Anurag Agrawal: Summer in the milkweed patch

AgrawalAnurag Agrawal is a professor in the Department of Ecology and Evolutionary Biology and the Department of Entomology at Cornell University. He lives in Ithaca, New York. His latest book, Monarchs and Milkweed, is available now.

It’s peak season for milkweed and the village of insects that make milkweed its home.  In my book on Monarchs and Milkweed, I devote an entire chapter to these diverse and fascinating other milkweed insects.  Below are photos from two days last week (July 6 and 7th), one set from my front yard and the other from Shawangunk National Grassland Preserve, both in NY State. All but two of the 11 specialized milkweed herbivores was seen on these four species of milkweed. Do you know which two species are missing?

1
The butterfly weed, Asclepias tuberosa.  Likes it dry.
2
Common milkweed, Asclepias syriaca, >90% of monarchs that make it to Mexico eat this as a caterpillar.
2b
A. syriaca, mis-named because it was thought to be from Syria.
3
The purple milkweed, Asclepias purpurescens, rare in NY State, this spectacular individual was near the shawangunks.
3b
Vegetative swamp milkweed, Asclepias incarnata.
4
Flowering swamp milkweed, Asclepias incarnata, complete with the swamp milkweed beetle, Labidomera clivicollis.
5
The poke milkweed, Asclepias exaltata, loves the partial shade. Note the nearly mature monarch.
6
The four-eyed milkweed longhorn beetle, Tetraopes tetrophthalmus. Note the four functional eyes!
7
Like all chewing insects on milkweed, Tetraopes deactivates the latex by clipping the veins.
8
Drippy toxic gooey stuff.
9
A little egg laid upon a leaf.  Monarch inside.
10
The first day or a monarch’s life, it makes a latex-free island before starting to feed on the leaf tissue inside the circle.
11
A week later, the monarch has grown 2000 times its original size.  This caterpillar has parasitic wasps eating it from the inside out.
13
The only fly known to eat milkweed, a leaf miner, feeds between layers of the leaf (larva is hidden here): Liriomyza asclepiadis.
14
Euchaetes egle, the milkweed tussock moth, a misnomer since it’s in the woolly bear family, Arctiidae.  Egg clutches hatch into hundreds of caterpillars… note the foamy fluff that the egg mass was delivered in.  These turn into large hairy orange and black caterpillars. Hmmmmm…. same colors as adult monarch butterflies.
15
A dead bee, like so many that get stuck in milkweed’s flowers. Why do they get stuck?
16
A tourist, not a real herbivore of milkweed.
17
Adult of the milkweed leaf beetle, Labidomera clivicollis, here on common milkweed.
18
Larva of the milkweed leaf beetle, Labidomera clivicollis.  Larvae of this species are apparently polymorphic, with grey or orange coloration. Closely related to the Colorado Potato Beetle.
18b
An adult of the elusive milkweed stem weevil, Rhyssomatus lineaticollis, chewing on apical leaves of common milkweed.
19
Sometimes they poke the stem, as here on the poke milkweed, A. exaltata. No egg inside this one.
20
Other times eggs are laid in a row in the stem.
21
A trenched stem with milkweed stem weevil, Rhyssomatus lineaticollis, eggs.
22
Inside the stem, larval feeding and frass of the milkweed stem weevil, Rhyssomatus lineaticollis.
23
The milkweed stem weevil, Rhyssomatus lineaticollis, also deactivates the latex.  All the chewing herbivores of milkweed do it… more or less the same way, but with there own special twist.
24
No seed pods yet, but the small milkweed bug, Lygaeus kalmii, feeds on last year’s seeds and sucks milkweed’s sap (not the latex!) … The large milkweed bug has not yet arrived to NY State… it apparently cannot overwinter in the frozen north.
25
Aphis asclepiadis, one of three aphids that eats milkweed.  This species is greenish to brown to grey, typically lives on top of the plant, and is nearly always tended by ants.
26
And the Oleander aphid, Aphis nerii, usually bright yellow-orange.  Here with a winged adult, just founding a colony in Ithaca, NY.

Anurag Agrawal: Monarchs vs. Milkweed

by Anurag Agrawal

Coevolution is a special kind of evolution. And monarchs and milkweeds exemplify this special process. In particular, what makes coevolution special is reciprocity. In other words, coevolution is one species that evolves in response to the other, and the other species evolves in response to the first. Thus, it is a back-and-forth that has the potential to spiral out of control. In some arms races, the two organisms both benefit, such as that between some pollinators and flowering plants. But coevolution is more common among antagonists, like predators and their prey.

When biologists first described coevolution, they likened it to an arms race. An arms race, such as that between political entities, occurs when two nations reciprocally increase their armament in response to each other. So how does an arms race between monarchs and milkweeds, or between cats and mice, or between lions and wildebeest, or between plants and their pathogenic fungi, proceed? When coevolution occurs, it proceeds with “defense” and “counter defense.” And one of the few rules of coevolution is that for every defense that a plant or prey mounts, the predator mounts a counter defense, or an exploitative strategy to overcome the defense.

