The monarch butterfly is nature’s picky eater.
The iconic orange-and-black insect — one of Ontario’s most recognizable species — is facing serious pressures, one of which is tied up with its intimate relationship to the milkweed plant.
Monarch butterflies follow temperature signals in their migration, flying north until they reach an area with the ideal climate. Because of the warmer temperatures resulting from climate change, monarchs have been forced to migrate ever farther north.
“[Climate change] has the potential to change the whole migratory pathway of monarchs,” says Ryan Norris, associate professor at the University of Guelph and director of the Norris Lab. “If you imagine that all of a sudden the south becomes totally unsuitable for monarchs but then northern Ontario becomes very suitable, essentially it's making the difference between their breeding grounds and overwintering grounds longer.”
The problem is that the range of the milkweed plant — the only thing monarch caterpillars eat — extends only 160 kilometres into Canada, so the butterflies have begun travelling to areas that don’t have the species of vegetation they depend on.
“It's their host plant. They lay eggs on milkweed, and their larvae develop on milkweed, so they need milkweed to reproduce,” says Norris.
The butterflies can also face challenges finding milkweed even in areas where it once commonly grew, as agricultural and urban development have reduced the plant’s population.
Monarchs have only enough energy to take them to their traditional migratory destinations. When the distance they have to fly is lengthened, they have to expend more energy to travel — that means they’re exhausted when they arrive, which makes it more difficult for them to lay eggs and avoid predators.
The monarch population is declining globally. In 2017, the World Wildlife Fund’s annual count at the butterfly’s overwintering areas in Mexico showed that the population had fallen by 15 per cent in one year. Since the first count in 1996, it has declined by 86 per cent.
But the monarch can benefit from human help.
“Anything you can do to make it easier to be a butterfly is going to be helpful. There are things you can do to make it easy to be a butterfly — plant milkweed, make it easier for monarch butterflies to breed the next generation,” says Jeremy Kerr, professor and research chair in macroecology and conservation biology at the University of Ottawa.
More milkweed means a more robust monarch population; if it’s planted farther north as an annual crop, it could help sustain the butterflies sent there by the effects of climate change.
There have been some indications that butterflies in the wintering ground respond positively to warming temperatures, Norris says. But, he adds, the warming climate has been associated with ice storms in the high-latitude regions where the monarchs spend their winters, and those take a major toll on the population.
“The monarch will, in some periods, some places in the year, respond positively — and in other periods, it will be a total disaster,” Norris said.
“I wouldn't say all the pieces of the puzzle have been put together.”
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