Agriculture Reference
In-Depth Information
phase dispersal of seeds of
Petalostigma pubescens
(Euphorbiaceae) in northern Australia.
The first or frugivorous phase is ingestion of the fruit by the large flightless emu,
Dromaius novaehollandiae,
which may retain the fruit within its intestinal tract for more
than 72h. After the partially-degraded fruit is voided, it dries and the second phase takes
place as an explosive dehiscence of the endocarp which scatters the seeds 1.5 to 2.5 m
from the point of deposition. The final dispersal phase is transport of the seed by
ants attracted to their elaiosomes. Up to 1200 seeds may occur in a single scat and the
advantage to
P. pubescens
of the two final phases apparently lies in the reduced seedling
competition consequent on wider dispersal of the seeds.
Nectar feeding
While certain ants may feed regularly on floral nectar (Haber
et al.,
1981), few plants
appear to be regularly pollinated by ants possibly because of the toxicity of their
metapleural gland secretions (Peakall
et al.,
1991). Perhaps of equal importance to the
biology of both the plants and the ants is the utilisation by the latter of extra-floral
nectaries. As their name suggests, these nectar-producing structures have evolved in
a wide diversity of plants and vary from single to multiple celled structures; they may
occur on many parts of the plants including the stems, leaves or petioles. Their secre-
tions are largely carbohydrates although amino acids may also be present (Bentley, 1977;
Beattie 1985; Baker
et al.,
1978). Such secretions are attractive to the many ants that
feed regularly on them and which may defend the extra-floral nectaries.
Food bodies
Food bodies are small epidermal structures of varied origin found on certain plants that
provide nourishment to ants; they form one further component of the mutualisms that
have developed between certain ants and plants. Beattie (1985) lists them as providing
lipids, proteins and glycogen to the ants in return for their potential plant protection services.
Honeydew
Honeydew is an energy-rich product excreted by a range of homopterous insects.
These insects feed by inserting their suctorial mouthparts (proboscis) into the phloem
vessels of the host plant. The internal turgor pressure within these vessels obliges
such insects to imbibe sugar-rich fluids substantially in excess of their own metabolic
requirements (Chapman, 1982). This excess is excreted in the form of small droplets
and it is this material that is imbibed by the ants.
The material excreted by the homopterans is only slightly modified by its passage
through the insect gut. It contains a high proportion of sugars together with some
nitrogenous components in the form of amino acids and amides. A range of insects
(honey and other bees, wasps, flies, etc.) exploit this readily-available energy source
although it is the ants that have specialised in its utilisation (Wheeler, 1910).
While ants feed on the honeydew, the homopterans are protected by the ants from
predators and parasitoids; they may also be transported by the ants and sheltered in
specially-built structures. The homopterans taking part belong to the families
Aphididae,
Coccidae, Pseudococcidae, Psyllidae, Membracidae, Cicadellidae, Cercopidae, Fulgoridae,
