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A Cape Cod Notebook can be heard every Tuesday morning at 8:45am and afternoon at 5:45pm.It's commentary on the unique people, wildlife, and environment of our coastal region.A Cape Cod Notebook commentators include:Robert Finch, a nature writer living in Wellfleet who created, 'A Cape Cod Notebook.' It won the 2006 New England Edward R. Murrow Award for Best Radio Writing.

One More Sign of Spring: Aphid Herding on a Maple Tree

Dawidl / WikimediaCommons

For the past month or so, I have been watching the slow development of the flower buds on the red maple tree outside my study. This weekend, the maple blossoms came down in showers of wind, falling like small dark red stars onto the ground.

The leaf buds now stand up like little green candles waiting to ignite.

This morning, examining those branches stretching toward the deck, I noticed large black ants perched motionless on the buds. They were not numerous, but there were one or two ants on every terminal twig of the tree that I could see. The ones I examined were about a dozen feet off the ground, and they had climbed perhaps twenty feet of tree limb and branch to get there. Presumably there were others on the highest branches 30 to 40 feet off the ground. What were they doing so far up into the air?  What had caused them to make such an effort?

Then I noticed that the ants were gathered around clusters of tiny, translucent, tear-drop-shaped insects less than 1/8” long. These were aphids, and I suddenly realized that I had come upon one of the most fascinating relationships in the insect world, one I had often read about but never actually seen.

Ants and aphids have developed an extremely complex form of mutualism, or a mutually-beneficial relationship. Ants, as everyone knows, love sugar. Aphids often feed on the sugar-rich sap of plants like this red maple in order to obtain necessary nitrogen. But because the nitrogen in the plant is very dilute, they must consume large amounts of sap. The excess liquid is then excreted in the form of a high-sugar-content syrup known as “honey-dew,” or less lyrically, aphid-poop.

What I was watching now was an ant behavior known as “aphid-herding.” The ants “herd’ the aphids into tight clusters and then obtain the sugar-rich honeydew by stroking the aphids with their antennae, much as dairy farmers milk their cows. In return the ants protect the aphids from insect predators such as lacewings and lady beetles. They’ll even seek out and destroy the eggs of aphid predators. But ants’ care of their insect “cattle” doesn’t end there. In winter some species of aphid-herding ants will take the eggs of aphids home and store them away in their own nests. In spring, when the eggs hatch, the ants move the young aphids back to the host plants.

Sounds like a pretty cushy deal for the aphids, doesn’t it?  But, as with human dairy farmers, these ant-farmers always have the bottom line – or, in their case, maximum sugar production – in mind. Aphids are normally wingless, but if the populations get too dense or the food supply is insufficient, they will produce a generation of winged aphids to fly to new host plants. But the ants, not wanting to lose their food source, have been known to tear the wings off these aphids in order to keep them there. A recent study even shows that ants can produce chemical tranquilizers that keeps the aphids from wandering off. In other words, they drug them. From this perspective, their relationship can seem less like “mutualism” and more like, well, slavery.

As usual, any attempt to describe or judge relationships in the insect world in human terms or with human values is bound to fail. Nonetheless, one could do worse than spend a month watching the slow unfolding the foliage on a maple tree with all its attendant and auxiliary activity.

Robert Finch is a nature writer living in Wellfleet. 'A Cape Cod Notebook' won the 2006 New England Edward R. Murrow Award for Best Radio Writing.