Down in the farm

What, at first sight, could be more specific than agriculture? To become a farmer entails a series of familiar processes, from maintenance of gardens, transport, weeding, application of herbicides, manuring, cropping, to the exchange of cultures. That is effectively how we pursue our agriculture. So, too, and convergently, do the leaf-cutting ants (.Acromyrmex and Atta)1 that flourish in Central and South America.2 It was they who were the subject of my Vorstellung at the beginning of this chapter. As their common name suggests, these remarkable insects (Fig. 8.1) use their sharp mandibles to excise pieces of leaf3 that are then transported back to the communal nest. Although typically associated with forests, these leaf-cutting ants have managed to extend into desert environments.4 In some species there is a remarkable division of labour, in which a rather small number of worker ants leave the nest early, climb the trees, cut away the leaf pieces, and then drop them to the ground. There another group, arriving slightly later, find the pieces, cut them smaller, and carry them to the 'road', where a third group of ants transports the vegetation back to the nest.5 Another example of such division is either the 'bucket brigade' whereby some species of ant hand over plant material at junctions of the 'road' system6 or the formation of caches when supply exceeds the rate of processing the leaves within the nest.7 Travel is often via well-defined and more-or-less permanent paths,8 which may stretch for considerable distances, sometimes 100 metres. On the main thoroughfares the ground may be cleared to form a smooth highway that in turn maximizes walking speeds. In these social insects three distinct castes are recognized, and in addition a miniaturized variety (the minims) may also occur. Some of these minims spend time working in the gardens, as well as riding on the larger ants, where they engage in grooming and cleaning activities. Another important role, only recently appreciated, is to patrol the edges of the trails along which the foragers march, on the lookout for danger and when appropriate issuing an alarm.9 Yet others 'hitch-hike' on the leaf fragments as they are being carried back to the nest, in part probably to defend the leaf-carriers from attack by parasitoid flies (known as phorids) and possibly also to begin the preparation of the leaf prior to its arrival at the nest.10 In the nest itself, the soldier caste is vigorous in defence.

The collected leaves are not eaten directly11 but are used to provide a mulch for fungus gardens that are located within the nest. The leaves, of course, are fresh and the initial preparation, which as already noted may start during transport, includes a stripping away of the outer waxy layer. This process, achieved by a sort of licking, also appears to inhibit the activity of associated microorganisms, the control of which is a central necessity to the health of the fungus farm. Thereafter the leaves are shredded and pulped, and at this stage are ready for fungal innoculation.12 The saprophytic activities of the fungi break down the plant material, especially the resistant cellulose, and so provide an edible crop for the ants.13 The gardens are subject to careful and ceaseless maintenance. Weeding,14 especially of infected areas, is undertaken principally by tiny ants (the minims). In weeding several minima typically loosen the offending item before it is removed by larger ants. In addition to weeding, the minima also engage in grooming, that is the removal of alien spores. If necessary the ants will also transfer the fungi to parts of the nest with more suitable humidity15 and temperature.16 In addition to weeding there is also the activity of pruning, again to encourage the harvest.17 The application of fertilizers is in the form of a manure, an excrement rich in nitrogen as well as an enzyme supplement.18 The harvest is ready and, in the more advanced types of cultivation, cropping involves the cutting off of the knob-like ends of the fungus, which are rich in protein, sugars, and other compounds.19

As on our own farms there is, however, a recurrent danger of invasion by pathogens. For the ants' fungal gardens the principal risk comes from a virulent parasite, also a fungus, Escovopsis. If it is not controlled, the garden is soon converted into a mouldering and blackened ruin.20 How do the ants avert disaster? They apply the equivalent of a herbicide, specifically in the form of an antibiotic.21 This is derived from streptomycetean bacteria, the filaments of which grow attached to the bodies of the ants.22 In addition to these fungicides it is likely that other chemicals, including again antibiotics, secreted by the ants themselves, also help to inhibit the growth of unwanted bacteria and fungi.23 Both provide defences against infestation, but interestingly the ants show a trade-off whereby newly imported leaves are protected by the ants (mostly minims) using their own secretions. This helps to provide an all-round microbial defence. However, in the older parts of the fungus garden, where the fungal biomass is higher and presumably more vulnerable to attack by the parasitic Escovopsis, the larger ants responsible for this area of maintenance rely on the bacterial defences.24 Nor are these the only risks faced by the ant-gardeners: on occasion a specialized predator aggressively sweeps in, expels the attine ants, and usurps the garden.25 These so-called 'agro-predators' are a particular species of myrmicine ant, unrelated to the leaf-cutters and related attine ants. In addition to usurping the gardens these myrmicines have been observed to place their larvae adjacent to those abandoned by the vanquished attines, where most probably the latter larvae act as a convenient source of protein. Yet more terrible fates await other attine nests when their citadel falls, after stiff resistance and appalling casualties on both sides, to army ants. Here part of the resistance entails plugging nest entrances and building barricades with leaf fragments.26

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