Biology Reference
In-Depth Information
proline (Carter et al., 2006), and glycine influences their learning behaviour
(Kim & Smith, 2000).
3.4.3
Ants
Liquid carbohydrate is a major food of ants. The main sources are honeydew
and extrafloral nectars, and sometimes secretions from the caterpillars of
Lycaenidae, about half of which are involved in mutualistic associations with
ants (Baylis & Pierce, 1993). Ants are generally considered to be poor polli-
nators for several reasons, including their flightlessness, frequent grooming
behaviour, and production of antibiotic secretions that inhibit pollen germi-
nation; in addition they deter other pollinators and rob nectar (Peakall et al.,
1991). Ants competing with honeybees for
Eucalyptus
nectar have the ad-
vantage of foraging at night when most of the nectar is secreted (Buys,
1987). One way to exclude these undesirable visitors from flowers (even if
nectarless) is to attract them to extrafloral nectaries, and the distraction hy-
pothesis suggests that extrafloral nectaries function to reduce ant visitation to
flowers (Wagner & Kay, 2002). Mexican ant acacias have very active extra-
floral nectaries on the leaves; they secrete a daily pulse of dilute nectar,
which ensures that ants and pollinators are largely separated spatially, although
they have similar temporal patterns of activity (Raine et al., 2002). Ant repel-
lents secreted by the acacia flowers further deter ant visits, and this chemical
repellency of floral tissue may be a widespread phenomenon (Ghazoul,
2001).
The more commonly accepted alternative is the protection hypothesis, the
extrafloral nectar being a reward for ant (or wasp) guards for the anti-herbivore
protection that they provide. Extrafloral nectaries function for much longer
than the floral variety (Koptur, 1992), and herbivory can affect both the vol-
ume and composition of the extrafloral nectar. Leaf damage in
Macaranga
tanarius
(Euphorbiaceae) causes increased secretion of extrafloral nectar, via
a signalling pathway involving the plant stress hormone jasmonic acid, and
this is later reflected in reduced herbivory (Heil et al., 2001). Similarly, the
leaves of
Catalpa bignonioides
increase production of extrafloral nectar after
attack by caterpillars, and ant attendance also increases (Ness, 2003). These
studies show a causal link between herbivory, nectar production, and body-
guard recruitment. Smith et al. (1990) found dramatic increases in the amino
acid concentration of extrafloral nectar in
Impatiens sultani
after defoliation
(simulated herbivory). There was no simultaneous change in sugar concen-
tration, which suggests a specific rather than a general physiological
response. The production of extrafloral nectar is stimulated by its repeated
removal as well as by real or simulated herbivory (Heil et al., 2000).
