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Occurrence of Common Milkweed (Asclepias Syriaca) in Cropland and Adjacent Areas Robert G

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Occurrence of Common Milkweed (Asclepias Syriaca) in Cropland and Adjacent Areas Robert G Agronomy Publications Agronomy 6-2000 Occurrence of common milkweed (Asclepias syriaca) in cropland and adjacent areas Robert G. Hartzler Iowa State University, [email protected] Douglas D. Buhler United States Department of Agriculture Follow this and additional works at: http://lib.dr.iastate.edu/agron_pubs Part of the Agricultural Science Commons, Agronomy and Crop Sciences Commons, Entomology Commons, and the Plant Breeding and Genetics Commons The ompc lete bibliographic information for this item can be found at http://lib.dr.iastate.edu/ agron_pubs/32. For information on how to cite this item, please visit http://lib.dr.iastate.edu/ howtocite.html. This Article is brought to you for free and open access by the Agronomy at Iowa State University Digital Repository. It has been accepted for inclusion in Agronomy Publications by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. Occurrence of common milkweed (Asclepias syriaca) in cropland and adjacent areas Abstract Interest in the population dynamics and geographic distribution of common milkweed (Asclepias syriaca L.) has recently increased due to the importance of common milkweed in the life cycle of the monarch butterfly (Danaus plexippus). A survey of common milkweed occurrence in various habitats was conducted in Iowa in June and July of 1999. Common milkweed was found in 71% of the roadsides and approximately 50% of the corn (Zea mays L.) and soybean (Glycine max L. Merr.) fields. Corn and soybean fields had 85% fewer patches than roadsides. Conservation reserve program fields had the greatest average area infested. While common milkweed was frequently found in corn and soybean fields, average frequency and patch sizes were much greater in noncrop areas. Keywords Bt pollen, Monarch butterfly, weed distribution, maize, soyabean Disciplines Agricultural Science | Agronomy and Crop Sciences | Entomology | Plant Breeding and Genetics Comments This article is from Crop Protection 19 (2000): 363, doi:10.1016/S0261-2194(00)00024-7. Posted with permission. Rights Works produced by employees of the U.S. Government as part of their official duties are not copyrighted within the U.S. The onc tent of this document is not copyrighted. This article is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/agron_pubs/32 Crop Protection 19 (2000) 363}366 Occurrence of common milkweed (Asclepias syriaca) in cropland and adjacent areas Robert G. Hartzler! *, Douglas D. Buhler" !Department of Agronomy, Iowa State University, 2104 Agronomy Hall, Ames, IA 50011-1010, USA "US Department of Agriculture, Agricultural Research Service, National Soil Tilth Laboratory, Ames, IA 50011, USA Received 11 October 1999; received in revised form 28 December 1999; accepted 23 January 2000 Abstract Interest in the population dynamics and geographic distribution of common milkweed (Asclepias syriaca L.) has recently increased due to the importance of common milkweed in the life cycle of the monarch butter#y(Danaus plexippus). A survey of common milkweed occurrence in various habitats was conducted in Iowa in June and July of 1999. Common milkweed was found in 71% of the roadsides and approximately 50% of the corn (Zea mays L.) and soybean (Glycine max L. Merr.) "elds. Corn and soybean "elds had 85% fewer patches than roadsides. Conservation reserve program "elds had the greatest average area infested. While common milkweed was frequently found in corn and soybean "elds, average frequency and patch sizes were much greater in noncrop areas. ( 2000 Elsevier Science Ltd. All rights reserved. Keywords: Bt pollen; Monarch butter#y; Weed distribution; Maize; Soyabean 1. Introduction Common milkweed is adapted to a wide range of climatic and edaphic conditions (Bhowmik and Bandeen, Common milkweed is native to northeastern and 1976). Infestations may be found under a wide range of north central United States and adjacent areas of Cana- soil conditions in any textural group, but are most preva- da (Bhowmik and Bandeen, 1976). This perennial dicoty- lent on well-drained soils of loamy texture. In a roadside ledonous species is present in crop and pasture lands of survey, Cramer and Burnside (1982) found that infesta- many areas, including Iowa. Common milkweed and tion frequencies in Nebraska varied greatly by crop and other members of the Asclepiadaceae family are the land use. Over 70% of the soybean, oat, (Avena sativa L.) sole food source of monarch butter#y larvae (Brower, and sorghum [Sorghum bicolor (L.) Moench.] "elds 1969), whereas the adults feed on a wide range of #owers. were infested with common milkweed, while infestation Common milkweed is the most prevalent Asclepias frequencies for corn, wheat (Triticum aestivum L.), and