Farm Horizons, August 2013

Farming practices affect the ability of monarch butterflies to thrive

Jennifer Kotila

Much like the honeybee, monarch butterflies are disappearing at an alarming rate.

Also like the honeybee, numerous factors can contribute to its disappearance, including modern farming practices.

The use of genetically-modified, Roundup Ready corn and soybean plants has created a loss of habitat for the monarch butterfly in the Midwest, according to a study conducted by monarch experts at the universities of Minnesota and Iowa. Specifically, GM corn and soybeans have allowed farmers to more easily eradicate milkweed from their fields. Female monarchs lay their eggs exclusively on milkweed plants, and monarch larvae feed primarily on milkweed. The study directly correlates the loss of milkweed from agricultural fields to the loss in population of monarch butterflies.

Milkweed in fields has always been troublesome for farmers, reducing crop yield, according to the study. Infestation of milkweed in agricultural fields in the 1970s and 1980s was seen to be increasing, but the most effective herbicide – glyphosate, also known as Roundup – also harmed the crops farmers were trying to grow. The introduction of Roundup Ready soybeans in 1996, and Roundup Ready corn in 1998, allowed farmers to begin increasing their usage of Roundup to control weeds in row crops. By 2011, adoption levels of Roundup Ready soybeans and corn had reached 94 percent and 72 percent, respectively.

There was a 58 percent overall decline in milkweed from 1999 to 2010, with a reduction of 81 percent in agricultural fields and 31 percent in non-agricultural habitat. In 1999, a little more than half of all milkweed in the Midwest grew in agricultural fields. By 2010, a little less than a quarter of all milkweed grew in agricultural fields.

It is important to distinguish the location of milkweed plant decline because a study in 2000 found that monarchs lay about four times more eggs on milkweed in agricultural fields than on milkweed in non-agricultural habitat. That being said, the overall loss of 58 percent of milkweed leads to an even greater impact on the loss of resources for the monarch – 72 percent. Relative monarch production on agricultural milkweed declined from 82 percent in 1999, to 55 percent in 2000, according to the study.

Between 1999 and 2010, monarch egg production dropped by 81 percent, according to the study. If monarchs had increased their use of non-agricultural milkweed, such a sharp decline would not have occurred. However, egg-density on non-agricultural milkweeds did not increase as agricultural milkweed declined, according to the study.

Removing small patches of milkweed from the matrix (the area between larger patches of milkweed) makes it harder for female monarchs to achieve their egg-laying potential due to longer search times, according to a study published in 2010. Therefore, removing the small patches of milkweed that may be found within agricultural fields may significantly reduce the lifetime number of eggs laid by individual females, according to the study.

Because Roundup Ready crops have become so prevalent, the disappearance of milkweed from agricultural fields is inevitable – meaning the resource base for monarchs in the Midwest will be permanently reduced, according to the study. A new, lower ceiling for monarch population size will be set, and the population will be more vulnerable to deforestation at their overwintering sites, and extreme weather events and climate change at their breeding areas and on migratory routes.

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