LAWRENCE — Butterfly enthusiasts in the eastern half of North America will have to look harder this spring and summer to spot the winged migrants that have become an international symbol of insect conservation.
At a news conference in Mexico City on Wednesday, conservation scientists revealed that the second smallest population of monarchs on record reached their overwintering sites in the mountains of south-central Mexico last fall.
Scientists gauge the size of the roosting population in terms of how much forest the clustering butterflies occupy.
The size fell to 2.2 acres this winter, the second smallest since scientists began tracking in this manner in the early 1990s and down from a peak of 45 acres in the mid-1990s. The lowest recorded size was about 1.7 acres in the winter of 2013-2014.
The news could intensify debates among monarch scientists over whether the species deserves federal protection, how much risk the species (or its migration) faces, the size of the species’ population historically and how to increase current numbers.
“A lot of people may see very few this year,” said Kristen Baum, a professor of ecology and evolutionary biology at the University of Kansas. “It could be a rough couple of years.”
Baum directs Monarch Watch, a conservation, education and research program at the Kansas Biological Survey and Center for Ecological Research. It typically distributes more than 350,000 butterfly tags each year to thousands of volunteers across the continent to help study the species’ fall migration.
Why so few butterflies this winter?
So, how many butterflies are roosting this year? Translating acres of roosting insects to numbers of individuals is tough, but a 2017 study suggests calculating about 21 million butterflies per hectare (2.47 acres). That translates to fewer than 19 million roosting monarchs this year.
Widespread use of agricultural herbicides that kill milkweed, the spread of cities and suburbia, insecticides, parasites, climate change and logging in Mexico all get blamed for declining numbers of migrating monarchs in recent decades. Drought years deal an extra blow.
Last year’s drought along key parts of the migration route, such as Oklahoma and Texas, meant the insects likely struggled on their way south in late summer and fall to find enough flower nectar to complete the journey and survive the winter in the mountains of Mexico, according to Monarch Watch.
The good news: Insect populations can bounce back fairly fast when conditions improve, so roosting numbers could climb a year from now if rainfall and temperatures cooperate.
“That’s kind of the nature of insects,” Baum said. “There is the capacity to build back up.”
Rainfall impacts flower production. Temperatures affect how quickly or slowly eggs develop into adults that can carry the baton northward for the species’ annual migration to the northern U.S. and Canada.
“The thought is, the population will rebound,” Baum said. “But it’s going to take time and it really depends on all those other conditions.”
