A monarch on milkweed. (Samantha Knight)

As odd as it may seem, mowing down milkweed can help monarch butterflies.

University of Guelph researchers have found that cutting down milkweed where monarch butterflies lay their eggs and where caterpillars feed benefits the vulnerable insects — but only if mowing is timed strategically.

According to a new paper published in the journal Global Ecology and Conservation, researchers studying milkweed in southern Ontario found the plant should be cut down once in mid-July, before most monarchs have begun laying their eggs, and then not again for the rest of the summer.

That, they say, is the best way to allow monarch populations to grow, as the butterflies prefer to lay eggs on younger milkweed plants.

A monarch caterpillar eats a milkweed leaf
(Samantha Knight)

“Milkweed is absolutely essential to monarch butterflies; it’s the only plant where female monarchs will lay their eggs. It is also the exclusive food source for the larvae, or caterpillars, that emerge from those eggs,” said Samantha Knight, the study’s lead author and a former researcher in U of G’s Norris Lab.

“Understanding the most strategic way to manage the areas where milkweed grows will go a long way to giving monarch butterflies a chance to reproduce.”

The research received coverage on CTVNews.ca.

The insect’s numbers have been in decline for years – so much so that it is now classified as “special concern” under the Ontario Endangered Species Act. With monarch numbers up in southern Ontario this year, the research findings are significant – particularly to those who manage the fields where milkweed tends to flourish.

Knight and two other U of G researchers led by Prof. Ryan Norris from U of G’s Department of Integrative Biology experimented with different timing for mowing milkweed on roadside rights-of-way where the plant is typically cut.

They compared different mowing times with no mowing to determine which practice resulted in the most monarch eggs.

Samantha Knight

Mowing milkweed benefited the butterflies, as trimmed plots generally yielded more eggs than unmown plots.

“The butterflies probably see younger milkweed as higher quality and potentially more nutrient-dense for their caterpillars that will emerge from these eggs,” said Knight.

They also found that the best time to mow in southern Ontario is during the second and third weeks of July.

“That’s right before monarch egg-laying will have reached a peak. Any mowing of milkweed after that point will likely result in the deaths of hundreds of butterfly eggs and caterpillars,” said Norris, who was recently named the Weston Family Senior Scientist for the Nature Conservancy of Canada.

Because some monarchs may be ready to lay their eggs immediately after a plot of milkweed is mowed down, managers should leave some plants standing while the rest of the plot regrows, say the researchers.

Smiling portrait of biology professor Ryan Norris
Prof. Ryan Norris

With the number of milkweed plants in southern Ontario having dropped by nearly half since the 1990s, and with monarchs struggling to adapt to habitat loss, the researchers say their findings should help anyone managing milkweed fields who and wants to see more of these delicate insects.


Samantha Knight

Prof. Ryan Norris