continent, dozens of ecosystems, hundreds of cities and roads, and, depending on the route, vast expanses of water. Each leg of the trip is a constantly shifting mosaic, subject to the whims of natural disasters and human development.
Possible culprits have ranged from deforestation in Mexico where they overwinter to invasive species, pesticides, and disease. But a new study that recreated monarch butterfly populations and migrations in silico—in a computer model—found that the orange-and-black beauties are most sensitive to the loss of milkweed, the plant that nurtures monarch caterpillars and imbues them with an effective chemical defense against predators.
Kate Prengaman, writing for Ars Technica:
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.
Compared with other threats, monarchs were most affected by the loss of milkweed. The study’s authors report the insects were four times more sensitive to the loss of their nursery plant than the loss of overwintering habitat in Mexico.
These findings mesh nicely with field studies that report a steady decline of milkweed abundance in the real world. The primary reason why there’s less milkweed seems to be the intensification of agriculture. As farms are pressed to produce more food from the same amount of land, they have been pushing the boundaries of their fields into old fencerows, which used to be prime milkweed habitat. Increasing use of pesticides also appears to contribute to the problem, though not in a way you might expect. Rather than killing monarchs directly, herbicides have been reducing the number of milkweed plants that grow between crop plants.
The study paints a grim picture, but there’s still hope. Preserving the wintering grounds in Mexico can help stem losses, as can setting aside more parkland in the United States. And perhaps most importantly, by planting milkweed in gardens and along roadsides, we all can play a part in saving one of North America’s most charismatic insects.