Aphids: Nature’s Most Incredible Bug
The following is an excerpt from Ingredients. To buy the book from Amazon, click here, or click here for a list of all retailers.
Remember that plants are constantly pumping what is essentially syrup from the leaves to the rest of the plant via sieve tubes buried deep within their tissue. If you want access to this sugar rush, you cannot just take a bite of a plant. Leaves, shoots, stems—in other words, the parts of the plant that house most of the sugar superhighway—are not sweet. (Think celery stalks.) That’s because when humans take a bite of plant with our gigantic gnashing teeth, we’re not just getting the sieve tubes; we’re getting all the other parts of the plant that don’t have a constant stream of sugar whizzing through them, and that tends to cancel out the sappy parts. We’re also getting bitter chemicals the plant makes specifically so they don’t taste good. Unfortunately, we just don’t have the delicate machinery required to dip into a plant’s sugar superhighway. But there is a creature that does: the humble little aphid.
Aphids, also known as plant lice, are quite small, usually green, and absolutely terrible for plants. We’ll start our story with a single lady aphid—let’s call her Mabel—landing on a plant. Mabel is about 5 millimeters long, but she’s big for an aphid. Most species are about 2-3 millimeters long. Once Mabel finds a spot she likes, she spits out a small bead of saliva that quickly hardens to the consistency of peanut butter. As it’s hardening, Mabel unfurls her “stylet,” which is kind of like a hypodermic needle, except it’s flexible and has two channels instead of just one.
The stylet is basically Mabel’s mouth: her face just sort of stops being a face and starts being a long, flexible needle.
Mabel penetrates the gel saliva that she’s just spit out with her hypodermic needle-face, and soon the tip of her stylet arrives at the surface of the plant. Unlike the metal needles that doctors jab you with, Mabel’s stylet doesn’t punch through plant cells; it worms its way between them. Mabel pushes her stylet into the plant in gentle pulses: before each pulse, she spits out a small glob of gel saliva, then penetrates it, and when the tip of her stylet pokes out the other side of the glob, she spits out another glob, penetrates that one until her stylet tip comes out the other side, and so on. These globs of gel saliva harden, creating a sheath that protects (and lubricates) her stylet as she pushes it between plant cells, farther into the plant.
Every so often, Mabel needs to get her bearings. Her stylet doesn’t have eyes—it has no way to know where it is inside the plant—so she pokes the tip of her stylet into a nearby cell. Once inside, she takes a “sip” of the cell’s contents. In other words, she sucks up some of the cells’ guts into one of the two channels in her hypodermic needle-face, and “tastes.” We don’t really know what “tasting” is like for Mabel, but we think she’s checking to see how sweet or sour it is. If it’s not sweet enough and/or too sour, she retracts her stylet, changes direction, and moves on, deeper into the plant. Eventually she penetrates the holy grail of plant anatomy, the sugar superhighway that is a sieve tube.
As you might guess, plants do not want to be penetrated. Especially not in the sieve tube, because they know what’s coming next: large-scale theft of the sugar they’ve worked so hard to create. Plants are not ungenerous. They have no problem making a fair trade with an insect or an animal, something along the lines of:
Hey! You, thing that can move! I’m stuck here, but I’ve just had sex and I need you to take all these fertilized embryos I’ve made far away from here so they can go forth yonder unto this world. (In consideration of said services, you may drink nectar from my sweet flower or eat my sugar-sweet fruit.) Sound good? Great, done deal.
But when something tries to take sugar without giving anything in return, the gloves come off. When a caterpillar, for example, chews, rips, and tears plant tissue, plants do a bunch of things in response. Electrical and chemical signals travel to the rest of the plant, alerting it to damage. Long, thin proteins inside the sieve tube called forisomes double or triple in width, partially blocking the tube. The cell starts producing a sugar called callose that also helps to plug up the tube.
But Mabel knows that this defensive dance is coming. So as soon as she confirms that the cell she’s penetrated is a sieve tube, she spits out a different kind of saliva that pretty much stops the plant’s defensive response in its tracks. Now she’s basically set. She’s suppressed the plant’s sieve tube defense system, and because the tube is under pressure, she doesn’t even have suck up the sap. She just opens or closes a valve in her head to control the flow.
