3 years ago



100 Percent of starting

100 Percent of starting milkweed density 90 80 70 60 50 40 30 20 10 non-ag Iowa ag Hartzler Iowa ag Pleasants 0 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 Year Figure 21. Change in milkweed density in Iowa: agricultural and non-agricultural habitats (updated from Pleasants and Oberhauser 2012, Figure 1, supplied by authors). Data from Minnesota also indicate widespread milkweed decline. Extensive milkweed surveys were conducted from 2003 to 2005 in Minnesota crop fields (Koch 2005). The survey covered 72 Minnesota counties with appreciable acreage planted to corn and soybeans, with an average of six to seven fields surveyed per county. Each year 453 fields were surveyed on average, equally divided between soybeans and corn. Averaged over the three years, milkweed was detected in just 3.4 percent of surveyed fields, and those fields harbored 0.084 milkweed plants/m 2 . Averaged over all fields (including those with no milkweed), milkweed density came to just 30 plants per hectare. Milkweed plants were much more numerous in this area just three to five years before the Koch surveys. In the year 2000, Oberhauser et al. (2001) studied milkweed in five cornfields in east central Minnesota/west central Wisconsin, finding on average 2,850 milkweed plants per hectare, roughly two orders of magnitude (100-fold) higher than the level found in the Koch (2003-2005) surveys. Although these sites were not necessarily representative of landscape milkweed prevalence because candidate fields with less than 10 milkweed stems/ha were excluded, the authors report that the majority of sites visited during their site selection process had some milkweed (Karen Oberhauser, personal communication to Bill Freese, 3/20/14), as opposed to only 3.4 percent of fields with milkweed in the 2003-2005 Minnesota surveys. Dr. Oberhauser reported that the study fields in 2000 had never been planted with herbicide-resistant soybeans or corn, and attributed the drop in milkweed numbers by 2003-2005 to the widespread planting of genetically engineered, glyphosate-resistant soybeans and corn (personal communication to Bill Freese, 3/20/14, Pleasants and Oberhauser 2012). Monarch ESA Petition 54

The Iowa and Minnesota surveys exemplify the broader picture of milkweed decline throughout the major monarch breeding grounds in the Midwest due to the similarity in land use. The entire region is dominated by corn and soybean fields (Figure 22), the vast majority of which are Roundup Ready varieties. Figure 20 shows that adoption trends for genetically engineered, herbicide-resistant corn and soybeans (nearly all Roundup Ready) are quite similar in the 12 Midwestern states, with 89 to 97 percent of soybeans, and 81 to 94 percent of corn, herbicideresistant by 2013. Anecdotal evidence reported by farmers and scientists of common milkweed’s absence from or rarity in crop fields in Nebraska, Kansas, Michigan and North Dakota—all states where it was once quite prevalent—provide further corroboration of the near eradication of milkweed from cropland by glyphosate use with Roundup Ready crops (Center for Food Safety 2014a). Figure 22. Corn and soybean production in the United States 2013. Source: USDA CropScape (2013). Green represents corn, blue represents soybeans. Depth of color signifies intensity of cultivation. The extensive loss of milkweed from croplands has contributed significantly to the dramatic decline in monarch abundance since the mid-1990s. Common milkweed in crop fields is of particular importance to monarchs because it produces considerably more monarchs per plant than milkweeds growing elsewhere. Oberhauser et al. (2001) analyzed milkweed distribution and per-plant monarch productivity and found that in Iowa, Minnesota, and Wisconsin, the number of eggs deposited per milkweed plant was higher on milkweeds in corn fields than on milkweeds in old fields, pastures and field edges. Pleasants and Oberhauser (2012) extended this analysis Monarch ESA Petition 55

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