• Andy Davis

The squeaky-clean milkweeds of Nova Scotia, Canada – shangri-la for monarchs

Hello everyone,

I'm back from a long break from blogging! So far, my summer has been full of lots of monarch experiments, vegetable gardening, and some travels. Today, I’m going to tell you about a recent trip I made to Nova Scotia, where I saw something that got me thinking about the monarch migration.

As the title of this blog suggests, I just got back from a trip to see my folks in Nova Scotia. For Americans who are not geographically-inclined, there’s a map below showing where this is. It’s ok if you don’t know- I get this a lot. And hey, I still have to look at a map to see where Wyoming is! This region of Canada is pretty much the extreme northern edge of the monarch breeding range, and for monarchs making the trip from there to the Mexican overwintering sites, it’s a heck of a journey. But we do know that it’s possible, thanks to a recent tag recovery from there, which I blogged about already.

Anyway, it’s way up there. It takes several generations for monarchs to reach it in the spring. But monarchs do make it there most summers in low to moderate numbers. In fact, I typically judge the success of the summer breeding population on whether or not monarchs have made it to that region that year. When they do, it’s usually a good year. That’s just my yardstick though – very unscientificky (hey, I just made up that word!).

Based on my yardstick, this has been a very impressive year for the breeding population. Have a look at this map of monarch sightings, which I grabbed from the citizen-science program, e-butterfly (www.e-butterfly.org). By the way, I’ve been recently playing around with this site, and it’s super-cool. You can pull up maps and data from any butterfly you can think of. It’s like Journey North, on steroids.

So if you look at this map for monarch sightings from this summer, you can see that there are lots of monarchs in the Atlantic provinces right now. In fact, when I was up there last week, I saw at least one male during a 10min walk.

I also saw lots and lots of milkweed, which is what I wanted to talk about today. There were thousands of common milkweeds along one stretch of highway that my family and I traveled. I would estimate there were at least 100 shoots every 50 feet. My relatives also had some stands of swamp milkweeds in their yards. All of it was in bloom and very tall, so I would guess it was in its peak for the summer. I had the chance to look at about 50 plants up close over my trip and I think I saw about 5 or 6 eggs all told. But what really struck me was that there was nothing else on the milkweed. No aphids, no milkweed bugs, no beetles, wasps, or whatever. Nothing at all. Check out these pictures I took to prove it. The only insect of any kind I saw was a few honeybees visiting the flowers.

(that's my kid)

In all of these shots, look closely at the milkweed leaves and shoots. Notice how clean they are. In the 50+ shoots I examined up close I didn't see a single aphid!

When I got back I decided to ask an expert on milkweeds (Anurag Agrawal) on whether this was normal for this region. Anurag is no stranger to the monarch world – he has been studying milkweeds for many years, including the critters that live on them. Below is his response.

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Hi Andy,

Yeah, this makes some sense to me. We studied some of the northern populations, although not extensively. Our results are summarized in an Ecological Monograph from 2012: http://www.eeb.cornell.edu/agrawal/documents/woodsetal2012ecolmonographs.pdf

In our New Brunswick sites, there were relatively few insects and most damage was by mollusks (see Fig. 10 in the paper). I also went back to the raw data just for fun, and see that we surveyed several natural populations around Fredericton, NB, with very low densities of leaf miners (Liriomyza asclepiadis), aphids (only Myzocallis ascelpiadis), and a few individuals of the milkweed tussock moth (Euchaetes egle). No monarchs, Tetraopes, or Lygaeids in the two years we were out there.

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After getting this reply I looked up Anurag's paper and skimmed it (it was a lengthy paper). It looks like their team had examined common milkweed from various latitudes, including in New Brunswick (that's the province right next to Nova Scotia), and found that the more northern milkweeds appear to be more resistant to insect damage, and there appears to be fewer milkweed insects in the northern milkweeds they examined. There was a lot more in the paper than just this, and it is downloadable, so please check it out to see more. But anyway, I'm happy because it looks like my observation is consistent with the science - yay.

The thing that makes this observation very interesting, at least to me, is that it demonstrates why monarchs migrate in the first place. Bear with me here. Consider a monarch migrating north in the spring from Mexico, and it reaches, say, Oklahoma. Why not just stop there? There’s milkweed there. The same goes for sites further north, like Ohio. There’s milkweed there too. Why does the progeny of each generation in the spring continue to push further north? What is at the end of the migration that they can’t find along the way? The answer just might be these squeaky-clean milkweeds in Canada. Imagine being a monarch larvae on one of these beauties – there are no insect predators, there’s no competition for leaves, there’s no aphids sucking the nutrients out of the plants, and there’s a tonne of milkweed to go around! It’s paradise for monarchs! Ooh wait, but then there’s that extra-long trip back to Mexico in the fall… So is it worth it?

Think of the migration, any migration really, as a risk-reward kinda thing. There is no doubt that the migration (any migration) is risky – a lot of monarchs perish along the way. But look at the payoff – if you can make it farther than anyone else in the breeding range, then you get to enjoy all of the resources to yourself. Contrast this migratory lifestyle with any non-migratory critter, be it a butterfly or what-have-you. Sedentary critters never have to undergo a risky, death-defying migration, but then they also have to compete with everyone else who shares their patch of habitat. Which lifestyle is better?

Interestingly, the monarch is capable of either lifestyle – around the world there are many populations of monarchs. We all know about the famous migratory one in eastern North America, but there are others that don’t migrate, and some that even have partial migrations. So the monarch seems capable of enjoying whichever strategy is suitable for its region!

Well, that’s all the musings I have for now on this subject. Thanks for listening (or reading).

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Direct link to this blog entry:

http://akdavis6.wixsite.com/monarchscience/single-post/2017/07/14/The-squeaky-clean-milkweeds-of-Nova-Scotia-Canada-%E2%80%93-shangra-la-for-monarchs

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The science of monarch butterflies

A blog about monarchs, written by a monarch scientist, for people who love monarchs