John Pleasants* estimates the numbers of milkweed plants lost to changes in agricultural practices and the extent of the conservation effort required to replace them. Almost 900 million milkweed plants have been lost from corn and soybean fields (mostly) and from grassland conversion to crops. The decline from 1999 to 2014 is about 40%, from 2.2 billion to 1.3 billion milkweed plants. Plants in corn and soybean fields can produce more monarchs than plants in other habitats. Thus, more than one plant must be added to grasslands for every one lost from a corn field.
Pleasants calculates that the current population of 1.3 billion plants can only support about 3.2 ha of overwintering population in Mexico, about half of the 6 ha goal of conservationists. 425 million addition milkweeds in grasslands would support an additional hectare of overwintering population and an additional 1.6 billion would be required to reach the 6 hectare goal. A massive effort would be required to meet this goal.
JOHN PLEASANTS. Milkweed restoration in the Midwest for monarch butterfly recovery: estimates of milkweeds lost, milkweeds remaining and milkweeds that must be added to increase the monarch population. Insect Conservation and Diversity (2017) 10, 42–53.