monarch-esa-petition-final_61585

Brower et al. (2012a) identify loss of breeding habitat due to land development as one of the
primary factors implicated in the drastic downward trend in monarch abundance in recent years
(in conjunction with other threat factors, including severe weather events and loss of milkweed
due to increased herbicide use caused by the cultivation of genetically-engineered, herbicideresistant
crops) (p. 96). Flockhart et al. (2014) also identify urbanization as a contributing factor
in the land-use change that is driving monarch declines (p. 4).
Development of roads causes direct loss of monarch habitat, and chemicals sprayed on roadsides
can also be harmful to monarchs including herbicides. Road maintenance and other related
activities may also impact the butterflies. For instance, the application of road salt to melt snow
and ice during winter can affect butterflies the following summer. Road salts are applied widely
during winter months. For example, in Minnesota in the metropolitan area of Minneapolis and
St. Paul, approximately 300,000 tons of sodium chloride are applied to roads each winter (Snell-
Rood et al. 2014, p. 1).
Sodium is important for the function of neural and muscle tissue and influences brain size and
other traits, but can have varying effects at different life stages. Sodium availability is limited in
most ecosystems, which likely led to the evolution of sodium cravings and sodium foraging
behaviors. For example, adult male butterflies of many species engage in “puddling” to get
sodium that they then transfer to females as part of mating practices (Snell-Rood et al. 2014, p.
1). Changes in sodium availability translate into physiological effects on butterflies including
effects on neural and muscle tissue development.
Excessive sodium, however, appears to have detrimental impacts on monarch larvae. Snell-Rood
et al. (2014) reared monarchs on milkweed collected from roadsides or milkweed collected from
prairies and found that milkweeds readily take up roadside sodium which is then taken up by
larvae. They found that the survival rates of monarch caterpillars were significantly lower on
roadside milkweed leaves than on milkweed leaves from prairies (40.5% vs. 58.2%, P = 0.02). In
surviving butterflies, the fitness effects of the induced physiological changes were unclear. They
also reared cabbage white butterflies (Pieris rapae) on diets with varying levels of sodium and
found that butterfly survival was significantly lower on a high-sodium artificial diet than on a
medium- or low-sodium diet (high: 10.9%; medium: 34.3%; low: 41.7%; P < 0.0001).
Due to widespread loss of milkweed in agricultural fields attributable to increased use of
herbicides resulting from near-ubiquitous planting of genetically engineered, herbicide-resistant
corn and soybeans, roadside milkweeds are becoming increasingly important habitat for
monarchs. Flockhart et al. (2014) estimate that roadside habitats now harbor 10 percent of all
milkweeds in eastern North America (p. 16). It is estimated that in Iowa, which is representative
of the monarch’s Midwest breeding grounds, roadsides harbored 13 percent of milkweed in
1999, and 36 percent of milkweed in 2012 (based on data supplied by John Pleasants). Reduced
caterpillar survival due to road salt could thus have significant effects on monarch populations,
particularly so given the newly heightened reliance on roadside milkweed for recruitment.
Loss and Degradation of Overwintering Habitat in Mexico
The eastern monarch population primarily overwinters in oyamel (sacred) fir (Abies religiosa)
forests in the mountains of the Trans-Mexican Volcanic Belt in Central Mexico. The high
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