Monarch butterflies and herbicide resistant crops
by Bob Hartzler

Update - May 25, 1999 - Research from ISU indicates a potential relationship between monarch butterfly mortality and Bt corn pollen.   More information on the ISU Pest Management and the Environment Web site.

January 26, 1999 - 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, Saskatchewan conducted a study to determine the range of monarchs during their summer stay in the US and Canada (Science, 8 Jan., 1999.   283:171).  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 crop)?

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.

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