monarch butterflies and latex -- 5/12/21

Today's selection -- from The Language of Butterflies by Wendy Williams. Monarch caterpillars are killed by -- but also protected by -- latex:
"Female monarchs lay eggs about the size of a pinhead on milkweed plants -- and only on milkweed plants. They deposit the eggs on the underside of a leaf, usually one egg to a leaf. Monarchs are not the only insects that use milkweed. Well over a hundred other species do, implying that at one time milkweed plants were much more common than they are today. Still, the competition isn't quite as tough as it might seem, as not all of them use the same parts of the plant as the monarchs.

"After three to five days, depending on the weather and the temperature and the time of year (more about this later), a minuscule caterpillar emerges. The emergent life-form is so small I looked directly at one but did not see it.

"The caterpillar spends the next nine to sixteen days desperately eating milkweed, starting with the leaf on which the egg was laid. The caterpillar must eat milkweed -- and only milkweed. Poor thing. The creature has no choice in the matter; no other plant but milkweed will do. For the monarch caterpillar, the intensity of survival begins immediately upon emerging from the shell.

"After first eating the egg's shell, which is full of nutrients, he takes a long drink, like a 'cat drinking milk,' according to the entomologist Dame Miriam Rothschild. In fact, he goes on a milkweed-sap bender, drowning himself in the stuff -- sometimes literally, as the ecologist Anurag Agrawal has documented.

"Most of us, if we have had any kind of experience of nature as children, are familiar with the ickiness of milkweed sap. Tear open a milkweed leaf, and out the rubbery goo would 'spew,' monarch biologist Lincoln Brower told me. And then it would dry, and then your fingers would be stuck together. It was fun as a kid to pretend that your fingers had been captured by this mysteriously strong and compelling blob-like substance, which would stay gummy and mucilaginous and annoying and never come off your hands until you washed it off painstakingly in a nearby stream.

"As adults, we would learn that this material is called latex. Latex is by no means rare. Roughly ten percent of all plant species have evolved to use latex. Latex from rubber trees makes our automobile tires. Syn­thetic rubber has been developed, but is not as durable as natural latex. There is nothing else quite like it on the planet.

"Milkweed latex is all-around nasty stuff, full of poisons. The famed monarch researcher Lincoln Brower once tasted some: 'It just about knocked me over. It was so foul-tasting. I drooled and I almost vomited.'

"Interesting, I thought. I myself am not in the habit of putting strange materials in my mouth. But field scientists seem to display a certain machismo, which they can then crow about over beers. Even Charles Darwin succumbed to this derring-do: 'One day, on tearing off some old bark, I saw two rare beetles and seized one in each hand; then I saw a third and new kind, which I could not bear to lose, so that I popped the one which I held in my right hand into my mouth. Alas it ejected some intensely acrid fluid, which burnt my tongue ... '

"Most scientists survive these experiments. ...

"The fact that the just-emerged caterpillar must eat milkweed is a strange irony: the caterpillar's first drink might well be its last, since the latex that glued your fingers together when you were a kid can also glue caterpillar jaws shut. Hence, death by starvation. Agrawal suggests that roughly 60 percent of caterpillars die because of this first meal. That's a pretty high casualty rate. If their jaws are not overwhelmed, then the animals may well die because of the rather mundane fact that their feet get stuck.

"Sometimes, to lessen the danger, a caterpillar will succeed in sepa­rating the leaf from the plant by chewing at the joint between the leaf stalk and the main stalk of the plant. This makes his job less onerous, since the latex spewing out of the leaf will do so with less pressure, and the caterpillar can partake of the nourishment in smaller sip-size swal­lows. Or sometimes he will even carefully chew a circle in the leaf, then eat from inside the circle, thereby vastly reducing the pressure of the gushing latex. Mostly, though, he will just begin chomping away and hope for the best.

"His compulsion to eat of the leaf of the milkweed plant and to drink the bitter latex seems terribly cruel, the stuff of Greek tragedy: Con­demned to embrace the source of our own demise. Fatal attraction. There's more to this dire truth than just the stickiness of latex and the fact that it tastes atrociously bitter.

"Latex is deadly. Another irony: the more latex a caterpillar eats, the more reduced that caterpillar's overall growth, but the better protected that same caterpillar will be from birds and other predators. If, that is, the caterpillar survives. Many die from the poison they are required to ingest.

"This is perverse.

"It's also essential: overall, predatory birds cut caterpillar numbers in half.

"That latex is poison has been known by humans for centuries. Ro­mans extracted it from plants and used it to assassinate enemies. It is something like digitalis, affecting the heart and nervous system of all animals.

"So here's something to chew over, perhaps the ultimate perversion: the caterpillar drinks the latex precisely because it's poisonous. That which does not kill us makes us stronger. If it survives the poisoning, it's way ahead in the life-and-death match it will be playing for its entire existence.

"The caterpillar can store the toxin away in several specific locations in its body. When predators try to eat the caterpillar, they get mouthfuls of noxious, repulsive poison. Birds find this discouraging. Often their response is to regurgitate. Predators learn quickly. Most never again try to eat a monarch. Or, as Darwin and Bates showed, other butterflies that just look like monarchs. These stored toxins have lasting power. When the caterpillar transforms into a butterfly, the stored toxins remain, still able to deter predators."



Wendy Williams


The Language of Butterflies: How Thieves, Hoarders, Scientists, and Other Obsessives Unlocked the Secrets of the World's Favorite Insect


Simon & Schuster


Copyright 2020 by Wendy Williams


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