to be similar to those of honey-bees
(Wäckers, 1999, 2001), the composition of
floral nectar is probably suitable for
hymenopteran parasitoids. Parasitoid
species have been reported to feed on vari-
ous types of floral nectar (Kevan, 1973;
Jervis et al., 1993; Idris and Grafius, 1995;
Baggen and Gurr, 1998).
Extrafloral nectaries include a wide
range of nectar-excreting structures
(Zimmerman, 1932). Extrafloral nectaries
have been described in approximately 1000
species from 93 plant families (Koptur,
1992; Whitman, 1996). They occur on a
range of plant parts, including stems,
leaves, fruits and flowers. Extrafloral nec-
taries are distinguished from their floral
counterparts by the fact that they are not
involved in pollination. Instead, they are
thought to serve a role in an entirely differ-
ent type of mutualism, in which plants use
nectar to recruit predators or parasitoids.
The latter return the favour by safeguard-
ing plants against herbivory.
In a number of plant systems, it has been
demonstrated that the presence of extrafloral
nectar can translate into both reduced plant
damage (O’Dowd and Catchpole, 1983;
Wagner, 1997) and increased plant reproduc-
tive fitness (Rico-Gray and Thien, 1989;
Oliveira, 1997). The above-mentioned stud-
ies have all focused on the role of EFN in
plant–ant mutualisms. However, extrafloral
nectaries are also frequented by a range of
other carnivorous arthropods (Bugg et al.,
1989; Koptur, 1994; Whitman, 1996). The pro-
vision of these food supplements may serve
to enhance the effectiveness of plant–spider
(Ruhren and Handel, 1999), plant–predatory
wasp (Torres-Hernández et al., 2000) or
plant–parasitoid interactions (Lingren and
Lukefahr, 1977; Stapel et al., 1997).
Honeydew is a generic term for sugar-rich
excretions of phloem-feeding Sternorrhyncha.
It is generally accepted that sap-feeding
insects have to excrete carbohydrates to bring
the high carbohydrate/amino acid ratio of the
ingested phloem sap in balance with their
nutritional requirements. Honeydew is an
exception to the above-mentioned sugar
sources, as it is a waste product, rather than
having a primary function in mutualistic
interactions. However, depending on its com-
position, honeydew can be eagerly collected
by ants (Stadler and Dixon, 1999; Völkl et al.,
1999). The general tendency of ants to defend
and protect sugar sources has resulted in
mutualistic interaction between some honey-
dew producers and ants. In these instances,
honeydew production has to some extent
become an analogue to EFN.Sugar-source Characteristics
Nectar and honeydew contain various sug-
ars, amino acids, lipids and other organic
compounds in more or less aqueous solu-
tions (Baker and Baker, 1982b; Kloft et al.,
1985). The nutritional and energetic value of
a particular nectar or honeydew is deter-
mined by its volume, its composition and the
component concentrations.VolumeThe volume of floral nectar excreted and its
composition are primarily a plant character-
istic. In addition, however, they may be
affected by other factors, such as the age of
the nectary, irradiance, temperature, soil con-
ditions and water balance (Búrquez and
Corbet, 1991) and state of pollination (Gori,
1983). The duration of nectar secretion is lim-
ited by the – often brief – flowering time.
The often copious nectar volume
secreted by extrafloral nectaries can exceed
floral nectar production. This is in part due
to high production levels, as well as to
extended periods of production. As in floral
nectar, the production of EFN is affected by
abiotic factors (Bentley, 1977). In addition,
plants can raise the secretion of nectar in
response to two biotic mechanisms. Nectar
production can be induced both by ant
attendance (i.e. nectar removal) (Koptur,
1992; Heil et al., 2000) and herbivore feed-
ing (Koptur, 1989; Wäckers and Wunderlin,
1999; Heil et al., 2001; Wäckers et al., 2001).
This sophisticated two-pronged mechanism
allows plants to actively distribute their
investments in a way that optimizes their
defence.62 F.L. Wäckers