Many Monarchs – Too Much of a Good Thing?

No, this article is not about royal weddings, but it is about monarchs – Danaus plexippa, the monarch butterfly, to be precise.

migrating monarchs in my pollinator garden

A year ago I spoke to the Toronto Horticultural Society about “Wild Things in your Garden”. I mentioned monarchs in that talk. Did you know that monarch studies have a special link to Toronto? Fred A. Urquhart, a University of Toronto scientist, studied monarchs since 1940 tracing their migration routes, but he couldn’t find their winter roost in the rough mountains of Mexico. So, he placed ads in Mexican newspapers. An US engineer living in Mexico, Kenneth C. Brugger, saw the ad and in 1975 told a delighted Prof. Urquhart that he’d found the roost. 35 years of work had paid off! Spectacular pictures ensued in places like National Geographic, and eventually the Mexican government created a Monarch Preserve to protect the trees in which the monarchs roost. More details about this quest are at the site of the Urquhart Memorial Garden, and you can read a detailed history of Urquhart's story in this 1999 Vanity Fair article by Alex Shoumatoff.

In my 2010 talk I highlighted declines in monarch butterfly numbers during the winter of 2010, when one quarter the number overwintered in Mexico compared to 2009.  All this gloom had one good effect: it mobilized many people, including me, to think and act to preserve Canadian habitat for pollinators, including monarchs. With the support of the Hort and many of our members, this has grown into our Pollinator Gardens (PG) project. As part of the project, I planted a number of milkweed plants (the only food for monarch caterpillars) in my cottage garden with an eye to finding out which species were most attractive to the butterflies and which grew easily. I can now report preliminary results from one year’s observations.

monarchs on swamp milkweed, my PG

My veggie garden is surrounded by a wild garden with common milkweed (Asclepias syriaca) and butterfly weed (A. tuberosa). In previous years the wild garden had raised respectable numbers of monarchs, and I thought perhaps the presence of lots of common milkweed might make the butterflies ignore the swamp milkweed (A. incarnata) and Scarlet milkweed (A. curassavica) I planted among the veggies. Nothing could have been farther from the truth! The new species were covered with caterpillars by August, so much so that some of the plants would have been eaten down to nubbins if I hadn’t moved caterpillars to less crowded quarters. Most popular was the Scarlet milkweed (with both caterpillars, adult butterflies, hummingbirds, and people, all of whom liked its bright red and gold flower – to eat, in the case of the caterpillars!). However, by the end of August the stand of swamp milkweed had caught up more than a dozen caterpillars visible on any sunny noontime, among the big pink, fragrant flower clusters.

caterpillars eat scarlet milkweed

I did some research, and found that monarch moms prefer to lay their eggs on milkweeds with (a) soft leaves, and (b) high levels of the noxious compounds which make birds throw up when they eat monarchs. Sure enough, scarlet and swamp milkweeds both have softer leaves than the common species and higher levels of icky goo. So far, so good.

Because I had many, many monarchs in one area I had much more luck in locating the critical stage between caterpillar and butterfly: the beautiful sea-green, gold-speckled pupa. These can be very hard to find in wild sites, as the caterpillars wander far from their milkweed restaurants before pupating. But I had enough to actually learn to recognize their habits. I found that caterpiggles typically wiggled about 2 to 5 meters away from their plant before pupating. I found many pupae under horizontal surfaces about 30 cm to one meter above ground level. They liked the bottom of  the board fence that rings the garden, and especially the underside of wild grape leaves in the wild garden. My first research conclusion for the PG is that in settings like a school garden, planting milkweeds a few meters from benches or horizontal boards on posts may allow children to see the stunning pupae when they return to school in September.

monarch pupa, about 1 day since pupation

Although monarch adults have few natural enemies, caterpillars and pupae are eaten – not by birds, but by tiny fly or wasp parasitoids whose eggs eat the larval stages from the inside out. Ugh! I was worried that concentrating good food plants in a small area might increase losses to these parasitoids, but encouraged by reports that they are not nearly as destructive in Canada as in the US (where up to 90% of monarchs are killed in larval stages by them).

parasitoid wasps on caterpillar ready to pupate

There are programs to monitor the number of caterpillars attacked by fly parasitoids, but because pupae are so hard to find, not much study has been done on the tiny wasps that attack them. So, with my newfound search pattern for pupae, I did some preliminary research. I’ll need a second season to make the results scientific, but I can tell you today that monarch pupae are indeed attacked by wasps in Ontario, that the damage is minor in July but rises in August, to the point where almost half the pupae in my garden in September were parasitised. Was this because I had created an ideal breeding site by planting very attractive milkweeds densely? That’s a question I hope to follow up this year, by moving some of the milkweeds to another location in lower densities and monitoring pupae there, compared to the main cluster. Then I’ll be able to tell you whether many, many monarchs are too much of a good thing.

The goals of the PG project include education and scientific research in how best to make a pollinator garden. As the example I’ve just told you shows, you can contribute to both these goals in your own garden with nothing more complicated than some milkweed plants and a pair of sharp eyes. Nothing I did required a microscope or test tube! Plus, I got to do all my research out of doors in my garden in some of the finest weather of the year…unlike, for example, studying the effect of vanishing Arctic ice on polar bears.

p.s. all photos in this blog post were taken by me, Clement Kent, in my country Pollinator Garden during summer/fall 2010.