Biology Reference
In-Depth Information
exudate attracts insects that disperse the spores. The cost of fungal spore dis-
persal is, however, at the expense of the reproductive function of the plant.
Wäckers (2002) gives other examples, such as rust fungi that produce, near
their spores, sugar droplets consumed by dispersing insects.
Honeydew is the greatest non-floral source of sugar, and was probably a
flight fuel for insects before the appearance of flowering plants. It is the
excretory product of homopteran insects, such as aphids, whiteflies and scale
insects, which must feed more or less continuously on phloem sap in order to
obtain sufficient nitrogen. The excess sugar and water in their diet is
excreted as honeydew, which differs from nectar in containing oligosaccha-
rides synthesized by the insects from the dietary sugars. The sugar compo-
sition of honeydew depends on both the sap-sucking homopteran and its host
plant and, in addition to sugars, amino acids from the phloem are also
excreted to some extent (Byrne & Miller, 1990; Völkl et al., 1999). It was
suggested by Downes and Dahlem (1987) that honeydew use may have pre-
ceded nectar feeding in early Diptera, which appeared long before the
angiosperms: the pseudotracheate labellum of flies would have been ideal for
dissolving and then imbibing dried films of honeydew on leaves. These
sponging mouthparts are likewise suited for drinking stigmatic secretions
which, like honeydew, are shiny fluids which would be visually attractive to
flies. The fossil history of surface fluid feeding involves a wide range of
imbibed fluids, not necessarily involved in pollination (Labandeira, 2002).
Many small insects such as flies, ants, and parasitoid wasps meet their
carbohydrate requirements from a mixture of floral nectar, extrafloral nectar,
and homopteran honeydew, although in laboratory experiments it has been
found that the oligosaccharides in honeydew are less likely to elicit feeding
responses and are of less value nutritionally (Wäckers, 2000, 2001). Ants in
Australian rainforests obtain sugar and amino acids from many different nec-
tar and honeydew sources (Blüthgen et al., 2004). Ants frequently tend
phloem-feeding homopterans, protecting them from natural enemies in ex-
change for carbohydrate-rich fluids (Völkl et al., 1999). Honeydew is also a
valuable sugar source for honeybees, particularly in forests when nectar is in
short supply, and honeydew and other sugary fluids may substitute for nectar
in the diets of nectarivorous birds (Paton, 1980; Gaze & Clout, 1983). Finally,
the manna mentioned in the Biblical chapter Exodus was apparently honey-
dew produced by a scale insect (
Trabutina mannipara
) associated with
tamarisk; it accumulates when attending ants are absent (Bodenheimer,
1947; Ben-Dov, 1988).
