Sunday, 16 September 2012

Green Tree Ants

Every northern gardener can tell you about Green Tree Ants. "They are pesky and numerous and get all over you when you try to garden". So true but when you look into them in a bit of detail, they are marvels of organisation.

The Green Tree Ant, Oecophylla smaragdina, in the same genus as the Weaver Ants of Africa. Our species is widespread in the tropics. Its geographical range includes northern Australia and much of tropical Asia.

We seldom see  GTA's in the rainforest because they are ants of the "edge" or of disturbed or open areas. You can usually find them on the vegetation of recently cleared areas or under power-lines where the vegetation is regularly cut and the habitat is more open. These ants are principally predators. They feed on other insects or whatever protein they can manage. I heard recently of an observation of hundreds of ants moving a small road-killed python up a tree to their nest where it would be dissembled and eaten.
Recently GTA's have been showing up at my light sheet. They work night and day removing any poor insect that takes their fancy. They do it in a systematic way. Once a prey has been selected, a group of ants stretches it out but they don't dissemble it. Instead they just hold it in that position for many minutes before taking it away. If you free the prey, you discover that it is dead. So how do they kill it? When the ants bite, they do not sting. When they bite us they often recurve the abdomen and squirt a small amount of formic acid in to the wound, just to make it more painful! But they do not seem to do this to potential prey. They just systematically hold it in a stretch.

When I consulted the "Ant Bible" I found numerous references to GTA's but nothing on their prey-capturing techniques.

This book is well worth having. It is 732 pages of fascinating information about the world of ants.

When the GTA's bite, they may inject a substance from their mandibular glands that renders the prey immobile. Or could the experience so shock the nervous system of the prey that is causes death? Someone may know the answer.
Here the ants have a female of a bibionid that is "protectively-coloured" meaning that it is advertising that it is distasteful to vertebrates like lizards and birds. However, this usually does not apply to other insects.
The GTA nest is a marvel in itself. They can be as large as a rugby football. This is a small one in the process of building. The ants tie leaves together in a community effort. They "sew" the leaves together with silk. But since the adults cannot do this themselves because they do not produce silk, they use their larvae for that purpose. A "string" of workers holds the leaves together while others come along bearing larvae that spin the silk that holds the leaves together to form chambers for the queen and brood.

The GTA's have a cadre of commensals that live with them and serve a variety of purposes. Butterfly caterpillars live within the nests affording them protection from predators and parasites. Their excretions provide the ants with sugars they need in their diets. The ants also "farm" in the sense that they promote and cultivate certain bugs like aphids and scales that also provide them with sugars for their diet. The ants no only protect their "cattle" from predators but they move them to better sites if they find them. So having GTA's in the garden because they consume pest insects is not always the case.

GTA's seem to be seasonal in our rainforest garden. We are in the midst of a very dry Dry Season and the forest has opened up a bit. Thus the ants. I suspect as the rains approach and the vegetation becomes a bit more dense, the GTA's will disappear and other more adapted rainforest species will take over.


Piotr Naskrecki said...

The African Oecophylla longinoda use a similar stretching technique of killing their prey. This species is also more common in disturbed habitats than in undisturbed forest or woodland savanna. I wonder if perhaps the stretching, combined with spraying with the formic acid, causes the prey to release so much stress hormones that it dies.

randomtruth said...

Wild. I think I tasted these ants while visiting the Daintree. Lemonade, right?

Mr. Smiley said...

Good comments. Thanks
The indigenous population used to eat the abdomens of GTA's and it is said they provided some vitamins as well as formic acid to their diet. They also roasted the large nests over the fire and ate the contents. That could be a bit prickly. Tour group leaders often encourage visitors to "try one". They do have a citrus flavour.