species and is considered the preferred food source for alfalfa (Medicago sativa L.) were 36, 28, and 6% respec- monarch larvae in Iowa. The importance of common tively. Railroad, roadside, and pasture areas had infesta- milkweed populations in the central United States to tion percentages of 70, 51, and 14%. the monarch (Wassenaar and Hobson, 1998) and toxicity Corn pollen may be dispersed 60 m or more by wind of the pollen of Bt (Bacillus thuringiensis) transformed- (Raynor et al., 1972) and deposited on other plants in corn deposited on common milkweed leaves (Hansen and around corn "elds and can be ingested by non-target and Obrycki, 1999; Losey et al., 1999) to monarch larvae organisms that feed on these plants. Recent research has generated interest in the distribution of common (Hansen and Obrycki, 1999; Losey et al., 1999) has shown milkweed in crop lands and adjacent areas. that the expression and dispersal of the Bt toxin in the pollen of transgenic corn may be toxic to non-target Lepidoptera species. In a laboratory assay, larvae of the monarch butter#y reared on common milkweed leaves * Corresponding author. Tel.: #1-515-294-1164; fax: #1-515-294- dusted with pollen from Bt corn, ate less, grew more 9985. slowly, and had higher mortality than larvae reared on E-mail address: [email protected] (R.G. Hartzler). leaves dusted with untransformed pollen or on leaves 0261-2194/00/$- see front matter ( 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 1 - 2 1 9 4 ( 0 0 ) 0 0 0 2 4 - 7 364 R.G. Hartzler, D.D. Buhler / Crop Protection 19 (2000) 363}366 without pollen (Losey et al., 1999). Hansen and Obrycki cells were mapped with the 1 : 100,000-scale US Geologic (1999) found that common milkweed leaf samples taken Survey Digital Raster Graphic map series for navigation from within and at the edge of corn "elds caused 19% and "eld map selection. If a 10 km grid cell was split mortality of monarch larvae with Bt pollen compared between two 1 : 100,000-scale map sheets, the sample was with 0% with non-Bt pollen and 3% in no pollen con- shifted one cell south and two cells east. trols. Within each 10 km grid cell ten 0.5 ha (50 m;100 m) Each fall (autumn), millions of monarch butter#ies sampling areas were surveyed for the presence of com- from eastern North America migrate to 10}13 discrete mon milkweed. County plat maps were used to locate colonies in the Oyamel forests of central Mexico (Was- the sampling areas within grid cells. Sampling areas were senaar and Hobson, 1998). Although monarch migration identi"ed prior to going to the "eld by arbitrarily specify- has been extensively studied, the migration patterns to ing a distance from a landmark, usually a road intersec- these locations from speci"c areas in North America has tion or railroad crossing. This method eliminated not been characterized. Using an isotopic labeling tech- sampling bias since the sampling areas were selected nique, Wassenaar and Hobson (1998) determined that all without prior knowledge of terrain or vegetation. Each monarch wintering colonies were composed of indi- sampling area was immediately adjacent to the road, viduals originating mainly from the Midwest United extending 100 m into the "eld. The majority of sampling States. The importance of the Midwest in the monarch areas consisted of publicly owned roadside adjacent to butter#y life cycle raises concerns over common milk- the road and a second area of private property. The weed populations in corn and soybean "elds as the use of width of roadside in most situations was approximately glyphosate-resistant crops increase. Glyphosate provides 10 m and the dominant vegetation was smooth brome an additional herbicide for the control of common milk- (Bromus inermis Leyss.). weed in the growing crop (Owen and Hartzler, 1999) and Each sampling area was divided into sub-samples may reduce common milkweed occurrence in crop "elds. (sites) based upon the land use (i.e., roadside and corn). In areas of the North America where the landscape is Data collected included size of site, vegetation type, dominated by row-crop production, a large proportion crop growth stage, number of distinct common milkweed of the common milkweed may be either in corn/soybean patches, and size of individual common milkweed "elds or in adjacent areas within several meters of the patches. Common milkweed stems within 1 m of each "eld edge. The objective of this research was to determine other were considered be part of a single patch, and patch the relative distribution of common milkweed in di!erent size was estimated as the area encompassed by the con- habitats in Iowa. The results will help assess the potential tiguous stems. Solitary stems were assigned a patch size impact of Bt pollen on monarch populations and propor- of 1 m. tion of milkweed that occurs in crop "elds that may be Data were analyzed using analysis of variance to allow a!ected by changing herbicide use patterns.
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