Insectes soc. Vol. 48, 2001
Research article
367
1971a). Moreover, the tendency to expose the gland greatly
differs among individuals of the same colony: some bees
never expose their Nasonov gland even after performing
numerous consecutive foraging trips, while others consis-
tently expose this gland after a new profitable source is found
(Free, 1968). Additionally, factors such as the food intake
rate of the hive (Pflumm, 1969; Pflumm et al., 1978), the
presence of a floral (Free, 1968; Wenner and Wells, 1990), or
the Nasonov gland scents (Free, 1968), and environmental
conditions (Núñez, 1971a) may affect Nasonov gland ex-
posure.
Materials and methods
Experiments were performed at the experimental field of the University
of Buenos Aires (34°32¢S, 58°26¢W) from March to April 1997 when
competition with natural nectar sources is reduced. A colony of nearly
2000 European bees (hybrid descendents of Apis mellifera ligustica
bees) was located in a single-frame observation hive (see Frisch, 1967).
Apparatus
The experimental food source was located 160 m from the hive in an
open and shady place, conditions that allowed us to observe clearly the
scenting flights of the experimental bees. It consisted of an artificial
feeder delivering sucrose solution scented with almond essential oil
(80 µl/L). Scented solutions were used both during training and assay.
They were delivered at a constant rate of 5 µl/min through the center of
a 40-mm-diameter disk of yellow paper. A synchromotor activated a
pumping system, with a syringe delivering the sugar solution through a
cannula (Núñez, 1971b). The syringe containing the sugar solution
could be replaced when the bee was in the hive. In this way, the same
‘artificial flower’ was able to offer different sugar solutions (0.6, 1.2 or
2.4 M). Because these were provided at the same flow rate (5 µl/min),
different amounts of sugar were delivered per unit time (1, 2 and
4 mg/min). During the assays, the feeder was switched on when the bee
arrived at the feeding area, and off when it left for the hive. Therefore,
sugar solution did not accumulate between foraging visits.
When collecting nectar honeybees may expose their
Nasonov gland before and/or after probing the sugar solution
at a feeding place (Frisch, 1967; Free, 1968; Pflumm, 1969;
Núñez, 1971a). Pflumm (1969) described three different
behavioral sequences for bees exposing the Nasonov gland
for different profitability ranges: 1) when foraging at low-
profit food sources, bees fly straight to the artificial feeder
and land there without flying around it. When they land,
the scent organ is not exposed. 2) For higher food source
profitability at the feeding place, bees fly in circles around
the food source while exposing their Nasonov gland before
feeding. 3) If the food source profitability is even higher, the
bees expose their Nasonov gland while flying and they keep
the organ everted for part of the feeding time. Moreover,
there may be two phases of Nasonov gland exposure after
landing: in the first, the arrived bee fans its wings with the
gland exposed before it begins to feed. In the second phase
the bee exposes the Nasonov gland during part of the feed-
ing, with or without fanning (Pflumm, 1969). Although some
works studied Nasonov gland exposure during the flight and
the feeding stages (Frisch, 1967; Free, 1968; Pflumm, 1969;
Núñez, 1971a), none has identified a possible relationship
between them. Free (1968) observed that bees that extensive-
ly exposed their glands before feeding also spent more time
exposing them during nectar feeding. Therefore, our first
goal was to quantitatively analyze both stages of the Nasonov
gland exposure, before and during feeding, in bees con-
fronted with different nectar rewards and determine if a
relationship exists between them.
Procedure
A group of bees was trained to collect sugar solution from the artificial
feeder and marked with a colored tag on the thorax. During each ex-
perimental day only one bee was selected for the measurements, while
the others were caged for later use. The selected bee was allowed to
freely visit the feeder and was recorded during 12 successive visits. The
measurements took place between 1030 and 1330 h.
At the end of an assay, the caged bees were released at the feeding
place, offering the same solution flow rate and sugar concentration than
the following experimental day. Out of the experimental time, the ap-
paratus was controlled automatically, switching on at 700 and off at
1800 h. Thirty minutes before the beginning of the experimental period
(i.e., three complete foraging cycles), only a single, marked test bee
was allowed to collect solution at the feeder under the same conditions
as during the training period. This procedure eliminated perturbations
caused by other bees, which may have influenced the performance of
the test bees.
It is well known that the reward expectation that bees
acquire throughout successive foraging visits may affect
the estimation of food source profitability (Greggers and
Menzel, 1993). For example, bees elicit different dance
behaviors according to their previous foraging experience
at the feeding site (Raveret Richter and Waddington, 1993).
Thus, foragers may modify their Nasonov gland exposure
according to not only the current food source profitability but
also their previous experience. In order to examine this
possibility, we analyzed whether abrupt fluctuations in food
source profitability affected Nasonov gland exposure before
and during feeding. These quantitative changes in profit-
ability would allow us to understand how coupled both
behavioral stages are.
Experimental series
Three constant and two variable reward programs were alternatively
offered during the two months of the experimental period. All of them
delivered sugar solution at a rate of 5 µl/min during 12 successive
foraging visits. For constant-reward programs the concentration of the
solution was 0.6, 1.2 or 2.4 M. For variable-reward programs the con-
centration was changed after every four visits by the test bee. One
program was 0.6–1.2–0.6 M and the other was 0.6–2.4–0.6 M.
Behavioral responses
Based on the behavioral sequences described by Pflumm (1969) the
following behavioral parameters were recorded:
1) Duration of Nasonov gland exposure before feeding: the time the
individual bees flew in circles close to the feeder. When bees fly around
the food source it is only occasionally possible to clearly observe the
gland everted, nevertheless, the scenting flights are very distinctive (see