The mystery of the monarch and Canadian Tiger Swallowtail
We're joining forces to relate the curious tale of two butterfly species. One is a long-distance migrant, the other a stay-at-home local.
Though they have many great differences, they share on curious thing in common. Each year, we first see them within only a few days apart!
Of course, the long-distance migrant is the familiar monarch. The local resident is the Canadian Tiger Swallowtail (CTS, for short).
Monarch caterpillars feed on milkweed. It takes at least two generations to reach here from where their ancestors, who overwintered in Mexico, mate and lay eggs in Texas. No stage can survive below-freezing conditions.
On the other hand, the CTS uses a variety of host plants, such as birch, popular and cherry. After overwintering as a pupa when temperatures can dip to -45 degrees, the adult CTS emerges in the spring.
Though the date of their first appearance here can vary widely from year to year, they show up about the same time in northern Minnesota in any given year.
Now we'll relate our stories of how we discovered this curious phenomenon.
In 1996, I started working for the USGS Shingobee Headwaters Aquatic Ecosystems Project (SHAEP).
My boss said I was a very observant and needed to start keeping phenology notes. He further tried to explain it to me, but said just write everything down. So I started keeping track. The two butterflies I knew were the monarch and the CTS, just called "swallowtail" then.
After several years, I noticed they both showed up about the same time. So I contacted John Weber and John Latimer to confirm.
For reasons that are still unclear, in 1995 I started keeping track of every butterfly I encountered on my walks. (I started doing the same thing for dragonflies in 1997.) So far I've tallied 110 butterfly species and over 266,000 individuals.
Though Dallas started keeping his phenology sightings only on paper, for a number of years now he has them neatly organized in computer spreadsheets.
On the other hand, I'm still in the paper-era. So it took me a little time to respond to Dallas' Monarch-CTS request.
He was very excited that my data did confirm what he had been seeing.
For the 25 years of my sightings, there has been a gap of three days or less for 14 of those years. In fact, eight times the monarch and CTS showed up on the same day.
My earliest first and latest "first sightings" both are among the eight years with no gap. Even more curious is that my earliest for both was May 10, 2012, while the latest first was 29 days later on June 8, 2013. Just think, almost a full month's difference, but both showing up on the same date!
My overall 25-year average is May 27 for CTS and just a day later, May 28 for the monarch!
Latimer's been the volunteer host for Northern Community Radio's KAXE weekly phenology show since 1983. He told Dallas that once he sees one, either a monarch or a CTS, he starts looking for the other.
Some years ago, he told Dallas, "I I think you are on to something."
Though Latimer lives 75 miles east of Dallas in Grand Rapids, he's roughly at the same latitude as both Dallas and John Weber.
What we've learned
The 23-year graph displays the CTS and monarch first showing up at the same time. The average for the CTS is May 25 — just three days before the first monarch on May 28.
Though they arrive about the same time, that "same time" stretches for 29 days (i.e. from May 10 to June 8).
For a quarter of a century, we've witnessed this CTS-monarch pattern at this latitude.
Dallas has computerized his and Weber's flight seasons for all butterfly species for this area. So far, only the CTS-monarch pair exhibit this appearance pattern.
What we don't know
We really don't know what causes this phenomenon. Dallas speculates it could be tied to the number of degree-days it takes to get milkweed to about a 6-inch height that coincides with the first arrival of migrant monarchs and locally-emerged CTS. We have no way of determining what that "degree-day" calculation would even be, if it is a driving factor at all.
As you can see in the graph, a divergence in the lines begins in 2015. The CTS now precedes the monarch by two to five days. Is this a manifestation that climate change is already having an impact?
We're at the southern edge of CTS' range. As climate change pushes plants and animals further from the equator, will a CTS-monarch pattern still exist or will it be replaced with an Eastern Tiger Swallowtail (ETS)-monarch pattern? Dallas has checked with a phenologist in the Twin Cities who did not find a ETS-Monarch pattern.
The CTS-monarch pattern may just fade away with continued climate change.
In the meantime, marvel that the curious tale of a long-distance migrant coupled with a stay-at-home local even still exists at all in our neck of the woods.