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monarch-esa-petition-final_61585

monarch-esa-petition-final_61585

Figure 24. Projected Use

Figure 24. Projected Use of 2,4-D With and Without USDA Approval of 2,4-D-Resistant Corn and Soybeans by 2020. Source: CFS (2014a), based on projections made by Dow in USDA APHIS (2013), Appendix 4. Scenarios 1, 2, 3 and All Enlist represent 2,4-D use based on various adoption scenarios for 2,4-D-resistant corn and soybeans by 2020. Scenario 1: 30% of corn and soybean acres are 2,4-D-resistant and sprayed with 2,4-D; Scenario 2: 40.5% of corn and 45% of soybean acres are 2,4-D-resistant and sprayed with 2,4-D; Scenario 3: 80% of corn and 68% of soybean acres are 2,4-D-resistant and sprayed with 2,4-D; All Enlist: 85% of corn and 89% of soybean acres are 2,4-D-resistant and sprayed with 2,4-D (representing complete displacement of glyphosate-resistant varieties by 2,4-D-resistant corn and soybeans). See CFS (2014a) for more details. A second study explored the impact of a range of drift-level dicamba doses on the plant and arthropod communities in agricultural “edge” habitats (Egan et al. 2014b). The most striking result was a significant decline in the abundance of broadleaf plants over time and with increasing dicamba dose. Impacts were observed at substantially lower levels (about one percent of the dicamba field application rate) than have been reported to affect plant communities in other studies. This study was conservative in design: dicamba alone was applied just once per year over two years. More severe impacts would be expected with longer-term use, and with the dicamba-glyphosate mix to be used with dicamba-resistant crops, which could be applied up to three times per year according to the proposed label (CFS 2012c). In general, the complementary Monarch ESA Petition 64

action of glyphosate and either 2,4-D or dicamba, applied in the form of Enlist Duo or Roundup Xtend to resistant crops, would kill or injure a broader range of plants more effectively, and over a broader range of plant growth stages, than either component alone. The implications of these studies are plain for use of dicamba and 2,4-D with crops engineered for resistance: these are herbicides that selectively kill broadleaf plants, the main nectar source for adult butterflies, including monarchs. Dicamba and 2,4-D will be used more often during a season, more extensively in an area, and more continuously over years with resistant crops than they are currently used in agriculture. This is precisely the use pattern that the studies discussed above suggest would have long-term, harmful effects on butterflies and other species. Herbicide drift thus poses a present and increasing threat to monarch habitat. Remnant Monarch Habitat Insufficient to Sustain Monarch Populations Remnant monarch habitats have become increasingly important, because of the overwhelming loss of milkweed from crop fields and CRP lands. Remnant habitats include pasturelands, roadsides, and field edges, though milkweeds in these habitats produce fewer monarchs per stem than milkweeds in crop fields (Oberhauser et al. 2001, Pleasants and Oberhauser 2012). All of these habitats are threatened by pesticide drift or direct application. Pastureland represents the most abundant non-cropland habitat for milkweed, but milkweed is very sparse in pastures (Hartzler and Buhler 2000), probably because it does not compete well with long-established grasses. The already-low milkweed density in pastures in 1999 declined by half by 2012, and it is estimated that milkweeds in pastures now account for just three percent of monarch production in the Midwest breeding range (based on data supplied by John Pleasants). Pastures are also often sprayed with broadleaf herbicides (Johnson and VanGressel 2012), which kill flowering plants that provide nectar to monarch adults and may also be a factor in milkweed decline. For instance, the largest single use of 2,4-D and one of the major uses of dicamba is on pasturelands (US EPA BEAD 2012, Monsanto 2010, Table VIII-12, p. 199). In light of milkweed loss from other areas, roadsides have become an important component of remnant monarch habitat (Flockhart et al. 2014). When crop fields had more milkweed in 1999, roadside plants accounted for only six percent of monarchs (based on Hartzler and Buhler 2000 and data supplied by John Pleasants). Because of the decimation of cropland milkweed, roadsides now produce 35 percent of Midwest monarchs, second only to CRP lands (based on data supplied by John Pleasants). Monarch habitat on roadsides is threatened by aggressive management (e.g., mowing and herbicide applications) of roadside vegetation (Commission for Environmental Cooperation 2008), and also potentially by application of road salt (Snell-Rood et al. 2014). Field edges that do not abut roads may also harbor milkweed, but increasing farm and field size has sharply reduced such fencerow habitat (Doll 1998; R. Hartzler personal communication to Martha Crouch, January 21, 2014), which becomes incorporated into cropland planted primarily to Roundup Ready corn and soybeans, where any milkweed is eliminated through glyphosate use. In sum, the resiliency and extinction risk of monarchs is largely driven by availability of milkweed and nectar sources and appropriate weather conditions on the breeding grounds in the Monarch ESA Petition 65

MONARCH CONSERVATION
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