Basic observations

Small foraging areas

Several studies showing that social insects use spatial memory in foraging date back to the penultimate century (e.g. Fabre 1879; Müller 1882). After many decades of detailed research on spatial memory of bees (e.g. Chittka et al. 1995; Menzel, this volume), most pollination biologists accept that such memory exists, but most associate it with finding the nest rather than finding food sources. Optimal foraging theory is partially responsible for this (Healy & Hurly, this volume): some adherents of this theory proposed that pollinators forage using essentially the same rules as protozoans. The numbers of places visited during a foraging bout seemed to many biologists too high (often, several thousand flowers must be visited to fill the stomach of a bee) for bees to memorize much detail of the complex flight path.

Yet, if one catches bee workers at a patch of flowers, marks them, and releases them, one will frequently see some of them return to the site (Ribbands 1949; Heinrich 1976; Free 1993). This indicates that at least some individuals have established small foraging areas to which they return for all or most of their feeding. In one study, 37 plants of Penstemon strictus were planted in a meadow in a hexagonal pattern with 1.5 m between plants (Thomson et al. 1997). We marked bees and followed some of them intensively from 23-28 July 1990. Several bees did all of their foraging in this area; one worker in particular, Bombus flavifrons "Blue," worked the array for our entire period of close observations (23 July through 5 August 1990). She would visit the 37 plants (and some of other meadow species that grew interspersed with the Penstemon) essentially all day, disappearing for only a few minutes at c. half-hour intervals to drop off collected rewards at the nest. Bumble bees of other species have performed comparably on other plants (Thomson et al. 1987), but we do not know whether this site fidelity is typical.

Fig. 10.1. Frequencies of visits in one day to a focal plant of Penstemon strictus by a number of marked bumble bees (Williams & Thomson 1998). Bee names mostly indicate painted color marks (e.g., RY = red-yellow), although a few distinctive bees were identified by natural attributes (e.g., HUGE).

Individual bees

Fig. 10.1. Frequencies of visits in one day to a focal plant of Penstemon strictus by a number of marked bumble bees (Williams & Thomson 1998). Bee names mostly indicate painted color marks (e.g., RY = red-yellow), although a few distinctive bees were identified by natural attributes (e.g., HUGE).

In another study of Penstemon strictus, Williams & Thomson (1998) videotaped all visits to a single potted plant in a circular array of 27 plants. We had marked visitors on previous days. Four individuals made over half of the plant visits (Fig. 10.1); these bees returned to the focal plant at statistically regular intervals, with mean interarrival times of 5.36, 5.90, 7.07, and 7.91 min. Unmarked bees might have been vagabonds with no site fidelity, site-faithful bees that evaded marking, or site-faithful bees that were new arrivals.

Individual bumble bees may maintain more than one foraging area. Brian (1952) noted that Bombus agrorum (now pascuorum) workers tended to leave the nest in characteristic compass direction, but that some individuals had more than one departing direction. These bees also came back with different pollens when they left in different directions. Karen Goodell (personal communication) found that certain workers of B. ephip-piatus collected one of two different sets of several pollen species on different trips in a montane Neotropical habitat. The most likely explanation for the covariation of several species is that the bees were going to two different localities, then foraging inconstantly in each place.

Traplining behavior

If bees do return frequently to foraging areas, they may also tend to visit a set of plants within those areas in a particular, somewhat repeatable circuit (Manning 1956; Heinrich 1976; Thomson et al. 1982, 1987, 1997). In fact, such traplining is a case where pollinator individuality manifests itself par excellence. In one study, we let bumble bees (Bombus impatiens) forage in an arena with six artificial flowers at fixed positions. The nectar rewards were adjusted to bee crop capacity, so that bees had to visit all six flowers (but not more) to fill their stomach once. Each bee was tested individually and encountered an absolutely identical array during 40 successive foraging bouts. Yet, each bee found a unique solution to the problem of linking the six flowers, and used this solution repeatedly (see Fig. 10.2).

Although we lack comparative studies that would indicate how often bumble bees show trapline behavior, or what circumstances tend to elicit it, it seems likely that traplining is most likely to emerge (1) when nectar or pollen rewards are replenished rapidly after being drained by a visitor, and (2) when there are spaces between plants, with sufficient landmarks to allow bees to orient. Bumble bees, especially Bombus ternarius, showed clear traplining behavior on scattered plants of Aralia hispida in central New Brunswick (Thomson et al. 1982); in dense stands of Solidago spp. (goldenrods) nearby, however, bees of the same species showed no discernable tendency to repeat their flight paths, although they were using small foraging areas (J. D. Thomson & W. Maddison, unpublished data).

Two aspects of bumble bee traplining are most relevant to this paper. First, although traplines are quite flexible - bees do not slavishly follow a fixed route, but rather add new plants and drop old ones as conditions change - there is a conservative tendency for bees to keep using accustomed flight paths (Thomson 1996) and to keep returning to plants that have been particularly rewarding in the past (Thomson 1988). For example, Manning (1956) described how bees that had been trained to visit potted plants still returned to those locations after the pots had been completely removed. Second, bees return to plants on their traplines at surprisingly brief intervals, c. 10 min in both Penstemon strictus and Aralia hispida.

Variation in working speed

When following marked pollinators, one is frequently struck by variations in the speed of individuals. Some of this variation is caused by differences in the nectar offerings of plants on which these individuals forage.

For example, bees and butterflies will fly more rapidly when more nectar is available, an observation with several possible explanations (Nunez 1970; Kunze & Chittka 1996). But there is also variation between individuals who are using the same resources at the same time. Some such variation can be explained by size: larger bees are faster fliers (Spaethe et al. 2000). In addition, some sensory attributes correlate with size and influence the speed with which bees detect flowers. Spaethe et al. (2000) recently found that larger bees have better visuo-spatial resolution, and are therefore substantially more accurate and faster at detecting small flowers. Furthermore, foraging speed is dependent on colony needs in bumble bees (Cartar 1992a).

In studies with numerous marked bees, Thomson has frequently encountered a few individuals that seem to fly much faster and to handle inflorescences very quickly. Because such bees are hard to observe for long bouts, they may be underrepresented in certain types of observational data.

Even among the more stolid bees for which data are available, however, there are individual differences in working speed and in other aspects such as flower constancy (Table 10.1). The mean flower-handling times of 17 bees in the 1994 data varied two-fold. Recall that all of these data come from the same plant on a single day. Bees also varied about twofold in the duration of their plant visits (measured as the mean number of flowers visited per plant visit), but plant-visit durations varied so much within bees that the variation among bees was insignificant. In addition to showing variation among individuals, the data for "Blue" suggest that this bee's foraging tempo slowed over the two weeks she was observed.

Variation in foraging mode

Different bees may adopt different ways of using flowers. One of the more conspicuous differences involves the type of floral reward - pollen or nectar - being actively sought. On Penstemon strictus, for example, most Bombus workers enter the large flowers rightside-up and tongue the paired nectaries at the filament bases. These bees usually accumulate small pollen loads, but they never fill their corbiculae, presumably because their honeystomachs fill first. Other bees, mostly B. bifarius, ignore the nectaries, turning upside-down to grasp the anthers and sonicate pollen from them. These bees accumulate very large pollen loads. Some bees combine the two behaviors, but most individuals tend to stick with one type of behavior over at least a few days. Still, changes occur; bees

Bee 1

Sequential position

Flower

Sequential position

Flower

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