Biology Reference
In-Depth Information
and nectar simultaneously. The intensity of their foraging is illustrated by the
carpenter bee
Xylocopa capitata
, which forages almost exclusively on the
trees of
Virgilia divaricata
(Fabaceae) in Cape Town during spring (Louw &
Nicolson, 1983). In suitable weather conditions, a female carpenter bee visits
eight flowers per minute; the nectar of two of these flowers covers her flight
costs, while the nectar of the remaining six is available for larval provisions.
However, it is pollen rather than the concentrated nectar (53% w/w) that de-
termines the number of foraging trips. Pollen is collected simultaneously,
and a female
X. capitata
must visit about 1,700 flowers of
V. divaricata
to
collect enough pollen to raise one offspring.
Bees lick nectar with hairy tongues. The importance of tongue length was
emphasized by Harder (1986): it determines the maximum flower depth
from which a bee can feed, the volume ingested per lick, and the licking rate.
Bees are commonly divided into two broad categories based on their tongue
length and resulting ability to forage at shallow and deep flowers. Short-
tongued bees include the Andrenidae, Halictidae, Colletidae, and Melittidae;
long-tongued bees include the Megachilidae and Apidae. The long-tongued
orchid bees (Euglossini) differ from most other bees in using suction feeding
(Borrell, 2004). Kingsolver and Daniel (1995)
predicted that a shift from
lapping to suction feeding would increase the effect of nectar viscosity on
rates of ingestion, and in fact orchid bees do collect less concentrated nectars
than other bees (Roubik et al., 1995; Borrell, 2004). The optimal concentra-
tion of 30-40% for these bees coincides with that predicted for suction-
feeding Lepidoptera. For lapping bees the optimal concentration is higher
(Table 3): the fastest energy intake occurs at 50-65% sucrose in bumblebees
(Harder, 1986), and 60% in four species of
Melipona
(stingless bees) and
introduced
Apis mellifera
in a tropical forest in Panama (Roubik & Buch-
mann, 1984). However, colonies of all four
Melipona
species were found to
gather more concentrated nectar as the day progressed. Sampling the crop
contents of bees can be used to provide information about nectar harvesting
on a community basis (Roubik & Buchmann, 1984; Roubik et al., 1995), but
this method depends on the assumption of Park (1932) that nectar carried in
the crop of bees is not dehydrated. The nectar collected by honeybee fora-
gers is regurgitated to their nestmates in a process known as trophallaxis, and
the rate of transfer, like the rate of ingestion, depends on sugar concentra-
tion, being maximal at 30% w/w and decreasing at higher concentrations due
to the increased viscosity (Tezze & Farina, 1999).
Although honeybees have been shown to prefer nectar concentrations of
30-50% (e.g., Waller, 1972), in practice they collect from a much wider
range: for example, Seeley (1986) measured concentrations of 15-65% in
