parasite

For years, we have heard that the iconic monarch butterfly is in great peril. Monarchs east of the Rocky Mountains undertake a 4-generation, 3,000-mile migration in late summer to their winter home in central Mexico. Since the 1990s, the overwintering monarch colonies began a steep decline causing scientists to fear for the sustainability of the species. Trying to help, ordinary citizens have been planting milkweed – the food for the caterpillars – and have been rearing eggs and caterpillars on their porches and releasing adult butterflies.

A number of recent studies have rocked the world of monarch science. A growing group of scientists now believe that monarchs are not in fact imperiled and that the home rearing and commercial breeding of monarchs is actually a major threat to the butterfly.

According to a comprehensive 2022 study, there has been no sharp decline in Eastern monarch populations across their summer breeding range. The long-term declines in winter colony sizes are real but do not appear to be affecting the collective breeding population of Eastern monarchs.

An undisputed major threat to monarchs is a parasite known as OE, which weakens butterflies and also makes it unlikely for them to complete migrations. It turns out that overcrowding of butterflies resulting from the efforts of backyard and commercial breeders is increasing the spread of the disease. The butterflies are in need of social distancing.

Monarch scientists are split into two camps. One group calls for the protection of the butterfly under the Endangered Species Act. The other says that what monarchs most need is for us to leave them alone. It is a confusing situation.

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Web Links

Rethinking Monarchs: Does the Beloved Butterfly Need Our Help?

Photo, posted October 1, 2021, courtesy of Renee Grayson via Flickr.

Earth Wise is a production of WAMC Northeast Public Radio

The monarch butterfly is one of the world’s best-known butterflies and has become the symbol for a whole class of imperiled pollinators. Populations of the iconic orange-and-black insects have declined dramatically in recent decades.

According to the U.S. Fish and Wildlife Service, the eastern monarch population has dropped 88% between 1996 and 2020. Habitat loss, pesticide use, and global climate change are some of the factors driving monarch butterfly population declines.

According to a new study led by scientists at Emory University, monarch butterflies are also increasingly plagued by a debilitating parasite. The study, which was recently published in The Journal of Animal Ecology, reviewed 50 years of data on the infection rate of wild monarch butterflies by the protozoan Ophryocystis elektrosirrha (or O.E.). The research team found that the O.E. infection rate had increased from less than 1% of the eastern monarch population in 1968 to as much as 10% today.

The O.E. parasite invades the gut of monarch caterpillars. If the adult butterfly leaves the pupal stage with a severe parasitic infection, it begins oozing fluids from its body and dies. If the infection is lighter and the butterfly survives, it will not fly as well or live as long as uninfected butterflies.

The study found that one of the contributors to the rise in parasitism is the increased density of monarchs in places where they lay their eggs. The increased density may be due to several factors, including the loss of habitat, the widespread planting of non-native milkweed, and by people raising large numbers of monarchs in confined spaces.

If the infection rate continues to increase, monarch butterfly populations will likely continue to plummet.

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Web Links

Monarch butterflies increasingly plagued by parasites

Plight of the Monarch

Saving The Monarch Butterfly

Photo, posted September 18, 2008, courtesy of Roy Niswanger via Flickr.

Earth Wise is a production of WAMC Northeast Public Radio.