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Foraging organisms (like bumble bees) move between resource points (like flowers) whose natural distributions vary enormously: from hyperdispersed to random to clumped. These differences in habitat structure may significantly influence the fitness of both plant and pollinator. To examine the effect of habitat structure on pollinator movement and fitness, we observed captive worker bumble bees collecting nectar from artificial flowers containing equal volumes of reward and arranged in two spatial configurations: a hexagonal array with constant distances between flowers ("constant"), and an "exploded hexagonal" array, with variable distances between flowers ("variable"). The mean nearest-neighbour distance was the same in both arrays, as was the general hexagonal appearance. The experiment therefore compares how resource dispersion, independent of nearest-neighbour distance, influences bee behaviour. Bees in the variable array showed decreased directionality, higher revisitation frequencies, and greater inter-flower flight distances than shown in the constant array. As a consequence, bees in the variable array had a 19% lower gross rate of nectar collection. Our results suggest that wild-foraging bees should prefer regularly spaced flowers (when all else, including mean nearest-neighbour distance, is equal), and that plants can decrease self-pollination by regular spacing between flowers, inflorescences, or individuals.

This study relates foraging behaviour and division of labour to a colony's energy state in wild-foraging bumblebee colonies whose energy stores had been experimentally manipulated. It examines: (1) colony-level effects of energy manipulation, to see if shifts in a colony's energy requirements produce shifts in the behaviour of its workers; and (2) the behaviour of individuals who changed tasks following manipulation and those that did not, to see if task specialization, based on learning, can explain differences between these groups. Individually marked bumblebees were observed at three colonies of each of three species ( Bombus flavifrons, B. melanopygus , and B. occidentalis ). Overall, colonies responded to changes in their energy stores, both by adjusting the relative sizes of the foraging and within-nest work-force, and by changing the behaviour of individual foragers. In response to energy manipulations, colonies allocated effort where demand was relatively greatest (foraging versus within-nest tasks), collected more of the resource that was in greater immediate demand (nectar versus pollen), and changed the rate at which each forager collected food (within-nest bout durations). Task switching behaviour was generally consistent with that expected by learning-constrained task specialization: (1) less efficient foragers switched to withinnest tasks when energy stores were enhanced; (2) workers who began foraging after energy depletion were less efficient than were continuing foragers and (3) workers who switched from pollen to nectar collection following energy depletion were less efficient foragers both before and after the manipulation. However, workers who switched from nectar to pollen collection were no less efficient at their old or new tasks than non-switching bees. In general, worker bumblebees show a flexible division of labour in the face of short-term changes in their colony's energy state.

Risk-sensitive foraging decisions (i.e., those based on both reward mean and variance) have often been observed in laboratory contexts, but there are few observations of risk-sensitive foraging in the wild. Indeed, wild foragers need not show risk sensitivity, since they can adopt alternative tactics for minimizing their probabilities of energy shortfall. I tested for risk-sensitive foraging in a natural context in coastal southwest British Columbia, Canada. Seablush (Plectritis congesta) and dwarf huckleberry ( Vaccinium caespitosum) are co-occurring plants that offer foraging bumble bees equivalent expected rates of net energy intake, but that differ in variability. This equalization of mean profitabilities of floral rewards results from the actions of nectar-collecting foragers. Risk-sensitive diet choice can therefore be based solely on the variances of the foraging alternatives. I manipulated the energy requirements of bumble bee colonies (Bombus melanopygus, B. mixtus, B. sitkensis) by draining or enhancing their honey pots. I then censused foragers from these colonies on the two flower species. If bees were risk sensitive in their diet choice, they should have increased their relative use of the more variable flower type (dwarf huckleberry) when their colonies were nectar depleted compared to when their colonies were nectar enhanced. This was indeed the case. Wild-foraging bumble bees thus appear to be sensitive to both the mean and the variance of the energy rewards offered by alternative flower species, and this sensitivity is affected by colony energy requirements. These results suggest that wild foragers may change their risk sensitivity.

