Adventures Among Ants - Mark W. Moffett [85]
TAILOR-MADE ANT ACCOMMODATIONS
Where do so many ants find homes in the trees? Many nest in hollowed twigs or galleries in bark, or the litter that accumulates among the roots of orchids and between the leaves of tree ferns.11 Cavities capacious enough to hold large colonies are rare, though, and the success of such colonies often depends on constructing nests, such as the weaver ant’s tents, with materials they find in the canopy or produce themselves. There are other ants that use silk, usually combined with leaves, to build their nests—mostly larval silk, though in one African species the adults synthesize silk of their own from a gland near their mouths, and an Australian ant steals its silk from spider webs.12 The ant-garden ants are among many species that use carton.
There are ants that nest terrestrially and forage in the trees, giving them a toehold in both environments. This is more common than the reverse, a canopy-nesting species that primarily forages on the ground. It’s a classic suburban commuter’s compromise between the best housing and the best income: the forest floor provides more roomy nesting opportunities, and the food and other resources found in the canopy make the transit worthwhile.
While the canopy species mentioned thus far live on any plant that offers a suitable nesting cranny, certain trees, epiphytes, and vines provide custom-fitted ant accommodations. Some of these so-called ant plants cater to a specific ant, providing food and board suited to no one else.13 Why? These residents are proficient at eating herbivores, and they kill anything that sets foot on their host. As a boy, I read how Cecropia trees house Azteca ants in spacious compartments in their trunk joints, feeding them pale, glistening “food bodies” more nutritionally balanced than nectar or honeydew, which exude from the base of each palmate leaf. On my first trip to the tropics as a college student I ran into one such tree—literally—and learned that Azteca don’t just pick on creatures their own size.
Weaver ants, though similarly aggressive, do not occupy specialized ant plants. They can live in any tree by creating their own nests and finding their own food (if the plant has nectaries, so much the better). Childhood experience guides the choice of plant homestead: workers and queens prefer to nest and forage on the tree species they grew up on as larvae and young adults, and, like humans, they become more set in their ways as they get older. Still, the ants show a special affinity for mango and citrus, a fact that encouraged the Chinese to use Oecophylla to control citrus pests as far back as A.D. 304.14 In parts of Africa and Asia, their use in biocontrol continues—though pity the laborers who climb those trees to pick the fruit!
Is the weaver ant’s presence good for the trees? To answer such a question, ecologists conduct a cost/benefit analysis. In some ways the benefits clearly outweigh the costs: weaver ants cull leaf-munching insects, and tree foliage lasts longer where the ants reside. One type of beetle, though fond of foliage, flies away from a tree the moment it senses weaver ant pheromones.15 Another benefit to trees might come from weaver ant hygiene, or rather the lack of it. Nutrition is a problem in tropical forests, where soils are thin—but a tree can absorb nutrients through its foliage as well as its roots.16 The workers in some ant-plant mutualisms use leaves and stems as toilets and trash chambers, thereby feeding their plant. The fecal droplets that weaver ants scatter over leaves to mark their territorial claims might serve as fertilizer, too.
Known as the “dinosaur ant” for her primitive appearance, this Nothomyrmecia macrops worker from Poochera, South Australia, is guarding scale insects that have exuded so much honeydew that they appear to be covered in sugar.
On the negative side, the leaves that the weaver ants incorporate into their nests may be lost prematurely to wear and tear, as the ants pull them from their ideal alignment for photosynthesizing.17 However, since only a tiny percentage