Loss of migration: Threat for Monarch butterflies

Monarch butterflies (Danaus plexippus), may be the most familiar in North America, perform annual migration across the continent. They travel from breeding grounds in the eastern U.S. and Canada to central Mexico to spend the winter every year.

Globally, these pretty butterflies are not considered as endangered species.  However in recent years their numbers have declined rapidly and change in their migration has been observed. This may be due to the reduced availability of milkweed plants as a result of changing agricultural practices and land use patterns. The monarchs lay eggs on the underside of young leaves of milkweed plants and also feed on milkweeds extensively at larvae stage. Thus to solve this, concerned gardeners have started planting milkweed to help replace some of the butterflies’ lost breeding habitat. However tropical milkweed is the most commonly available milkweed species. Monarchs love it, but, tropical milkweed does not naturally die back in fall like perennial milkweeds native to North America. This allows monarchs in those areas to stay back and keep breeding all winter without migration.

A recent study by ecologists of University of Georgia, demonstrates that these sedentary winter-breeding butterflies are at increased threat of disease.  They analysed more than 5,000 monarch samples collected from 100 sites across the U.S., Canada and Mexico, for a specific protozoan infection. The samples, collected by gently pressing clear tape against the butterfly’s abdomen, were viewed under microscope. They found that non-migratory, winter-breeding monarchs in the southern U.S. were five times more likely to be infected with parasites than migratory monarchs sampled in their summer breeding range or at overwintering sites in Mexico.

Previous studies from the same institute showed that for some wildlife species, including monarchs, long distance migration helps to reduce infectious disease transmission. This may be because of the fact that infected individuals falls off during the strenuous journey, or migrating animals get a chance to move away from contaminated habitats. Thus non-migratory monarchs don’t have the benefits of migration, thus infections in majority of monarchs at winter breeding sites are observed.

Experts think that transmission of some pathogens must be controlled historically by migration, which has now become a problem because of the changing of migration patterns due to human activities, whether or climate change, habitat destruction or barriers to migration.

The researchers thus advice to replace tropical milkweeds with native milkweeds to promote the migration of monarch butterflies.

This finding could be applicable to other migratory species as well.

The study is published recently in the journal, Proceedings of the Royal Society B.

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Arunima Maiti

Arunima Maiti

Biomedical scientist with special interest in reproductive biology.

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