Monarch Butterflies: The Victim Of Unintended Consequences

When you were a kid, did it seem like there were more monarch butterflies than there are now. How many did you see this past summer, compared to 10 or 15 years ago? If it seems like there are fewer around, that is the case, and one of the reasons is the increasing prevalence of crops that are tolerant to post emergent applications of glyphosate.

Common milkweed is not tolerant of glyphosate and it is the only source of food for the caterpillars that become monarch butterflies. Iowa State University weed specialist Bob Hartzler says common milkweed is a native plant in the Cornbelt, and while other species of milkweed once populated the prairies, their numbers diminished when crop cultivation began. He says common milkweed numbers did not diminish when cropland areas increased and their numbers were just as prevalent in all fields.

But Hartzler says when Roundup Ready crops were introduced, glyphosate began to reduce the populations of common milkweed in the Cornbelt, and negatively impacted the monarch butterfly. He has tracked that trend with the increasing use of glyphosate tolerant corn and soybeans over the past 10 years. Hartzler says the number of milkweeds did not change significantly between 1999 and 2009 along roadsides. In fact 71% of the roadsides that he sampled in 1999 had milkweeds growing, compared to 82% in 2009. However, when fields were sampled for common milkweed presence, there were 51% in 1999 and only 8% in 2009.

Hartzler reports, “Presence of milkweed in these areas probably has increased over the past 30 years due to reductions in mowing and spraying by county and state organizations. In contrast, there was nearly an 85% decrease in the number of corn and soybean fields infested with common milkweed between 1999 and 2009. The amount of milkweed present in infested fields decreased by a similar percentage. Although other factors may have played a role in the decline in common milkweed in crop fields, the widespread use of glyphosate associated with the planting of RR crops undoubtedly is a major cause.”

When is the last time you applied glyphosate to a field to specifically eradicate a group of milkweeds? You may not have done that, but an application of glyphosate will do its job and not select only certain weeds to eradicate. It does a good job at killing all plants, except those which are genetically tolerant and those few populations of a handful of weeds that have created their own resistance. Hartzler says the long term impact of the reduction in common milkweed on monarchs is difficult to access since many factors influence population dynamics of the butterfly. However he says a group concerned about the reduction in the number of monarch butterflies has initiated a new program, designed to promote the planting of milkweed to provide alternative plants to those that were formerly present in cropping areas.

Looking specifically at Iowa, Hartzler says when Roundup Ready crops were introduced; estimates were that Iowa corn and soybean fields produced 78 times more monarch butterflies than non-agricultural habitats. He says the decline in common milkweed found in corn and bean fields could negatively affect monarch butterfly reproduction anywhere there are similar cropping patterns.

The growth in popularity of glyphosate tolerant corn and soybeans has lead to a decline in the number of common milkweed plants, which are the only food source for monarch butterflies. Advocates for monarch butterflies are pointing to the fields that once feed the caterpillars which became monarchs, but say their existence is threatened unless milkweeds are intentionally planted to provide food for the larvae.

Source: Stu Ellis, University of Illinois

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