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Changes in common milkweed occurrence since the introduction of RR crops   
Bob Hartzler

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November 18, 2010 Common milkweed (Asclepias syriaca) is native to the Cornbelt and is the most prevalent member of the milkweed family in this region. Other milkweed species were more abundant than common milkweed prior to conversion of the tallgrass prairie to cropland, but common milkweed is more tolerant of disturbance than other milkweeds, and thus has increased in the past century. Larvae of the monarch butterfly feed exclusively on members of the milkweed family (Asclepidaceae).

The introduction of herbicides resistant crops has raised concern that these weed management tools could reduce populations of milkweed in the Cornbelt, therefore negatatively impacting the monarch butterfly. Of particular concern is the Roundup Ready (RR) trait since this trait allows postemergence applications of glyphosate, a herbicide especially effective on perennial plants such as milkweed. RR soybeans were first marketed in 1996, and in 1998 approximately 40% of the soybean acres were planted to these varieties. By the early 1990's RR soybean were planted on over 90% of the soybean acres, and the planting of RR corn hybrids dramatically increased in the late 1990's.

Surveys of crop fields and adjacent roadsides were conducted in 1999 (> 300 fields) and 2009 (> 200 fields) to determine the presence of common milkweed in these areas (Table 1). Common milkweed occurrence in roadsides adjacent to crop fields did not change significantly over the time frame of the surveys. 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.

Table 1. Occurrence of common milkweed in Iowa roadsides and crop fields. (Hartzler, 2010)

Land Use
% sites infested
Area infested (sq m per hectare)


Crop Field

Iowa’s landscape is dominated by cropland, with approximately 75% of the landmass dedicated to agricultural production. At the time of introduction of RR crops it was estimated that Iowa corn and soybean fields produced 78 times more monarchs than non-agricultural habitats. Thus, the decline in common milkweed found in corn and soybean fields could negatively affect monarch reproduction within Iowa and surrounding states with similar land use patterns. Farmers do not intentionally target milkweed in corn and soybean fields since it rarely is present at economic levels, but rather the milkweed is contacted by herbicide intended to control other, more serious, weeds. 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, Monarch Watch has initiated a new program , Bring Back the Monarchs, promoting the planting of milkweed to provide alternative plants to those formerly present in agricultural fields.


Hartzler, R.G. 2010. Reduction in common milkweed occurrence in Iowa cropland from 1999 to 2009. Crop Protection 12:1534-1541.


Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University

For more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
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Common chemical and trade names are used in this publication. The use of trade names is for clarity by the reader. Inclusion of a trade name does not imply endorsement of that particular brand of herbicide and exclusion does not imply nonapproval.