Foraging choices balanced between resource abundance and handling concerns: how the honeybee, Apis mellifera, select the flowers of Robinia pseudoacacia.

Nectar is a main resource harvested by foraging honeybees: their ability in selecting among flowers is the key to optimize resource collection. This ability is expected to be the result of co-evolutionary traits between the plant and the pollinator visiting it; notwithstanding, novel interactions may occur between native and invasive species. Analysing foraging efforts, flexibility and individual constrains has to be taken into account. The foraging pattern of the ubiquitous honeybee on Robinia pseudoacacia, a North-American species widely naturalized in European countries, grounds a perfect case study. The plant shows papilionate flowers especially reach in nectar, but their tripping mechanism is difficult for the small/light-weight honeybee. Yet Apis mellifera is known to pay frequent and constant visits to them: in fact, one of the most appreciated unifloral honey is produced out of R. pseudoacacia. The aim of this study was to understand when and how the bees overcome physical constraints to succeed in flower visits, and to what extent this flexibility extend from the individual to the species. Data were collected in Italy, through focal observations of foraging individuals, nectar content measurements and experiments with manipulated inflorescences. Results clearly indicate that nectar content changes accordingly to the state of flowers (visited or unvisited), which also show slight changes in appearance. Foraging individuals, able to detect these differences, perform active choices preferentially selecting already-visited flowers: lower in nectar content but easier to manipulate. Even if the choice is primarily driven by handling constraints, individual experience and strength of stimuli are prompting visits also to unvisited flowers, notwithstanding a higher risk of failure in resource collection. Behavioural plasticity matching a satisfactory compromise grounds the decision that maximizes the intake of resource balanced with the effort to gain it.

An energetics-based honeybee nectar-foraging model used to assess the potential for landscape-level pesticide exposure dilution.

Estimating the exposure of honeybees to pesticides on a landscape scale requires models of their spatial foraging behaviour. For this purpose, we developed a mechanistic, energetics-based model for a single day of nectar foraging in complex landscape mosaics. Net energetic efficiency determined resource patch choice. In one version of the model a single optimal patch was selected each hour. In another version, recruitment of foragers was simulated and several patches could be exploited simultaneously. Resource availability changed during the day due to depletion and/or intrinsic properties of the resource (anthesis). The model accounted for the impact of patch distance and size, resource depletion and replenishment, competition with other nectar foragers, and seasonal and diurnal patterns in availability of nectar-providing crops and wild flowers. From the model we derived simple rules for resource patch selection, e.g., for landscapes with mass-flowering crops only, net energetic efficiency would be proportional to the ratio of the energetic content of the nectar divided by distance to the hive. We also determined maximum distances at which resources like oilseed rape and clover were still energetically attractive. We used the model to assess the potential for pesticide exposure dilution in landscapes of different composition and complexity. Dilution means a lower concentration in nectar arriving at the hive compared to the concentration in nectar at a treated field and can result from foraging effort being diverted away from treated fields. Applying the model for all possible hive locations over a large area, distributions of dilution factors were obtained that were characterised by their 90-percentile value. For an area for which detailed spatial data on crops and off-field semi-natural habitats were available, we tested three landscape management scenarios that were expected to lead to exposure dilution: providing alternative resources than the target crop (oilseed rape) in the form of (i) other untreated crop fields, (ii) flower strips of different widths at field edges (off-crop in-field resources), and (iii) resources on off-field (semi-natural) habitats. For both model versions, significant dilution occurred only when alternative resource patches were equal or more attractive than oilseed rape, nearby and numerous and only in case of flower strips and off-field habitats. On an area-base, flower strips were more than one order of magnitude more effective than off-field habitats, the main reason being that flower strips had an optimal location. The two model versions differed in the predicted number of resource patches exploited over the day, but mainly in landscapes with numerous small resource patches. In landscapes consisting of few large resource patches (crop fields) both versions predicted the use of a small number of patches.

Flower choice by honey bees (Apis mellifera L.): sex-phase of flowers and preferences among nectar and pollen foragers.

Bees foraging for nectar should choose different inflorescences from those foraging for both pollen and nectar, if inflorescences consist of differing proportions of male and female flowers, particularly if the sex phases of the flowers differ in nectar content as well as the occurrence of pollen. This study tested this prediction using worker honey bees (Apis mellifera L.) foraging on inflorescences of Lavandula stoechas. Female flowers contained about twice the volume of nectar of male flowers. As one would predict, bees foraging for nectar only chose inflorescences with disproportionately more female flowers: time spent on the inflorescence was correlated with the number of female flowers, but not with the number of male flowers. Inflorescence size was inversely correlated with the number of female flowers, and could be used as a morphological cue by these bees. Also as predicted, workers foraging for both pollen and nectar chose inflorescences with relatively greater numbers of both male and female flowers: time spent on these inflorescences was correlated with the number of male flowers, but not with the number of females flowers. A morphological cue inversely associated with such inflorescences is the size of the bract display. Choice of flowers within inflorescences was also influenced predictably, but preferences appeared to be based upon corolla size rather than directly on sex phase.