Once a monarch butterfly lays an egg on a milkweed plant, the natural history of coevolution unfolds. For every defense that the plant mounts, milkweed mounts a counter defense. Once the caterpillar hatches, it must contend with a bed of dense hairs that are a barrier to consumption of the leaf. But monarchs are patient, and have coevolved with the milkweed. So their first strategy is to shave that bed of hairs such that the caterpillar has access to the leaves that lie beneath.

Agrawal

For every defense there’s a counter defense. But next, when the monarch caterpillar sinks its mandibles into the milkweed leaf, it encounters a sticky, poisonous liquid called latex. In this video we will see how the monarch caterpillar deactivates the latex bomb that the milkweed puts forward.

And so the arms race continues, with reciprocal natural selection resulting in coevolution between monarchs and milkweeds. In my book, Monarchs and Milkweed, I outline the third level of defense and counter defense between these two enemies. Milkweed next mounts a remarkable and highly toxic defense chemical called a cardiac glycoside. But, yes, again the Monarch has evolved the means to not only not be poisoned by the cardiac glycoside, but to sequester it away and put it to work in defense of the Monarch itself from its enemies, such as predatory birds. For more on the Monarch – Milkweed arms race see this video, filmed in Ithaca, New York outside of Cornell University where we conduct our research.

AgrawalAnurag Agrawal is a professor in the Department of Ecology and Evolutionary Biology and the Department of Entomology at Cornell University. He is the author of Monarchs and Milkweed: A Migrating Butterfly, a Poisonous Plant, and Their Remarkable Story of Coevolution.

Anurag Agrawal: The migration patterns of the monarch butterfly

by Anurag Agrawal

The plight of monarch butterflies if often in the news: many scientists around the world are working hard to understand their annual migratory cycle. How do the monarchs produced during summer in the northern reaches of America contribute to the overwintering population in Mexico? The origin of monarch butterflies that make it to Mexico has been hotly debated because it has profound consequences for how we approach monarch conservation.

A new study is remarkable in its use of historical collections over the past 40 years and modern isotopic analysis. The scientists address the most important regions in the U.S. for producing monarch butterflies that actually make it to Mexico. This sort of data has been very difficult to come by and there has been a lot of speculation. As outlined in my new book from Princeton, the midwest has dominated discussions as being the most important region in the U.S. for monarchs. In the study, the authors find that the Midwest contributes a whopping 38% of the butterflies that make it to Mexico.

Monarchs

The regions studied by Flockhart et al. separated to highlight their relative areas

I would add two points for discussion. The first is that the areas of land that the authors designated as Midwest, Northeast, etc., seemed totally reasonable, but also somewhat arbitrary. In particular, an issue arises when you consider that, as designated in the paper, the Midwest is about 2.5 times as big as the Northeast. It is therefore not surprising that the Midwest produces about 2.5 times as many butterflies that make it to Mexico (38% vs 15%). In other words, the butterflies that make it to Mexico have about an equal probability of coming from the Midwest and the Northeast when land area is considered. Yet another way to think about this is that two states that are about equal sizes in the two regions (for example, Indiana and Maine) will on average produce about the same number of butterflies that make it to Mexico.

Monarch2

The annual migratory cycle of the monarch butterfly from Monarchs and Milkweed. In my past research, we have opted for a three simple regions defined by the butterfly generations.

Quite interestingly, the North Central area (including my home in the Finger Lakes region of NY) is slightly more important for butterfly production given its size. When you factor out the area of the Great Lakes (where there are no monarch caterpillars), the area of North Central is small (36% of the size of the Midwest). Thus, about 20% more butterflies per square mile come out of the North Central than the Midwest or Northeast. Where does this leave us?  The agricultural Midwest is certainly important, but perhaps not as important as previously thought.

The other point worth thinking about is that the Southwest (read: Texas) comes out as big in terms of area (equal to the Midwest) and relatively less important in terms of contributing butterflies (11% of the total).  The critical importance of the Gulf States including Texas, however, is not in the last generation of butterflies produced in fall that migrate south, but rather in the first generation of butterflies that are produced in spring and that migrate north to the Midwest and Northeast.  In other words, the Gulf States are absolutely critical for the annual migratory cycle, even if that is not where fall migrants are produced.  Without a spring generation there, the Midwest and Northeast would be empty!  In chapter 9 of the book, I summarize the critical importance of Gulf States not only for the spring, but also in providing floral resources for fall migrating butterflies.

I hope we see more studies like this in the future, as it provides new important information and was inspiring to read.

AgrawalAnurag Agrawal is a professor in the Department of Ecology and Evolutionary Biology and the Department of Entomology at Cornell University. He is the author of Monarchs and Milkweed: A Migrating Butterfly, a Poisonous Plant, and Their Remarkable Story of Coevolution.