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(e.g., short-tongued bees, wasps, beetles, butterflies, flies; cf. Petanidou,
1991). On the other hand, high-sucrose nectars are better adapted to more
specialized pollinators such as long-tongued bees which are apt to perform
sucrose digestion (hydrolysis). The dominance of hexose-rich nectars in
winter coincides with the prevalence of non-specialized pollinator guilds
(e.g., syrphid, anthomyiid, and other flies). Similarly, sucrose-rich nectars
prevail in spring and summer together with their selective agents, the long-
tongued bees. The presence of any high-hexose nectars during spring and
summer is probably related to mixed guilds of insects that are active during
that period (Petanidou, 2005).
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Trade-off between plant water economy and co-evolution with insect diet.
For the same carbohydrate reward offered to pollinators, high-sucrose nec-
tars utilize less water than high-hexose ones. Considering that calorific
value is more important to bees (at least to honeybees; cf. Wells et al.,
1992) than the type of sugars contained in the nectar (i.e., mono-, disaccha-
rides), it could be concluded that in bee-dominated communities, such as
those of the Mediterranean, selection favours high-sucrose over high-hexose.
5.3
Nectar amino acids and pollinators
Phenylalanine (present in 9.5% of the study species) and GABA (present in
63% of the species) were the only amino acids in the phryganic community
that were consistently correlated with pollinator guilds and families (Petani-
dou et al., 2006). The effect was expressed as the relationship between the
phenylalanine content of plant nectars (= % of total amino acid content)
versus the number of species in pollinator guilds or families visiting them.
Phenylalanine appeared to be positively related to long-tongued bees and
megachilids. GABA could be correlated with to a broader array of insects—
long-tongued bees, anthophorid and andrenid bees, as well as anthomyiid
and syrphid flies.
On the other hand, several amino acids appeared to be sporadically repel-
lent to a few insect groups. Asparagine appeared to repel many insect groups:
beetles, bugs, anthomyiid flies, wasps, short-tongued bees and colletids, but
only megachilids among the long-tongued bees). These characteristics seemed
to be a result of co-evolution with bees—long-tongued bees, especially
Megachilidae, seem to have played the major selective role for phenyla-
lanine-rich nectars (Petanidou et al., 2006). This could be related to the fact
that phenylalanine is an essential amino acid in bee diets (de Groot, 1953),
an explanation that fits well with the classic ideas of Baker and Baker
(1973a, b, 1978, 1986). Petanidou et al. (2006), however, go further by
