Mechanisms behind the monarch's decline

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Science  22 Jun 2018:
Vol. 360, Issue 6395, pp. 1294-1296
DOI: 10.1126/science.aat5066

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Every year, monarch butterflies (Danaus plexippus) migrate several thousand miles between their breeding sites in eastern North America and their overwintering sites in Mexico. During the spring and summer, monarchs fly to their large breeding territory east of the Rocky Mountains and into Canada, where they lay eggs on milkweed plants (Asclepias spp.), the only food source for their caterpillars. After four generations, the butterflies fly south in late summer to overwinter in comparatively tiny high-elevation fir forests in central Mexico (1). Data released earlier this year by the World Wildlife Fund (WWF) Mexico and the North American Butterfly Association (NABA) suggest that reduced success during migration, due to environmental and anthropogenic factors, may help explain the falling monarch population (see the figure).