Monarch butterflies are beautiful visitors to our gardens, but over the last fifteen years there have been fewer and fewer making an appearance. During the winter of 2009-2010, the number of monarchs from eastern North America overwintering in Mexico reached an all-time low. The numbers came up somewhat for the 2010-2011 winter, but are still low. What's behind this decline?
Researcher Lincoln P. Brower and colleagues, in an article published in the March 2012 journal Insect Conservation and Diversity, suggest three factors that have contributed to the bust in butterflies. First is the degradation of the forests in Mexico that provide overwintering habitat to the monarchs. Much of this habitat has been lost due to illegal logging in areas that have been set aside as reserves.
Another big factor is the increased cultivation of genetically modified glyphosate-resistant corn and soybeans in the U.S., which has resulted in more extensive use of the herbicide glyphosate in farm fields. This herbicide use has led in turn to a sharp decline in the milkweed plants in the fields that provide breeding habitat for monarchs.
The third factor noted is the role of extreme weather interfering with winter survival or summer breeding. For example, overwintering butterflies experienced record-breaking precipitation over the 2009-2010 winter during what is normally the dry season, which caused flooding, landslides, and high winds that wreaked havoc on their habitat. In 2009 high spring temperatures in Texas limited breeding. As the climate warms, the expectation is that these types of weather extremes will increase, further harming butterfly populations. The study's authors suggest that only with better stewardship in both the U.S. and Mexico will we be able to count on having these lovely lepidopteron visitors continue to frequent out gardens.
To read the entire article on the decline of monarch butterflies, go to: Wiley Online Library.