Monarch butterflies raised in captivity may not migrate to Mexico

If you're raising monarch caterpillars in the hope that when they become butterflies, they'll fly to Mexico and help replenish endangered populations, a new study offers some warnings and some advice.

Monarch butterflies raised indoors as caterpillars don't head south towards Mexico when they emerge from their chrysalises, researchers at the University of Chicago have found.

"It's easy to mess up their development, it seems like, in becoming migratory butterflies," said Ayse Tenger-Trolander, lead author of the study published this week in Proceedings of the National Academy of Sciences.

Her research also found that monarch butterflies bred by commercial breeders — which are often bought by schools or for mass butterfly release events —  might not have the instinct or ability to fly to Mexico at all.

Monarch butterflies in eastern Canada and the United States fly thousands of kilometres in the fall to spend their winters in the pine and fir forests in the mountains of central Mexico. The wintering monarchs used to cover many hectares of forest. They've been declining and dropped to a low of just 0.67 hectares in 2013-14, although there has been some recovery since then. 

The decline has been blamed on deforestation and the loss of the plant their caterpillars feed on, milkweed, across North America. They're being considered and recommended for listing as an endangered species in the U.S. and Canada, respectively.

Store-bought simplicity

Tenger-Trolander has been studying butterfly migration for four years as part of her PhD thesis, but getting research subjects isn't easy — it's time consuming to catch and raise the number of butterflies she needs. Fortunately, these days, you can buy just about anything online.

"So I was like, 'Wouldn't it be great if I could order monarchs and not have to catch them anymore?'" she recalled.

She ordered some, but needed to make sure they actually were identical to wild monarchs in their migration behaviour. So she tested them in a flight simulator for butterflies.

In the fall, when they're getting ready to migrate, wild monarchs point south.

Tenger-Trolander found that the captive-bred monarchs pointed in random directions, like wild monarchs raised during the summer when they don't migrate.

At first she was surprised — until she thought more carefully about how important it is for wild monarchs to migrate successfully in order to pass on their genes to the next generation.

"If you don't make it to Mexico by November, December, and you're not part of this group of butterflies that's going to breed in the spring, you're kind of out [of the gene pool]," she said. "So annually, essentially, you have this incredible purifying selection of the population."

On the other hand, generations of butterflies might breed perfectly well at a butterfly farm without needing their migration genes.

That said, the butterflies study came from a single breeder that had been selling monarchs for at least 20 years, and not all captive-bred butterflies might have the same problem. In fact, Tenger-Trolander said, there have been reports of captive-bred monarchs with special tracking tags making it to Mexico.

But what about wild-caught monarchs raised in captivity? 

It turns out that can have an effect too — Tenger-Trolander found wild-caught monarchs raised indoors as caterpillars didn't fly south as adults in the fall even if exposed to cooler temperatures and shorter days.

"Clearly those … were not enough to convince them it was time to go to Mexico," she said.

In fact, even caterpillars raised outdoors whose chrysalises were brought indoors after that did not fly south as expected when they emerged as butterflies, the research found.

It appeared that even they didn't know that it was time to migrate even though it was autumn.

How to raise migratory monarchs

So what does this all mean for hobbyists or teachers who want to raise monarch caterpillars?

Tenger-Trolander said she certainly doesn't want to discourage people from raising monarchs, as she thinks it helps create a connection between people and nature.

But she recommends:

• Catching them in the wild rather than buying them from a breeder. (This practice is restricted by laws in some jurisdictions, such as Ontario).
• Rearing them outside at least in the fall. (Indoors is probably OK for monarchs raised in spring or summer).

Ryan Norris, a biology professor at the University of Guelph who studies monarch butterfly migration patterns, said in an email that the number of butterflies used in the study was quite small — about 100 in total — so the results should be treated with caution.

But based on the results, he said he would make similar recommendations.

Even if the monarchs raised by people can't migrate, however, it probably won't really make a difference to wild populations, he added. That's because very few monarchs are raised in captivity by people, compared to the overwhelming majority that lives their whole lives in the wild.