But there is one more plant sap defense Mabel has to deal with: sugar. Specifically, the can-of-Coke-high or sometimes even Aunt-Jemima-high concentration of sugar in sieve tube sap. As this incredibly concentrated syrup travels through Mabel’s digestive tract, it encourages water out of her cells,[1] so much so that other cells, deeper in Mabel’s gut, have to send their water to the front lines to replenish their fellow troops. Unfortunately, Mabel’s gotta eat, so she keeps gulping, and this water loss continues. The more sap passes through Mabel and out her butt, the more water is “sucked” out of her body. Eventually, if she doesn’t stop feeding on this plant syrup, Mabel will lose so much water to the sap that she’ll dry out, shrivel up, and die.
Or at least, she would . . . if she didn’t have two elegant methods to deal with this water loss problem. The first is the simplest: every once in a while, Mabel might retract her stylet from the sugar superhighway, find some xylem—which carries water from the roots upward—and take a delicious draft of water to restore her dehydrated tissues. Second, Mabel has an enzyme in her gut that bonds sugar molecules together, which reduces the sap’s ability to suck water out of Mabel’s cells. All this is great for Mabel but terrible for the plant, because it means Mabel can feed for basically as long as she wants to.
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Now let’s take a moment and appreciate how insane this is. A small creature smaller than your fingernail and lighter than a single four-inch-long strand of hair is able to
worm its flexible-needle-mouth-face between individual plant cells, down to millimeters beneath the surface of a stem (or even tree bark in some cases)
find and penetrate plant cells transporting a 30% sugar solution at 100-200 pounds per square inch of pressure
drink that plant sap almost undetected for basically as long as it wants without shriveling up and dying from the highly concentrated sugar solution sucking the water out of its body.
There is no human analogy for this . . . but if there were, it would involve you building a hypodermic needle roughly the width of a toilet paper roll and the length of your left leg, somehow clamping that needle to your mouth, sneaking up behind a firefighter in the middle of spraying a house, stabbing the firefighter’s hose with your toilet-paper-roll-size-needle-face, attempting to control the resulting surge of water into your digestive tract, and doing all this without the firefighter noticing.
But let’s get back to Mabel. She ain’t done.
Mabel doesn’t just sip on the sap. She chugs it. Why? For a surprisingly familiar reason: essential amino acids. Even if you don’t know what these molecules look like, you’ve probably heard the term. Amino acids are the building blocks of protein, and there are roughly 20 different amino acids in all of nature. Your body—and the bodies of most animals, including Mabel—can produce about half of these, so you don’t need to include them in your diet. As for the other half, you—and also Mabel—need to eat those, otherwise your body can’t make the proteins it needs, and all manner of bad things happen to you. Plant sap happens to contain every kind of amino acid Mabel could possibly desire,[2] but in oh-so-minuscule amounts. So to get enough essential amino acids, and also because the sap is under so much pressure that she doesn’t have a choice, Mabel has to drink a buttload of sap.
And that means Mabel craps a lot.
Aphid poop is not like your poop. Chemically it’s not all that different from sap; it’s a clear and colorless sweet, syrupy liquid. You might already know it by a different name: honeydew. When Mabel was a wee lass, she could poop her entire body weight every hour. As an adult, Mabel poops about a milligram per hour. That doesn’t sound like a lot, but remember that she only weighs two milligrams. Even if you were being force-fed like a foie gras goose and simultaneously had the worst case of diarrhea ever recorded, you would still be physically incapable of generating half your body weight of poo every hour.
All that is one aphid. When you start talking in terms of aphid colonies, it can be hard to wrap your head around the quantity of crap. In some forests in Germany, colonies can produce more than 110 pounds of dry honeydew per year on a single tree.[3] Depending on how dense the forest is and how many aphids there are, you could be talking about numbers in the hundreds-of-kilograms-of-honeydew-per-acre-per-year range.
But even after all this honeydew, Mabel still ain’t done. Aphids have a complicated life cycle and reproductive strategy. In the winter Mabel can choose to bone a male aphid, producing eggs with a mix of her and the father’s DNA. But in the summer, Mabel does not bone a male . . . but she still gives birth, this time to a live, fully formed, genetically identical copy of herself: a clone. And that baby clone is already pregnant with another clone when she’s born. Scientists have a great name for this: telescoping generations.
