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7-Misc: Study on effects of HR technology on Monarch butterflies

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Monarch butterflies and herbicide resistant crops 

Bob Hartzler 
Iowa State University

January 26 - What's the connection between these two organisms you ask? 
The monarch migrates each year from southern Canada and the eastern half 
of the U.S. to a few small sites in the mountains of central Mexico. 
Researchers in Saskatoon, Saskathewan conducted a study to determine the 
range of monarchs during their summer stay in the US and Canada. They 
found that approximately half of the monarchs were from a relatively 
narrow-swath from Nebraska to Ohio.

The researchers were surprised that so much of the population was 
concentrated in the heart of the cornbelt. They expressed concern about 
the rapid changes in weed control practices occurring in this region. 
Monarch larvae feed exclusively on milkweed plants, thus reductions in 
milkweed populations could have a dramatic impact on monach reproduction. 
The use of herbicide resistant crops (Roundup Ready, Liberty Link, IMI 
corn) could provide more effective control of milkweed than traditional 
herbicides, thus the concern.

Several factors need to be considered when looking at the impact of HRC's 
on monarch butterflies:
1) Will the rates and timing of herbicide applications made to control 
annual weeds have a significant impact on milkweed populations?
2) What percentage of milkweed in the corn belt is found in row crop 
acres vs in roadsides, pastures and other non-row crop areas?
3) Do monarchs have a site preference for egg laying (row crop vs non-row 

The researchers pointed out that their findings represent only a single 
year's distribution of monarch butterflies and may not represent 
historical patterns. However, it does illustrate the potential impact 
changes in weed management strategies could have on the ecosystem.

Monarchs and their roots. 1999. Science 283 (Jan 8):171.

Prepared by the author, 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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