Census counts of Monarch populations at their overwintering site in Mexico have document a population decline over the past 2 decades. Several explanations for this decline have been proposed but data is inconclusive. One popular explanation relates declines in Monarch population with a decline in milkweed in agricultural fields. Milkweed populations, and areas that support milkweed are reduced as a result of changes in farming practices. This idea has led to efforts to increase milkweed populations by altering the mowing and management of right of ways and other public spaces.
A paper in Oikos* challenges the milkweed decline explanation. Hidetoshi and colleagues analyzed data of Monarch Butterfly counts by citizen scientists that is archived by the North American Butterfly Association (NABA). Their analysis did not support the declining milkweed explanation. They found little evidence that the decline was occurring in the northern summer populations. Twenty-two years of data show a consistent expansion of Monarch population each summer in the North. The population decline is only captured in the overwintering population that migrates south in the Fall and reestablishes in early spring. This study points to the fall migration, overwintering and reestablishment as key issues. The severe drought in Texas from 2010-2015 likely had an effect of reducing the reestablishment monarch population that is at least in part responsible for recent declines. Other possible factors include: changes in disease and populations of natural enemies; pesticide use; and changes in populations of other flowering plants that serve as nectar sources for Monarchs. Whether or not milkweed population reduction is a primary cause of Monarch decline, increasing milkweed may buffer the Monarchs against further population decline. This study concludes that uncertainty surrounds the causes of Monarch decline. Further study to better understand and predict the effects of mitigation efforts is needed.
*Hidetoshi Inamine, Stephen P. Ellner, James P. Springer & Anurag A. Agrawal. Linking the continental migratory cycle of the monarch butterfly to understand its population decline. Oikos. Version of Record online: 27 APR 2016.