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Common milkweed in Iowa
by Bob Hartzler
and Doug Buhler (USDA/ARS)
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February 2, 2000 -  Common milkweed (Asclepias syriaca) is one of the few native prairie plants that can survive the annual disturbances that occur during production of corn and soybeans, thus this species is found both in row crop fields and in less disturbed habitats.  Common milkweed plays an important role in the life cyle of the monarch butterfly in that adult monarchs lay their eggs exclusively on species in the milkweed family and the larvae feed only on these plants.  Although there are 17 milkweed species found in Iowa, common milkweed is the most abundant and believed to be the most important food source for monarch larvae in Iowa and the northern corn belt.  This past year there has been increased interest in common milkweed and monarchs due to potential impacts of biotech crops on populations of both organisms.  To better evaluate these concerns, an increased understanding of the distribution of common milkweed in the midwest is needed.

We conducted a state wide survey to determine the occurrence of common milkweed in different habitats.  Forty 25 square mile cells were randomly selected across the state.  Ten 1.2 acre sites were selected within each of these cells to determine the presence of common milkweed.  All of the sites were located adjacent to a road and consisted of an area of roadside vegetation and an area in corn or soybeans (occasionally a site would be maintained in pasture or some other crop).

Common milkweed was most frequently found in roadsides, whereas pastures had the lowest infestation rate (Table 1).  Approximately half of the corn and soybean fields were infested with milkweed with an average of 2.8 milkweed patches per acre.  None of the fields surveyed (179 corn fields, 153 soybean fields) had a milkweed infestation that would be expected to impact crop yields.  Fields in the Conservation Reserve Program (CRP) had the highest percentage of land infested with common milkweed.

Table 1.  Distribution of common milkweed in Iowa, 1999.

Vegetation type Percent of sites infested
with common milkweed
Number of common milkweed patches per acre Percent of area infested with common milkweed
Roadside 71 19.4 1.0
Corn 46 2.8 0.3
Soybean 57 2.8 0.2
Pasture 28 2.8 0.1
CRP 67 3.6 2.1

Land maintained in CRP most closely resembles the native habitat of common milkweed, thus the high percentage of milkweed in this land is not surprising.  Much of the CRP land is found in the southern half of Iowa, and milkweed found in CRP probably is an important resource for monarchs in this part of the state.   There are approximately 800,000 acres of roadside in the state, and the uniform distribution of this habitat may increase its importance for monarchs.  While common milkweed was not found at high densities in corn or soybeans, it was present in over 50% of the surveyed fields.  The dominance of corn and soybean in certain parts of Iowa results in this being the primary habitat for milkweed in many parts of the state.

Corn pollen is relatively heavy and does not move long distances with wind, thus any risk to monarch larvae from Bt pollen is likely to occur with milkweed growing in or within a short distance (<10ft) of Bt corn fields.   Most corn fields share a border with a roadside, thus common milkweed in roadsides could accumulate significant quantities of pollen on their leaves.  We conducted a second smaller survey to evaluate whether milkweed was evenly distributed across the width of the roadside.  Significantly less common milkweed was found in the area of the roadside immediately adjacent to the cultivated field.  This could reduce the risk to monarchs using milkweeds found in this habitat.


% of roadside infested
with common milkweed

0-10 ft from field 0.9 b
10-20 ft from field 1.6 a
20-30 ft from field 1.9 a

Research conducted the past year has shown that pollen from most Bt hybrids is only moderately toxic to monarch larvae.  However, there continues to be debate on the risk of Bt corn to monarch populations due to an incomplete understanding of monarch behavior and the impact of Bt pollen on monarch fitness.  One important aspect of monarch behavior that remains unknown is whether the butterfly preferentially lay eggs on milkweed plants growing in different habitats.   If monarchs avoid milkweed growing in or near corn fields, then it is unlikely that pollen from Bt corn poses much of a threat to this species.  The EPA is requiring manufacturers of Bt corn to generate additional data on numerous aspects of this issue, so our understanding of the interaction between these two organisms will continue to increase.

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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