Monarch butterfly cluster

In “Monarch butterfly update,” Ernest Williams, the William R. Kenan Professor of Biology Emeritus, said that although the Northeast saw more monarch butterfly caterpillars and adults in 2019, the monarch population overall continues to decline. The article was published recently in the Adirondack Almanack (Saranac Lake, N.Y.).

Williams wrote that the region’s “thrilling abundance of monarchs in 2019 was unusual.” But, based on the official count recently released by the World Wildlife Fund-Mexico, “less than half as many butterflies were seen in Mexico this winter compared to the previous winter,” he said.

Noting that weather conditions in the Northeast last year were ideal for monarch growth and reproduction at each stage of development, Williams said that “monarchs from the Northeast reach the Mexican overwintering grounds at a much lower rate than do monarchs from the Midwest.

“In short, our Northeastern monarchs contribute less to the surviving and overwintering North American population than do monarchs from the Midwest,” he pointed out.

Williams discussed several factors that work against monarch survival. He explained how in the Midwest, loss of habitat and less milkweed has led to lower rates of monarch reproduction. Climate change led to a warmer fall, delaying the butterflies’ migration to the forests of Mexico, he said, adding that drought in Texas and northern Mexico lessened the availability of nectar that butterflies need to build up their energy supplies for the winter.

“Year 2019 was a fluke for the Northeast, but it serves as a reminder that long term trends are better indicators of their status than single good or bad years and that critical determinants of monarch abundance are available habitat and weather throughout the butterfly’s full annual cycle,” Williams concluded.

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