Agriculture Reference
In-Depth Information
Eurymelidae
and
Aleyrodidae
(Hölldobler and Wilson, 1990; Carver
et al.,
1991). The ants
included in this mutualism largely belong to the phylogenetically-advanced subfamilies
Myrmicinae, Dolichoderinae
and
Formicinae.
The variety of both ant and homopteran
species involved indicates that this mutualism has developed many times and this is
reflected in the degree of closeness of the relationship which may range from casual
to occasional to obligatory. While this mutualism is most apparent on the aerial surface
of plants, it also occurs widely in associations occurring on underground plant parts.
In a calcareous grassland in England, Pontin (1963) recorded a total of 16 subterranean
species of
Aphididae
and
Coccidae
tended by the formicine ants
Lasius flavus
and
Lasius niger.
Fungivory
Members of the tribe
Attini
(subfamily
Myrmicinae
) are distributed from 44°S in South
America to 40°N in the United States (Weber, 1982); the tribe includes approximately
190 species distributed among 12 genera (Hölldobler and Wilson, 1990). These ants
partake in a unique obligate mutualism with a fungus that they culture on organic
materials brought into their nests, in a similar association to that occurring in the
macrotermitine Isoptera.
As Hölldobler and Wilson (1990) have stated, the
Attini
are a particularly successful
group in the areas where they occur. For example, Cherrett (1986) has estimated that
they may remove from 12 to 17 % of annual leaf production in the tropical rainforests
where they occur and may influence the structure of the vegetation through selective
herbivory (Vasconcelos and Cherrett, 1997). They are also destructive agricultural pests
attacking a range of crops and grasses (including sugarcane) and may reduce the carrying
capacities of the pastures in which they occur by up to 50 % (Fowler
et al.,
1986).
Perhaps the best known species of this tribe belong to the genera
Atta
and
Acromyrmex
which are known as 'leaf-cutting' ants from their habit of cutting small sections of leaves
from their food plants with their specialised mandibles. These are returned to the nest
to be cultured with a symbiotic fungus. The remaining ants of this tribe culture their
fungi on other plant parts, insect faeces or carcasses (Weber, 1982). The fungal symbiont
associated with the attine ants appears to be
Leucocoprinus gongylophora,
a member
of the Agaricales (Basidiomycota) (Hölldobler and Wilson, 1990). However, it is not
entirely clear whether this is the only member of this genus routinely involved and one
species
Cyphomyrmex rimosus,
cultivates a yeast-like fungus (Wetterer, 1994).
As materials harvested by leaf-cutting ants are brought into the nest, they are
thoroughly chewed and moistened by the workers and, together with a drop of protease-
containing anal fluid, are deposited onto the fungal body and inoculated. Worker ants
eat the swollen tips of the growing hyphae although they derive most of their nutriment
from plant sap exuded from the leaf surfaces cut while foraging and during preparation
of the plant material for culturing (Quinlan and Cherrett, 1979).
The workers have a limited capacity to produce proteases and may acquire most
of those needed from the tissues of their fungal symbiont (Martin, 1984). The larvae, in
contrast, secrete their own proteases which they use to degrade proteins within the hyphal
walls of the symbiotic fungus. Workers derive glycogen and starch from, respectively,
the fungus and the food plant; degradation products from the pre-oral digestion of
