The flight and plight of the Monarch
Since 1995, my wife, Marlene, and I have had 23 annual accounts of our Fourth of July butterfly count seasons appear in the newspaper.
Earlier this month, Shannon Geisen invited me to submit periodic columns based on other butterfly and dragonfly sightings I've had. "Perfect timing," to quote one of my favorite phrases.
On March 5, the release of the data for 2017-18 overwintering Monarch in Mexico prompts my first column. I'll focus on that data, plus provide some of my Monarch observations in this neck of the woods.
Monarchs through the year
Monarchs are actually a tropical species. No stage — egg, caterpillar, chrysalis nor adults — can withstand freezing temperatures. However, they have developed an interesting relationship with milkweed as their host plant. Aided by various milkweed species, they surge north to repopulate much of North America within a few short months each year.
Then a single, migratory generation flies south to overwinter in highland groves of firs west of Mexico City. After loafing for some months, they fly to south Texas in late February/early March. There, the annual saga resumes as it takes one or two generations to reach our area, on average, by May 28.
During the course of a year, Monarch generations cover a 4,000-mile, round-trip journey. The lone migratory generation to Mexico accounts for about half of those miles.
I contend that this is the most important generation. Why? The overwintering generation provides the basis for the next year. A "large" population literally represents "strength in numbers" to meet the many threats, obstacles, etc. Monarchs meet during the course of that year. One the other hand, a "smaller" population has "the deck stacked against it" in more than one way.
Monarchs in Mexico
Winter sites in Mexico provide the best opportunity to estimate actual Monarch numbers, according to the U of M's Monarch Larva Monitoring Project. Late December is chosen since that is when the overwintering clusters are most compact and exhibit the least movement.
Less-complete measurements began in 1976. Consistent measurements have been conducted since 1993 and now incorporate GPS technology rather than using measuring tapes around the perimeters.
Results are reported in number of hectares occupied. Researchers estimate there are roughly 21 million Monarchs per hectare. (A hectare equals 2.471 acres.)
Winter 1996-97 was the banner year with 18.19 hectares occupied. For a number of years, that winter was considered a "billion Monarch" event. Using more refined analysis, they deflated that estimate to 382 million. But, by far, that has been the largest concentration of Monarchs in the past two decades.
The data released earlier this month for overwintering Monarchs showed a further shrinkage — from 2.91 hectares in 2016-17 to 2.48 in 2017-18. That equates dropping another 9 million Monarchs — going from 61 million to 52 million.
What's gone wrong?
A number of factors are ganging up on Monarchs over the past two decades. Most prominent are these:
• Ag practices in the corn/soybean belt where more than 90 percent of overwintering Monarchs originate. Heavy-duty herbicide applications have eliminated most of the milkweeds that used to be found in those fields.
• Habitat loss.
• Climate change with droughts, floods and so forth.
• Pesticide use.
In the 24-year history presented for winter data, only five winters have been lower than the 2.48 hectares measured for 2017-18. It is not encouraging that those five have been in the past six years.
Is a rebound in store? Monarchs are insects. In the past, insect populations have gone through tremendous peaks and valleys. However, as I eyeball that 24-year chart, I see a trend pointing to an ominous further drop.
My local data
Looking at data I've collected on Monarchs on my walks during the past five years reveals some interesting things going on.
Though I covered roughly the same miles on foot each Monarch season, their presence differed greatly. Likewise, the number of migratory Monarchs varied widely. However, I was struck by the constant 83 percent proportion the migratory represented for three straight years! Only when "my" migratory Monarchs exceeded 1,000 in a year did an increase in the overwintering area occur in Mexico. Did some of "mine" make it there?
After the first 2018 Monarchs show up in northern Minnesota, I plan to write a column examining first arrival history since 1995.
Things you can do
• Where practical, plant or allow milkweed to grow. Monarch caterpillars depend on it. Adults Monarchs and many other pollinators love to nectar on the blossoms.
• Encourage county and township jurisdictions to begin serious fall mowing after Sept. 15. Most migrant Monarchs will have passed through refueling on roadside nectar by then.
• Provide late-season blooms, such as blazing star, asters, goldenrods, etc. This will provide a necessary energy boost as Monarch head to Mexico.