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Distribution of Common Milkweed in Iowa
by Dawn Refsell

December 14, 2001 -  The North Central Weed Science Society held its annual conference recently in Milwaukee, WI.  The primary focus of this meeting is presentation of weed science research results, either in the form of oral presentations or posters.  The following is an adaptation of a poster displayed at the meeting.  Although this website reaches a much broader audience than attends the conference, we hope the information is presented in a manner that can be understood by persons without formal training in weed science.  

Occurrence of common milkweed (Asclepias syriaca) in Iowa
 Dawn Refsell and Bob Hartzler, Iowa State University, Ames and Doug Buhler, Michigan State University, East Lansing.


Larvae of monarch butterflies feed exclusively on members of the Asclepidaceae (milkweed) family.  Common milkweed is the species most utilized by monarchs in the Corn Belt due to its abundance.  The importance of common milkweed occurring in agricultural settings in the monarch life cycle is poorly understood.  A survey was conducted from 1999 to 2001 to determine the relative abundance and stability of common milkweed in different vegetative habitats across Iowa.  The initial survey in 1999 determined that approximately 50% of corn and soybean fields in Iowa were infested with common milkweed.  In subsequent surveys in 2000 and 2001, row-crop fields infested with common milkweed declined to approximately 35%.  In contrast, roadside right-of-ways infested with common milkweed were stable at approximately 75% from 1999 to 2001.  The initial 1999 survey determined a high infestation frequency in land enrolled in the Conservation Reserve Program.  A more extensive survey (100 sites) of undisturbed habitats in 2000 determined that 90% of these sites contained common milkweed.  Swamp, honeyvine and eastern whorled milkweed were present in 5, 4 and 1% of the undisturbed habitats, respectively.  This data, combined with information concerning land use patterns; can help determine the relative importance of milkweed occurring in agricultural land in the life cycle of the monarch.



Ten 10 km2 grid cells were randomly selected from within the state and then ten 0.5 ha sites within each grid arbitrarily selected.  Each site was divided into sub-samples (habitats) based on vegetation (roadside, pasture, corn, etc.).  Data collected from each site included size of habitat, number of distinct common milkweed patches, and size of common milkweed patches.  The sampling protocol used is diagrammed in Figure 1.  Common milkweed stems within 1 m of each other were considered be part of a single patch.  Patch size was estimated as the area encompassed by the contiguous stems, while solitary stems were assigned a patch size of 1 m2.  Data was collected in the summers of 1999, 2000 and 2001.  Data were analyzed using ANOVA and paired t-tests.  The 1999 milkweed survey data is published in Crop Prot. 19:363-366.

Non-disturbed Habitat Study   The initial 1999 survey found high common milkweed infestations in Conservation Reserve Program (CRP) fields, thus a more extensive sampling of non-disturbed habitats was conducted in 2000Four counties were selected for this survey (two in northern Iowa, two in southern Iowa).  Twenty 0.5 ha sites per county were sampled by randomly driving until appropriate habitats were located.  Sites included prairie wetlands, state and federal land reserves, and CRP land.


In 1999, approximately 70% of roadsides were infested with common milkweed, compared to 50% of corn and soybean fields (Figure 2).  Although a significant percentage of row crop fields were infested with common milkweed, the average area infested was relatively small and none of the sampled fields had an infestation that would be considered economic (Figure 3).  Common milkweed infestations were more stable in non-disturbed habitats than agricultural land, with corn and soybean fields infested with common milkweed declining from 51% in 1999 to 33% in 2001.  Although only 40% of CRP fields in the initial study were infested with common milkweed, these fields had a high density of milkweed.  The more extensive survey of non-disturbed habitats in 2000 found a high frequency of infestation and relatively high common milkweed densities (Figure 4).  Swamp, honeyvine and eastern whorled milkweed were present in 5, 4 and 1% of the undisturbed habitats, respectively (data not presented). 


Previous surveys in Iowa and Nebraska reported approximately 50% of row crop fields contained common milkweed (Cramer and Burnside, 1982; Fawcett, 1978).  Our initial survey in 1999 found a similar infestation rate, but the number of agronomic fields infested with common milkweed declined between 1999 and 2000.  This decline in common milkweed infestations in corn/soybean fields may reflect seasonal fluctuations in response to tillage or herbicides, rather than permanent reductions due to specific management practices (i.e. the use of glyphosate in Roundup Ready soybeans).

