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Monarchs that ‘drop out’ of the migration game pick up more parasites

There are few migrations as iconic as monarch butterflies’ annual flight from North America to Central Mexico and back each fall and spring. The butterflies (Danaus plexippus) take to the skies by the millions, braving storms, drought, predators, and humanmade barriers to make the 3500-kilometer journey each year.

Monarchs know when it’s time to head south based on several cues, including the death of milkweed plants, the monarch caterpillars’ main food source and a prime spot for springtime egg laying. However, as native milkweed plants grow scarcer, some gardeners in the southern United States have begun to plant tropical strains that don’t die in the fall. That’s had the unintended effect of making some monarchs “drop out” of their seasonal migration and establish year-round colonies near these permanent food sources.

But most monarchs still migrate, and these travelers are now mingling with the resident populations when they make pit stops on their treks to and from Mexico. Wanting to know more about the consequences of these interactions, scientists started to catch and analyze butterflies during the 2014 and 2015 spring and fall migrations, looking for signs of deadly parasites. They knew from past research that migrating helps the monarchs avoid passing on such infections because afflicted butterflies die off during the journey and the population gets to spend a portion of the year away from plants that harbor the parasites.

The researchers worked with citizen scientists to examine more than 500 monarchs from nine different sites in Texas, half of which had resident colonies, and half of which were exclusively pit stops for migratory monarchs. Migratory monarchs that stopped at resident sites were 13 times more likely to be infected by parasites and three times more likely to reproduce outside of the normal breeding season compared with other migrants, the team reported earlier this month in Ecology Letters.

This doesn’t necessarily mean the migrants at these sites became infected by mixing with residents. The scientists say butterflies that are already infected might simply drop out of the migration at the first sign of milkweed plants—often the tropical strains that host the resident colonies. Sensing the maturity of the tropical milkweed might also cause a biological change in the butterflies, telling them to cut their migration short to mate early, a ritual they usually put off until after the springtime migration north is completed.

The rise in parasitic infections isn’t the only reason monarch populations have plunged over the past 2 decades, but the scientists say it is concerning enough that all gardeners should steer clear of planting nonnative milkweed in their yards. Native plants that change with the seasons, they add, should help the monarchs regain their enthusiasm for their winter trips to Mexico.