Bumble bees rely on stored nectar to maintain high colony temperatures. This study examines some of the costs associated with exhausting stored nectar for a day in confined colonies of Bombus occidentalis Greene and B. melanopygus Nylander. Workers from energy-deprived colonies ceased incubating and allowed brood temperatures to drop to ambient levels. Workers from energy-rich colonies responded to a simulated vertebrate predator by actively moving about and buzzing loudly (apparently searching for the source of disturbance), but those from energy-depleted colonies mostly remained stationary on the comb. Workers from energy-rich colonies responded to an intruding social parasite (a Psithyrus insularis Smith queen) by chasing and attacking it, whereas those from energy-depleted colonies adopted stationary threat postures. In both cases, the more successful defensive strategy of simultaneous attack by several workers was not employed by nectar-depleted colonies. Cooling of final-instar larvae and pupae added to their development times (disproportionately so in the case of pupae), but low temperature per se did not affect their probability of survival. Hence, the costs of short-term energy shortfall include increased susceptibility to predators and parasites and a lengthened period of development. These costs probably relate to energy stores in a nonlinear manner, providing the foundation upon which risk-sensitive foraging decisions can be based.

This study examines whether the foraging behavior of worker bumble bees (Bombus: Apidae) collecting nectar on inflorescences of seablush (Plectritis congesta: Valerianaceae) is affected by colony energetic requirements, which were experimentally manipulated either by adding sucrose solution to honey pots or by removing virtually all available nectar from the pots. The competing hypotheses tested were: (1) no change; energetic requirements do not affect behavior, since there is a single best way to collect food in a given environment; (2) energetic currency; the energetic currency maximized by foragers changes according to colony energetic condition, with nectar-depletion causing a shift from maximizing long-term productivity to maximizing immediate energetic gain, thereby de-emphasizing energetic costs; and (3) predation; foragers devalue risk of predation as risk of starvation increaes, with colony nectar-depletion causing foragers to be less predation riskaverse in order to increase immediate energetic gain. Relative to when their colony energy reserves were enhanced, foragers from nectar-depleted colonies selected smaller inflorescences, visited fewer flowers per inflorescence, probed flowers at a higher rate while on each inflorescence, and walked between inflorescences less often, thereby spending a greater proportion of their foraging trip in flight. These behaviors increased a bee's energetic costs while foraging, and should also have increased its immediate energetic gains, allowing rejection of the no change hypothesis. Predictions of the predation hypothesis were generally not supported, and our results best support the energetic currency hypothesis. Foraging currency of bumble bees therefore appears to be a function of colony energetic state.

In a controlled laboratory experiment, we re-examined the question of bumble bee risk-sensitivity. Harder and Real's (1987) analysis of previous work on bumble bee risk aversion suggests that risk-sensitivity in these organisms is a result of their maximizing the net rate of energy return (calculated as the average of expected per flower rates). Whether bees are risk-sensitive foragers with respect to minimizing the probability of energetic shortfall is therefore still an open question. We examined how the foraging preferences of bumble bees for nectar reward variation were affected by colony energy reserves, which we manipulated by draining or adding sucrose solution to colony honey pots. Nine workers from four confined colonies of Bombus occidentalis foraged for sucrose solution in two patches of artificial flowers. These patches yielded the same expected rate of net energy intake, but floral volumes were variable in one patch and constant in the other. Our results show that bumble bees can be both risk-averse (preferring constant flowers) and risk-prone (preferring variable flowers), depending on the status of their colony energy reserves. Diet choice in bumble bees appears to be sensitive to the “target value” a colony-level energetic requirement.

1. The insect wing appears to degenerate with use, so that wing wear increases with increased flight activity. Wing degeneration may affect an insect's mortality. 2. The rate of mortality is known to increase with age in worker bumble bees. This study examines whether wing wear can account for mortality in foraging bumble bees. 3. Workers in eight wild-foraging colonies of bumble bee (Bombus melanopygus Nylander) were divided into two treatment groups: clipped (the outer margin of each forewing trimmed, reducing wing surface area by an average of 18%) and unclipped controls. The mortality and behaviour of foraging individuals was assessed with colony watches and night censuses. 4. Treatment did not detectably affect the proportion of foraging bees, the lengths of foraging or within-nest bouts of foragers, or pollen load sizes, but wing-clipping did cause bees with the greatest amounts of initial wing wear to stop foraging. 5. Mortality was positively related to natural wing wear among unclipped bees. In addition, relative to control foragers, foragers with clipped wing margins experienced significantly reduced life expectancies. 6. These results support the hypothesis that wing wear is a proximate factor responsible for an increase in mortality in older workers. The wing-wear hypothesis therefore provides one functional reason for foragers to adopt a currency that maximizes the ratio of net benefit to cost (i.e. 'efficiency').