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

Abstract

Abstract The loss of milkweeds in corn and soybean fields in the Midwest due to herbicide use has been implicated in the decline of the monarch butterfly population. In order to gauge the magnitude of the milkweed restoration effort necessary to compensate for this loss, it is necessary to know how many milkweeds have been lost and how many remain on the landscape. I estimate that since 1999, 850 million milkweeds have been lost from corn and soybean fields. In addition, since 2008, over 11 million milkweeds have been lost from grasslands due to their conversion into cropland, an annual loss rate of about 2 million milkweeds. Of the estimated 2.2 billion milkweeds present on the landscape in the Midwest in 1999, only 1.34 billion remained in 2014, a decline of almost 40%. But because each milkweed stem in an agricultural field averages 3.9 times more monarch eggs than a milkweed stem in non‐agricultural habitats, the potential monarch support capacity loss has been 71%. A conservation goal of 6 ha of overwintering butterflies has been proposed. It is estimated that the current milkweed population could support about 3.2 ha under average weather conditions in the breeding season. A total of 425 million milkweeds would need to be added to increase the monarch support capacity by just one more overwintering hectare and 1.6 billion would be needed to meet the 6 ha conservation goal. Thus, a massive milkweed restoration effort will be needed to produce a resilient monarch population.