Plants are the basis of all life, for only they can combine minerals in the ground with gases from the air and produce something worth eating. Insects, however, not only eat them, they also exploit them in much more devious ways.
Tropical rainforests are famous for being thick, tangled masses of vegetation. But in this one in Peru there are mysterious clearings, where only one, or at most two, kinds of trees will grow. The local people call such places as this “devil's gardens” and believe that spirits kill other kinds of trees. And the real killers of those trees? Well, they've only just been discovered.
The leaves of the surviving trees all have these swellings on their stems, and going in and out are armies of tiny, tiny ants. The swellings are their homes, specially developed for them by the tree. And in them, safe from predators, the ants keep their eggs and larvae. They even keep domestic livestock, white scale insects which, like aphids, supply the ants with drinks of honeydew.
Producing this accommodation also benefits the tree, for the ants provide their landlord with a valuable service: they guard it against its enemies.
All kinds of insects will eat a plant's leaves given the chance. But they don't get a chance—not on this tree—so the 9)caterpillar goes elsewhere.
The ants not only repel their host's animal enemies, they also, perhaps more remarkably, keep competing plants at bay. A squad of them leaves the barracks and sets out on one of their regular patrols of the neighbourhood. They've found a newly sprouted sapling. Perhaps it's grown from one of their landlord's seeds, in which case all well and good. But this one hasn't: it's an intruder. They go into action biting its stems. Reinforcements arrive. Hundreds of tiny jaws cut into its stems. The sapling begins to wilt.
But bites alone are not enough for the ants to achieve their ends. They lift their abdomens and inject formic acid into the crippled plant's wounds. The poison spreads through the plant's tissues hastening its death. And, within a few days of being comprehensively stung, all these plants are dead. And the ants, or the devils, have extended their garden still further.
But the benefit of this drastic gardening, of course, is not restricted to the plants. The ants also profit. They have ensured that their plant landlord can extend its territory without competition, and that provides them with more homes so they too can increase their numbers.