Milkweed shortage not the only reason for monarch decline

Monarch butterflies during migration. PHOTO COURTESY GENE NEIMINEN/USFWS.
Monarch butterflies at their overwintering grounds in Mexico. Photo courtesy Gene Neiminen/USFWS.

‘We have to get the science right’

Staff Report

The decline of milkweed may not be the main factor driving monarch butterflies toward oblivion, according to a new study by Cornell University scientists. Weather, habitat fragmentation and dwindling sources of nectar in the autumn are also critical, the new study reports.

“Thanks to years of data collected by the World Wildlife Fund and citizen-scientists across North America, we have pieced together the monarch life cycle to make inferences about what is impacting the butterflies,” said Cornell University Prof. Anurag Agrawal.

“Given the intense interest in monarch conservation, the blame being put on herbicide use and the national dialog about potentially listing monarchs under the endangered species act, we have to get the science right,” Agrawal said.

The study, published in the journal Oikos, did not find evidence supporting the “milkweed-limitation hypothesis” during the monarch’s summer breeding season in the midwestern and northeastern United States. Milkweed is the main food source for monarch caterpillars in summer, but not as the butterflies leave for their epic southern migration in autumn.

Instead, the new research suggests monarchs are facing challenges on the journey from  the U.S. and southern Canada to the overwintering grounds in Mexico.

In any given year, four generations of monarch butterflies traverse much of North America over a 2,000-mile trek beginning in early spring when they leave the Mexican wintering grounds. In the first generation, millions of monarchs flow through Texas and Oklahoma, with the subsequent generations moving into the Midwest and Northeast, until the start of fall, when the fourth generation returns to the mountainous, high-altitude Oyamel fir forests of central Mexico.

Despite the seemingly good news of annual population bounce-back on the return from the south each year, the scientists were clear that the monarch population has been dwindling.

“The consistent decline at the overwintering sites in Mexico is cause for concern. Nonetheless, the population is six times what it was two years ago, when it was at its all-time low.” Agrawal credits the population rebound to improved weather and release from the severe drought in Texas.

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One thought on “Milkweed shortage not the only reason for monarch decline

  1. The authors didn’t actually go into the field and determine “monarchs aren’t finding enough nectar to feed on as they travel long distances,” hence their nectar limitation hypothesis is hypothetical and imaginary.

    The authors also ignored UC Davis professor Art Shapiro findings that monarch butterfly numbers in California boomed last summer during the height of California’s extremely severe 4 year drought. Dr. Shapiro wrote this about the summer & fall of 2015: “The Monarch, Danaus plexippus, had its best year in a decade, breeding throughout the [Sacramento] Valley all season long, and reversing a decline lasting decades! http://butterfly.ucdavis.edu/…/LOOKING_BACKWARD_2015.pdf The monarch overwintering population in northern California during the fall winter of 2015-16 also boomed.

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