For monarch butterflies, the path to
endangered species status could be paved with good intentions.
Throughout the United States, monarch-lovers
are replenishing the supply of milkweed — the plants monarchs lay their
eggs in — by growing it in their gardens. However, they’re planting the wrong
species of milkweed. And in doing so, well-intentioned gardeners are actually
putting more stress on declining monarch populations by convincing them
to give up the annual migration altogether.
A
Really Good, Bad Idea
Every year, millions of monarchs
migrate from their summer breeding grounds in the eastern U.S. and Canada to
wait out the winter in central Mexico. However, since the 1990s, monarch
numbers have declined by nearly 90 percent. Humans, with all our farm fields,
roads and buildings, are largely to blame for the monarch’s decline.
Deforestation has decimated the monarch’s preferred habitats both north and
south. And herbicide-resistant crops allow farmers to spray stronger
weedkillers on their fields, which kills native milkweed in the process.
To counteract the loss of milkweed
on farm fields, sympathetic gardeners in southern states are giving monarchs a
meal by planting milkweed in their backyards. Unfortunately, the milkweed
they’re planting is an exotic tropical species called Asclepias
curassavica that grows year-round, unlike the native species, Asclepias
incarnata, that dies off seasonally. So, rather than fly south for the
winter when the milkweed dies off, monarchs are forgoing the Mexico migration,
living and breeding year-round in the south. And that’s trouble.
A
Protozoan Problem
That’s because of a protozoan
parasite called Ophryocytis elektroscirrha (OE). This parasite, if eaten
by monarch caterpillars, causes wing deformities in the adult butterfly and
shortens their lifespan. Infected butterflies shed spores that can then go on
to infect other youngsters.
OE isn’t usually a problem, though,
because of two things: the butterflies’ annual migration, and the type of
milkweed they eat. Infected monarchs will generally perish on the trip to
Mexico, which helps control OE’s spread. And by the time they return, the
native milkweed plants have died and new ones have grown up — another natural
check on spreading the infection. But the tropical type of milkweed survives
cold winters, and so does its pestilent guest.
Going
Nowhere, Going Down
To see just how migration was
affecting monarchs’ OE infection, researchers used a combination of field
studies and citizen science. They tested monarchs at various breeding sites in
the U.S. and Mexico over a period of two years, compiling more than 5,500
samples. Samples were taken by applying tape to the
abdomens of monarchs (none were harmed) to nab spores if they were
present.
The researchers found that monarchs
that overwintered in the United States were 5-9 times more likely to host the
parasite than were monarchs that spent the winter in Mexico. Researchers
published their results Wednesday in Proceedings of the Royal Society B.
How
to Help
Tropical milkweed is far easier to
purchase than the native species, but stores selling the native seeds are on
the rise. If you can’t get your hands on native milkweed, researchers suggest
cutting tropical milkweed down twice a winter to contain OE.
The U.S. Fish and Wildlife Service
is currently reviewing the monarch’s status under the Endangered Species Act,
and we should know this year if monarchs will be added to the endangered
species list. In the meantime, the monarch’s story just goes to show how
complicated our relationships with other species really can be.
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