Female Monarch Butterflies Disappearing

Female monarch butterflies are in decline, a new study finds.

Monarchs, the most iconic of butterflies, famously migrate from the eastern United States to spend each winter clustered on a few mountaintops in Mexico, where sometimes millions gather. (Monarchs west of the Rockies spend the winter along the California coast.)

But a new analysis of 70,000 eastern butterfly specimens caught at the wintering site over the last 30 years shows that female butterflies have dropped from 53 percent of the population in samples from 1975-1985 to 43 percent of the population in the last decade. The study appears in Biology Letters.

"It was really an accidental discovery," said study lead author Andy Davis of the University of Georgia in Athens. "I hadn't expected to find any decline over time. I expected just to figure out what the sex ratio was in the population."

Davis had been helping colleagues collect butterflies in Virginia during the fall migration in 2008, when he discovered that only 35 percent of the butterflies were females. This led him to scour published papers over the last decades to try to determine the sex ratio of monarchs captured at the overwintering site. He was surprised to see the steady decrease in females.

He also looked at the sex ratio of butterflies collected at points during the their southward migration and found a decline in females in those populations, too.

"That means the females aren't entering the migratory pool," he said. "Whatever it is is happening before or during the migration."

Why the decline is happening remains a mystery, but Davis has three possibilities in mind.

The first stems from an increase in numbers of tropical milkweed plants in the United States, especially in the South. Unlike native milkweeds, in which monarchs have evolved to lay their eggs, tropical milkweeds do not die back in winter.

These tropical milkweeds, available to monarchs when milkweed shouldn't be, may lure the females to stop migrating and lay their eggs. There is some evidence that supports that this is happening, Davis said.

Another possibility is that thanks to global climate change, the cold temperature cues that monarchs rely on to know it is time to migrate are missing. Without these, females might stay put and lay eggs.

The third theory is that a protozoan parasite called Ophryocystis elektroscirrha is killing females faster than males. "We know that this parasite affects females more than males," Davis said. "We know this parasite has been increasing."

Davis plans to continue monitoring the butterfly sex ratios in a more rigorous way, by collecting more butterflies along the migration route at various times.

"If this trend is real, and it looks real, it is pretty surprising," John Jaenike of the University of Rochester in N.Y. told Discovery News. Jaenike studies another butterfly parasite. "Population growth depends on the number of females, not males, and this could have some adverse impacts for the monarch population to grow."