A Great Season for Monarchs . . . Now the Bad News . . . Or is it?


A male monarch refueling on Heath Aster during the 2018 migration.

By all accounts, the monarch butterfly in the Midwest has had a phenomenal year. At Butterfly Ridge we raised and released 75 monarchs from eggs and caterpillars found on our site. We have heard of other enthusiasts around Ohio raising well over 100 monarchs in 2018. As comparison, in 2017 Butterfly Ridge raised and released 30 monarch butterflies. And for us, 2017 wasn’t a half-bad year either!

Based on population numbers we have seen here in Ohio, and reports throughout the Midwest, the overwintering population in Mexico this winter should be quite impressive! Will the size of the overwintering roosts top double digits in hectares, a population size not observed since 2004?

The answer to that question, and possibly the bad news, is a resounding ‘probably not’! While the number of monarchs in the Midwest has been outstanding in 2018, the odds of that translating into a large overwintering population in Mexico is slim, at least if past results resemble the future.

As I mentioned previously, at least in southern Ohio, 2017 was a pretty impressive year for monarchs as well. However, the size of the overwintering population in Mexico during the winter of 2017-18 actually declined by nearly twenty percent when compared to the previous winter. How could that be?! How can more monarchs in the Midwest equal fewer monarchs in Mexico?

In 2017, late summer/early autumn temperatures were unseasonably high in the Midwest, just as they are currently. These increased temperatures encouraged monarchs to stick around in the Midwest, rather than beginning their trip south in September as they should have. These late departures simply ran out of time to make it to Mexico. Some got caught in cold fronts, others decided, I am told, to hang out along the Gulf Coast instead.

2018 is playing out the same way. In fact, in Ohio, monarchs have tried to squeeze in yet another generation rather than flying south like they should. As I write this during the second week of October, people from throughout Ohio are reporting still having monarch caterpillars. Those caterpillars are still two weeks away from becoming adults. Those monarchs, optimistically, will not even start heading toward Mexico until, in a normal year, they should’ve already arrived in Mexico.

In addition, these ‘extra generation’ caterpillars growing in the wild will possibly not be able to find enough milkweed to completely finish their caterpillar-hood. At Butterfly Ridge, our milkweed has been expired for nearly a month now, either eaten by previous generations or succumbed to natural aging. This will result in either dead caterpillars or, at best, undersized adults.

So, the monarchs that will actually end up being the migratory generation, will be fewer in number than their parents who should’ve migrated, smaller, and getting a late start which will expose them to cold weather that their parents would have missed had they been on the ball! My suspicion is the size of the over-wintering population in Mexico this coming winter will be slightly larger at best than last year.

So is this the end of the migration? Probably not. Is this the first steps in the end of the migration, as we know it? Possibly. With a warming climate, warm autumns may become the norm. We may be seeing the beginning of a change of monarch behavior in general. Will squeezing in an extra generation become the new normal? Will migrating only as far as the Gulf Coast become adequate to survive warmer winters? Will monarchs start overwintering in the Midwest as pupa like so many other species of butterflies? Could we be actually observing the evolution of Danaus plexippus? Perhaps.