more variable. In general, EFN composition
shows relatively high levels of fructose and
glucose (Tanowitz and Koehler, 1986;
Koptur, 1994). This can be explained by the
exposed nature of most extrafloral nec-
taries, resulting in increased microbial
breakdown of sucrose. In addition to the
three main sugars, several other sugars
may be present (Table 5.1). Besides carbo-
hydrates, EFN may contain variable
amounts of proteins, amino acids and
lipids (Baker et al., 1978; Smith et al., 1990).
The particular amino acid composition can
increase the attractiveness of EFN as a food
source (Lanza et al., 1993).
Honeydew differs from floral nectar and
EFN as it often contains substantial amounts
of oligosaccharides (Kloft et al., 1985;
Hendrix et al., 1992). Even though the sugar
composition of honeydew reflects the origi-
nal composition of the phloem sap of the
host plant, the sugar components and their
relative quantities can be altered during the
passage through the gut of the phloem
feeder. On the one hand, phloem sugars such
as sucrose and maltose are broken down by
digestive enzymes, while, on the other hand,
the sap feeders may also synthesize more
complex sugars. The trisaccharides melezi-
tose and erlose (fructomaltose), as well as the
disaccharides trehalose and trehalulose, are
examples of sugars that are synthesized
through the action of gut enzymes on plant-
derived sucrose (Mittler and Meikle, 1991;
Hendrix et al., 1992). The resulting sugar
spectrum may range from honeydews that
are almost entirely composed of the phloem
sugar sucrose and its hexose components
fructose and glucose to those honeydews
that completely lack hexoses and are domi-
nated by insect-synthesized oligosaccharides
(Kloft et al., 1985; Hendrix et al., 1992; Völkl
et al., 1999).
Sugar concentrationsSugar concentration is an important factor
determining the uptake of a sugar source. At
low concentrations, gustatory perception
might be impeded (Wäckers, 1999), whereas
high sugar concentrations interfere with
sugar uptake (Wäckers, 2000). In floral nec-
tar, sugar concentrations can already range
from 5 to 75% at the time of nectar secretion
(Dafni, 1992). Environmental conditions may
further affect nectar concentrations, both
indirectly, through their effects on the nectar-
producing plant, and directly, through evap-
oration, hygroscopy or rain dilution.
Sugar concentrations of undiluted EFN
range from 5 to more than 80% (Koptur,
1992; Wäckers et al., 2001). In general, EFN
shows much more variation in respect of
sugar concentrations than floral nectar from
the same plant. When protected from rain,
EFN tends to be more concentrated, proba-
bly due to the fact that its exposed nature
increases evaporation.
The fact that honeydew is typically avail-
able as little droplets or as a thin film on the
substrate means that it is even more sub-
jected to evaporation. As a result, sugar con-
centrations are often at saturation. This is
likely to be a limiting factor in honeydew
uptake. This problem is accentuated by the
specific tendency of the honeydew sugars
raffinose and melezitose to crystallize
rapidly (Wäckers, 2000).Parasitoid Characteristics
Insects often show a tendency to visit sugar
sources of a certain composition (Baker and
Baker, 1982a). The sugar components are an
important factor determining patterns of
food utilization (Inouye and Waller, 1984;
Alm et al., 1990; Lanza et al., 1993; Josens et
al., 1998; Völkl et al., 1999). We have seen
that nectar and honeydew often vary
widely in respect of their sugar composi-
tion. As a result, one frequently investigates
an insect’s response to individual nectar or
honeydew components at well-defined con-
centrations, rather than studying a few arbi-
trary examples out of the broad range of
natural nectar or honeydew compositions.
In previous work, I have studied a range of
sugars occurring in nectar and/or honey-
dew (listed in Table 5.1), as well as lactose.
These 14 sugars were compared in respect
of their effect on parasitoid gustatory
response and longevity.64 F.L. Wäckers