Census shows monarch butterfly population not in decline, but scientists challenge findings, blaming milkweed loss

A monarch with a tag, which is used by citizen scientists in the yearly monarch butterfly census

Monarch butterfly populations have taken a nosedive over the last 20 years, according to researchers who monitor the number of butterflies that spend the winter in Mexico every year. But organizations of citizen scientists in the United States who conduct yearly censuses of monarchs in state parks and other locations in the summer have reported no consistent dip in the number of butterflies they see.

This discrepancy has led some to challenge the widely accepted belief that loss of milkweed on the U.S. landscape has driven the decline of the species.

[John Pleasants, an adjunct assistant professor in the Department Ecology, Evolution and Organismal Biology,] set out to pinpoint the reason for the discrepancy and found that it results from the fact that monarch activity has shifted out of agricultural fields, where milkweeds were once common. For example, roughly half of farm fields in Iowa used to have patches of milkweed, but the widespread use of the herbicide glyphosate has kept fields free of milkweed in recent years. That leaves monarchs with no choice but to concentrate in other areas with milkweed, Pleasants said. It is these other areas where summer censuses are conducted.

This increasing concentration effect masks the decline in population size, Pleasants said.

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