ST. PAUL — If you are seeing a lot of monarch butterflies this summer, you are not alone.
After a few seasons of startlingly low numbers, Minnesota’s monarch population has dramatically rebounded. The same goes for Wisconsin, Michigan and southern Canada.
“This is an unusually high spike,” said Kyle Johnson, a moth and butterfly specialist for the Minnesota Department of Natural Resources. “The last couple of years had definitely been a low point.”
Scientists are relying on anecdotal observations at this point to judge the size of the monarch population. But there’s also hard evidence to back it up.
Cora Lund Preston, a spokeswoman with the advocacy group the Monarch Joint Venture, says a survey of the monarchs’ wintering grounds in Michoacan, Mexico, found the highest population in more than a decade.
“We won’t get an official count for the whole monarch population until it is documented this winter in Mexico,” Preston said.
Annual fluctuations in monarch numbers are not unusual, but Johnson and Preston agree this year’s spike is promising.
“We hit the population goal this year for eastern monarchs,” Preston said. “But we are looking at long-term trends and we are still down on average.”
Each spring, monarchs make their way north from Mexico to much of the eastern U.S. and southern Canada. The insect has a two- to six-week lifespan, and it takes them between three and four generations to get as far north as Minnesota.
This fall, the caterpillars that emerge from chrysalis as butterflies will gorge on flower nectar before making the marathon migration 2,000 miles south to the mountains and hillsides of an oyamel forest in Michoacan.
One species of plant makes this daunting trip possible — milkweed. “Milkweed is the only plant monarch caterpillars can eat,” Preston says.
That means the butterflies have gardeners like Sondra Mann to thank for providing food and lodging. About 15 years ago Mann began planting native plants, including milkweed, in the yard of her Minneapolis home with the help of a city grant.
Now she has what she calls her own little nature sanctuary filled with native species.
“I would definitely recommend planting native plants, if you have the space to do so,” Mann said. “There’s so much you can learn by watching insects.”
There was a flurry of milkweed planting after monarch numbers plummeted in 2014, and Preston and Johnson believe it likely played a role in the recent population rebound. The more milkweed patches there are, the easier it is for monarchs to migrate and reproduce.
“Monarchs will use any available habitat,” Preston said. “It is important we have habitat out there in as many different places as possible.”
This year, monarchs also had the weather on their side. A cool spring isn’t great for farmers, but it is ideal for the migrating butterflies.
Preston says cool weather at the start of their migration encourages the first generation of monarchs not to travel too far north too fast. Pacing their way northward typically means hardier broods.
Monarch's future still faces challenges
A banner year for monarch numbers in the Upper Midwest doesn’t mean the species is in the clear. In fact, both eastern and western populations of the butterfly still face serious challenges.
While populations of the eastern monarch are up this year, the western numbers are at their lowest in recent history.
Habitat loss is the biggest threat to the monarchs that make their summer home in Minnesota. While there’s a lot of milkweed out there, Johnson says patches are not always close enough together to allow for easy monarch migration.
The butterflies’ wintering grounds in Mexico are also under threat from logging and development. Without a stable place to spend the winter months, the butterfly population could crater.
All these challenges will be taken into consideration next year when federal regulators decide whether to add the monarch butterfly to the endangered species list.
“It’s been a good year for monarchs,” Johnson said. “That doesn’t mean they are making a great recovery.”