Study challenges belief that milkweed is sole cause of monarch butterfly declines


Steep declines in the number of monarch butterflies reaching their wintering grounds in Mexico are not fully explained by fewer milkweeds in the northern part of their range, researchers report in a new study.

The research, published in the journal BioScience, reviews decades of studies of monarchs and includes an in-depth analysis of milkweed populations in Illinois, a state at the heart of the butterflies’ summer range.

Over several generations in their summer range, monarchs migrate north to breed – but only one generation makes the return trip to their wintering grounds in Mexico.

The new analysis confirmed that milkweed numbers have dropped by about 95 percent in cropland in Illinois over the last 20 years….

“But we have more milkweeds in natural areas than previous studies suggested,” [David] Zaya [Illinois Natural History Survey plant ecologist, one of the coauthors of the study] said. “I would say the milkweeds in natural areas are buffering the loss of milkweeds in the agricultural areas.”

Other lines of evidence challenge the notion that milkweeds are the sole cause of monarch declines, the researchers said. Despite smaller numbers leaving Mexico each spring, once they get to Illinois, the monarchs seem to quickly rebound.

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