All this means that—assuming they don’t get eaten by ladybugs or other predators—there can be twenty generations of aphids in a single season.
So, to recap, plants are basically a gigantic all-you-can-eat aphid buffet. If Mabel and her kin like what’s on the menu, they will:
gorge themselves on sieve tube sap for days, depriving plants of their essential nutrient flow
reproduce like it’s their job
coat everything below them in a sticky sugary mess, like a two-year-old with an ice cream cone.
If you happened to be a member of, for example, the Tübatulabal people living in California thousands of years ago, you wouldn’t have been all that bothered by any of that. In fact, you might have found a way to use it to your advantage . . .
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The first people who made the critical observation that aphids poop sugar were probably . . . not people. They were ants. And even after hundreds of millions of years, some species of ants still get their sugar fix from aphids. Today, if you let an aphid (of species P. Cimiformis) get its stylet into a plant, then put an ant (of species T. semilaeve) on the stalk, the following will probably happen:
The ant will bump into the aphid and perform what scientists call “antennal waving,” which looks roughly like fast-forwarded footage of a Pentecostal preacher faith healing a parishioner.
In response, the aphid will kick its rear legs, poop a droplet of honeydew, and point its honeydew-laden butthole at the ant. (Scientists call this “anal pointing.”)
The ant will then receive this droplet of honeydew with reverence and begin to drink.
The ant will then “antennate”—i.e., feel up with its antennae—the aphid’s butthole area, presumably to ensure that the flow of honeydew continues.
In return for the constant stream of honeydew, the ants protect the aphids from other predators. It’s a classic symbiotic relationship.
It’s all very well and good for ants to drink directly from the butts of aphids, but if you happened to a member of the Tübatulabal, Owens Valley Paiute, Surprise Valley Paiute, Yavapai, Tohono O’odham, or a number of other Native American peoples a few hundred years ago, you had to be a little more creative. Watching the aphids carefully over the course of the summer, you probably noticed that after a while, water evaporated from honeydew, leaving a coating of crystalized sugar on whatever poor plant they decided to infest—in California, usually a tule or reed or tall grass. And that was the basis of an ingenious processing method for something you could either call “Honeydew Spheres” or, somewhat more literally, “Aphid Poopballs.” In the late summer or early fall, before the rains began, people would cut the stalks of long summer grasses that aphids had been feeding on, let those stalks fully dry in the hot sun, and then thresh—i.e., beat the hell out of—them with sticks over bearskin or deerskin. As the grasses were “vigorously flayed,” the honeydew would fall off the stalk and onto the animal skin. Then the honeydew could be gathered and shaped into little cakes or balls and eaten as is or warmed up by the fire.
By the way, Native Americans were (and still are) extremely sophisticated when it came to processing things from nature. Aphid candy is one example among many of things that you or I couldn’t just wander out into a forest and make. Could you make baby diapers from lichen and exactly the right combination of algae and fungi? Or glue from sheep horns? Or gloves from mud hen skins?[4] No. You could not. Most of us wouldn’t last five days alone in a national park, whereas a woman from the Tongva people survived entirely by herself on an island 1/3rd the area of Washington, D.C., for eighteen years.
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[1]. By a process called osmosis. If you want to see osmosis in action, dissolve a couple teaspoons of salt in a cup of water and then throw in a crisp romaine lettuce leaf. Come back in 20 minutes and you should find a very limp leaf—because all that salt leached out the water in the lettuce’s cells. Same deal with Mabel, except instead of salt, sugar is the culprit, and instead of lettuce cells, Mabel’s cells are the things in danger of shriveling up.
[2]. To be super technical, the plant sap itself actually doesn’t have every amino acid Mabel needs. But there are bacteria in Mabel’s gut that convert amino acids in plant sap to the essential amino acids Mabel needs. Aphids have a microbiome, just like we do.
[3]. In case you’re wondering, “dry” honeydew means honeydew after the water has evaporated, which in turn means that the actual weight of poop exiting the colony’s collective buttholes would have been much higher.
[4]. You don’t even know what a mud hen is. (It’s a sort of duck-like creature.)