Milkweed loss in agricultural fields because of herbicide use: effect on the monarch butterfly population

Abstract

1. The size of the Mexican overwintering population of monarch butterflies has decreased over the last decade. Approximately half of these butterflies come from the U.S. Midwest where larvae feed on common milkweed. There has been a large decline in milkweed in agricultural fields in the Midwest over the last decade. This loss is coincident with the increased use of glyphosate herbicide in conjunction with increased planting of genetically modified (GM) glyphosate-tolerant corn (maize) and soybeans (soya).2. We investigate whether the decline in the size of the overwintering population can be attributed to a decline in monarch production owing to a loss of milkweeds in agricultural fields in the Midwest. We estimate Midwest annual monarch production using data on the number of monarch eggs per milkweed plant for milkweeds in different habitats, the density of milkweeds in different habitats, and the area occupied by those habitats on the landscape.3. We estimate that there has been a 58% decline in milkweeds on the Midwest landscape and an 81% decline in monarch production in the Midwest from 1999 to 2010. Monarch production in the Midwest each year was positively correlated with the size of the subsequent overwintering population in Mexico. Taken together, these results strongly suggest that a loss of agricultural milkweeds is a major contributor to the decline in the monarch population.4. The smaller monarch population size that has become the norm will make the species more vulnerable to other conservation threats.

Long-term Trends in the Number of Monarch Butterflies (Lepidoptera: Nymphalidae) Counted on Fall Migration at Long Point, Ontario, Canada (1995–2014)

“…Herbicide use for soy and maize cultivation since 1999 to 2010 in the United States reduced milkweed availability (main larvae food source) in 58% and butterfly reproduction dropped in 81% which is related to the decrease of butterfly density in the overwintering sites in Mexico (Pleasants & Oberhauser 2012). Deforestation and forest degradation in the core zones due to illegal logging within Mexican sanctuaries decreased drastically thanks to the commitment of ejidos and indigenous communities of the Monarch Reserve, the surveillance and payment of environmental services by Mexican authorities, the support of philanthropists and Mexican and international companies that promote locate alternative-income generation and employment to local communities ).…”

“…Monarch butterflies (Danaus plexippus) that hibernate in Mexico migrate between 1,200 to 2,800 miles from Canada and the United States to establish their colonies in the temperate fir and pine forests along the border between the states of Michoacán and the State of Mexico (Fig.1, Rendón-Salinas et al 2004-2013.…”

Density estimates of monarch butterflies overwintering in central Mexico

“…304 million stems, with an additional 1.34 billion stems remaining. Pleasants (2017) argued that 305 milkweed in corn and soybean fields produced 3.9 times more monarch eggs than milkweeds in 306 non-agricultural habitat (see Pleasants and Oberhauser 2013) and that, therefore, the loss of 307 Midwestern agricultural fields was especially hard hitting to monarchs; he suggested that the loss 308 of 850 million stems in corn and soy fields amounted to the equivalent of 3.31 billion non-309 agricultural stems of milkweed. Similarly, we estimate a 127 million monarch population would 310 require 3.62 billion stems of milkweed; with 1.34 billion stems remaining in the landscape, the 311 milkweed deficit could be as high as 2.28 billion stems, or ~700 million more stems than was 312 needed according to Pleasants (2017) to return the population to 6 ha of occupied overwintering At this time, we have too little information to posit an advantage of one density 340 estimation method over another, but it is likely that a combination, nay, a mixture, of reasons 341 contributed to differences in estimates.…”

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“…The largest population size recorded since monitoring 83 began in the early 1990s was 18.19 ha in winter 1996-1997. Since this peak in abundance, 84 monitoring suggests that the population has declined by over 90% (Brower et al, 2012; Vidal & 85 Rendón-Salinas, 2014; Rendón-Salinas & Tavera-Alonso, 2015), to a record low of 0.67 ha in 86 winter 2013 (Rendón-Salinas & Tavera-Alonso, 2015. These declines in abundance are 87 believed to be due, in large part, to declines in habitat availability in the breeding range of the 88 north-central United States, principally through loss of common milkweed (Asclepias syriaca) in 89 agricultural crops (Pleasants & Oberhauser, 2013;Pleasants, 2015Pleasants, , 2017.…”

“…These declines in abundance are 87 believed to be due, in large part, to declines in habitat availability in the breeding range of the 88 north-central United States, principally through loss of common milkweed (Asclepias syriaca) in 89 agricultural crops (Pleasants & Oberhauser, 2013;Pleasants, 2015Pleasants, , 2017.…”