Monarch butterfly population may be in peril in the Midwest

Indiana’s state insect may not be so easy to find in the future. Monarch butterfly populations have declined dramatically in the Midwest, a new study in the “Insect Conservation and Diversity” journal said.

The study found that during the past 15 years, the population of monarch butterflies in the United States east of the Rocky Mountains has been on a downward trend.

The population of monarch butterflies can be measured by the size of their overwintering colonies in Mexico. The size of the colonies during the 2010-11 season decreased 45 percent from the 17-year average size and 81 percent from its peak in the 1996-97 season.

While Ernest Williams, the author of the study and a professor at Hamilton College, does not suggest that the famous butterfly will become extinct, he does suggest environmental policies in Mexico and the United States may not be sustainable for the monarch population.

Every year, millions of monarch butterflies participate in one of the most famous migratory behaviors of any species. Of the female monarchs that lay their eggs in the eastern United States, around 90 percent lay them in milkweed fields, two studies cited by Williams found. The eggs hatch, devour the milkweed and mature.

Once grown, the butterflies that are born latest in the summer will fly thousands of miles across the United States and the Gulf of Mexico, into Mexico where they will spend the winter. In the spring, millions fly back to lay their eggs and start the process over again. “The entire life cycle of the monarch butterfly is dependent on milkweed,” according to the Indiana Department of Natural Resources website.

But milkweed fields, monarch butterflies’ favorite place to lay their eggs, are under threat from common American agricultural practices.

According to Monarch Watch, a conservation group devoted to the monarch butterfly, “monarchs face direct habitat destruction caused by humans,” citing destruction of milkweed fields.

Around 90 percent of soybeans planted and 23 percent of corn planted is resistant to pesticides, according to the study.

Farmers spray their fields with chemicals which kill all plants except their crops.

Consequently, milkweed fields declined between 80 and 90 percent in size between 1999 and 2009, two different studies found.

“We conclude that milkweeds will disappear almost completely from croplands,” Williams said in the study, speaking for himself and the other authors.  

The study also suggested extreme weather in both the U.S. and Mexico, and illegal logging in Mexico may be pushing the decline even lower.  

Another researcher, though, said he doesn’t think monarch populations are necessarily at risk for collapse.
Andrew Davis, an assistant research scientist at the University of Georgia, agreed in a study published in the same journal that milkweed fields have decreased in size and that logging and weather have decreased the size of their overwintering colonies in Mexico.

But Davis cites a discrepancy in the number of butterflies found in Mexico in the winter and a higher number found in the U.S. in the fall to suggest the butterflies are able to regain some of their lost population during the summer.

“Although there can be no doubt that the overwintering data show a statistical drop in colony size, the lack of a parallel drop in fall numbers shown here must also be considered,” Davis wrote.

“So even though the decline in suitable breeding and wintering habitat makes it a foregone conclusion that this unique and well-studied population of monarchs may someday collapse, I contend that it does not appear to be doing so just yet,” Davis said in his study.

— Zach Ammerman