Aphid herding: as sweet as manna from heaven

By Gary Raham
Nature Writer and Illustrator

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If houses are sprouting on the grazing lands around you and you miss watching the lazy meandering of cows across the meadow, take heart.

Some of those other highly successful social creatures on planet Earth, the ants, still tend their herds of domesticated aphids on the leaves of nearby plants. To see them just requires paying attention to smaller patches of real estate.

We tend to forget about ants because the average worker weighs only 1 to 5 milligrams (1/100th the weight of a Monarch butterfly) but two ant experts, Bert Hölldobler and Edward O. Wilson, estimate there are some 10 thousand trillion of them worldwide, which gives them a collective mass about the same as all the human beings on Earth.

Being social pays big biological dividends. Like us, ants cultivate veggie eaters to support their vast numbers. Ants form symbiotic relationships with a variety of arthropods including scale insects, mealy bugs, treehoppers, and caterpillars of lycaenid and riodinid butterflies, but their associations with aphids is perhaps the easiest to see since aphids are abundant on plants of all kinds.

Aphids pierce the phloem tubes of plants with their sharp mouthparts and suck out the sugary goodies in transit there. They process this food and excrete drops of what are euphemistically called honeydew that are rich in sugars (90 to 95 percent by dry weight), free amino acids (the building blocks of proteins), proteins, minerals and vitamins.

"Wild" or non-domesticated aphids shoot this waste away from their bodies, so that fungi don't germinate in it and cause health issues, but aphids that have learned to associate with ant species produce a drop on cue when an ant taps them with foreleg or antennae, then hold the drop at the tip of their rear ends and wait for the ant to consume it. Later, the ant will regurgitate part of the haul for the consumption of nestmates.

Wilson has successfully "milked" scale insects that demonstrate a similar symbiosis with ants by tapping individuals with a hair from his head. He claims the drop was quite sweet. You could try this someday if you're not worried about what the neighbors might think.

In the Mideast, people still collect the sweet excretions of scale insects that feed on tamarisk. They call it "man" and it is, most likely, the manna described in the Old Testament. Lest we feel superior in our culinary habits, Hölldobler and Wilson say that a large portion of bee honey is actually honeydew harvested from the surface of plants.

So, are aphids selfless good Samaritans? Hardly. Honeydew serves as protection money to hire ant armed guards. Ants drive off a host of potential aphid predators including parasitic wasps and flies, lacewing larvae, beetles and other creatures that give aphids grief. Aphids that have come to depend on such services sport smaller cornicles on their rears (the structures that expel honeydew in wild species) and produce thinner layers of protective wax on their bodies. They often have a basket of hairs on their hind ends to hold the honeydew and demonstrate behavioral differences designed to aid and abet ant access. For aphids, poop has become a valuable form of barter--a currency not unfamiliar in human political discourse.

Ants also will often raise the eggs of their domesticates with their own and transport their symbionts to the proper plants at the proper time in the insect's life cycle. Researchers studying a Malaysian ant/mealybug relationship demonstrated that the ants move their herds of bugs from "pasture to pasture" as each food source is used up.

Of course, as we know, relationships are never perfect. Some associations are more exploitive than others and, sometimes, a species that is symbiotic in one relationship may prove parasitic in another. Certain lycaenid butterflies (commonly called "blues") live in symbiosis with ants. (Some of this work was done by researcher Naomi Pierce at the Rocky Mountain Biological Lab in Gothic near Crested Butte.) Ants get honeydew from their caterpillars and, in turn, tend their eggs and larvae. At least one species of lycaenid caterpillar has been observed to munch on the aphid herds of alien ants with impunity. They somehow give off a chemical signal that disarms the normally vigilant shepherd ants.

About 500 ant scientists (myrmecologists) worldwide keep track of this fascinating nonhuman social behavior. It certainly seems like someone should. After all, ants have been fooling around with this social stuff for 100 million years or so. We might learn a few valuable tricks from such experienced pros.

Readers may enjoy Hölldobler and Wilson's 1994 book, "Journey to the Ants," which provides many more observations of these amazing creatures.