While roadsides and other non-disturbed habitats had higher densities of common milkweed than agricultural land, land use patterns across Iowa greatly affect the   distribution of milkweed across the state.  In northwest and central Iowa, the majority of common milkweed will occur in row crops since >90% of the total land mass is planted to corn and soybeans annually.  In southern Iowa,  more milkweed would occur in non-agricultural land than in row crops due to a greater amount of CRP and other types of non-disturbed habitats. 

The EPA and USDA/ARS recently concluded that Bt corn does not pose a significant threat to monarch butterflies.  Our interest in surveying milkweeds was sparked by the article in Science in which the impact of herbicide resistant crops on common milkweed infestations, and therefore monarchs, was questioned (Holden, 1999).   Our research has documented that common milkweed is prevalent in row crop fields, and other scientists have documented that these plants are readily utilized by monarchs.   Questions left unanswered include: 1) Will continued use of Roundup Ready soybeans and other herbicide resistant crops cause a permanent reduction in common milkweed infestations?, and 2)  Would a reduction in common milkweed in agricultural land impact monarch populations?

Literature Cited

•Cramer, G.L. and O.C. Burnside. 1982. Distribution and interference of common milkweed (Asclepias syriaca) Nebraska. Weed Sci. 30: 385-388.
Fawcett, R.S. 1978. New approaches to perennial weed control. Iowa State University, Fertilizer and Ag. Chemical Dealers. EC-1270:J
Hartzler, R.G., and D.D. Buhler. 2000. Occurrence of common milkweed (Asclepias syriaca) in cropland and adjacent areas. Crop Protection 19:363-366.
Holden, C. 1999. Random samples:  Monarchs and their roots. Sci. 283:171.
Malcolm, S.B., and M.P. Zalucki. 1999. Plant Latex and First-Instar Monarch Larval Growth and Survival on Three North American Milkweed Species. Journal of Chemical Ecology 25:1827-1842.

Oberhauser, K.S., M.D. Prysby, H.R. Mattila, D.E. Stanley-Horn, M.K. Sears, G. Dively, E. Olson, J.M. Pleasants, W.-K.F. Lam, and R.L. Hellmich. 2001. Temporal and spatial overlap between monarch larvae and corn pollen. PNAS:211234298.
Wassenaar, L.I., and K.A. Hobson. 1998. Natal origins of migratory monarch butterflies at wintering colonies in Mexico: New isotopic evidence. PNAS 95:15436-15439.


Additional commentsI have occasionally been asked that if a reduction in milkweed due to the use of herbicide resistant crops would threaten monarchs, should restrictions be placed on this technology in order to protect the monarch?  This is one of those questions to which there is no simple answer.  First, I don't believe we have enough information to determine whether herbicide resistant crops will permanently affect milkweed populations.  Like any weed, common milkweed is highly adaptive and it is extremely unlikely that it will be eradicated from agronomic fields due to a single change in production practices (adoption of herbicide resistant crops).  Second, very few farmers base their herbicide program on the presence of common milkweed in a field.  Annual weeds are much more competitive and spread more rapidly than milkweed, thus the herbicides used and herbicide application timing are based on the annual weed species present in a field, rather than common milkweed.  In most corn or soybean fields, milkweed could be considered somewhat of an 'innocent bystander' that happens to get sprayed by the herbicide.  Finally, one of the tenets of modern crop production is to maintain a 'clean field' in order to eliminate or minimize the negative impacts of weeds.   It would be a tough sell to convince farmers to change management practices in order to protect a weed present in their corn or soybean fields.

The continued use of Roundup Ready crops could cause permanent reductions in the amount of common milkweed found in corn and soybean fields.  However, it is important to remember that the current agricultural system dominated by row crops has existed for a short time period (<50 years).  Therefore, monarchs obviously are not dependent upon common milkweed found in corn and soybean fields.  Although the monarch butterfly has shown a preference to using common milkweed found in agronomic fields, the insect has the ability to survive on milkweeds in other habitats.  Some monarch specialists have questioned whether there is sufficient milkweed present in other habitats to maintain the monarch population, but this is getting way beyond the expertise of this weed scientist.  Bob Hartzler.


Acknowledgments:  Funding for this research was provided by the Leopold Center for Sustainable Agriculture, Iowa Agricultural Experiment Station, and the USDA/ARS.


Prepared by Dawn Refsell, graduate student, and 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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