As many as 1.8 billion additional stems of milkweed plants may be needed in North America to return imperiled monarch butterflies to a sustainable population size, according to a recently published U.S. Geological Survey study.
Monarchs rely on milkweeds for food and for breeding habitat, but over 860 million stems were lost in the northern United States over the last decade. Scientists with the USGS and partners examined the density of Eastern migratory monarch butterflies overwintering in Mexico from 1979-2002 and the amount of milkweed plants available to them in North America. The study found that 3.62 billion milkweed stems are needed to reestablish this monarch population, but only 1.34 billion stems remain in the U.S.
“Monarchs in eastern North America are a beloved insect, but they’re in jeopardy, partly due to the loss of milkweeds in cropland,” said Wayne Thogmartin, a USGS scientist and the lead author of the report. “Our study is important because it helps specify the conservation needs of this charismatic species.”
The Eastern migratory population of monarch butterflies, which spends the winter in Mexico but migrates to the eastern U.S. and Canada to reproduce during warmer months, declined by about 80 percent over the last decade. The U.S. Fish and Wildlife Service is considering a petition to list monarchs under the Endangered Species Act. The population is at risk of extinction unless its numbers increase significantly.
Because counting individual monarchs is challenging, scientists measure population size based on the geographic area that their colonies cover while spending the winter in Mexico. The population size of Eastern monarchs was 2.91 hectares, or 7.19 acres, in the winter of 2016-2017, which is a decrease from about four hectares during the 2015-2016 season. The U.S., Mexico and Canada aim to increase the number of Eastern monarchs wintering in Mexico so that they occupy about six hectares, or about 15 acres, by 2020.
Based on population estimates from 1993-2014, the scientists determined that the six-hectare monarch population goal represents about 127 million butterflies. Given that it takes about 28.5 milkweed stems to produce a single monarch overwintering in Mexico, the scientists calculated that a total of about 3.62 billion milkweed stems are needed in the U.S. to support 127 million butterflies.
Reestablishing milkweeds could help bring monarchs closer to the 15-acre goal, but restoring 1.8 billion milkweeds is difficult because location matters.
“Milkweeds in corn and soybean fields produce more monarch eggs than milkweeds located in non-agricultural areas,” Thogmartin said. “Competing demands for space in these agricultural locations limit the highly desirable habitat available to milkweeds and monarchs.”
The new study addressed milkweed numbers because milkweed is the only plant that provides food for monarch young. However, adult monarchs feed on a range of nectar sources, so the U.S. Fish and Wildlife Service recommends planting both milkweeds and nectar plants that are native to specific areas.
Reasons for monarch population declines are complex, although some evidence suggests that loss of breeding habitat is a primary factor. Other factors include adverse weather conditions in recent years, loss of overwintering habitat, disease and exposure to contaminants.