Laura Moquet

Early spring floral foraging resources for pollinators in wet heathlands in Belgium

In temperate regions, foraging resources for pollinating insects are particularly important in early spring, especially for social insects like bumblebees that are initiating colonies. Heathlands, protected open habitats under EU law, provide floral resources (pollen and nectar) for a range of pollinating insects. In early spring, in Belgian heathlands, only two floral resources are available: willows Salix spp. (Salicaceae) and bilberry Vaccinium myrtillus (Ericaceae). Our objective was to assess whether these two plant species provide quantitative and qualitative resources during early spring. We examined the springtime activity of flower visitors on both S. x multinervis and V. myrtillus flowers in relation to sugar concentration and composition in nectar as well as composition of polypeptides and amino acids in pollen. The chemical composition of pollen differed significantly between the two plant species. Salix x multinervis offered pollen with higher polypeptide and essential amino acid contents than V. myrtillus. However, nectar quantities of V. myrtillus flowers were relatively high compared to S. x multinervis. During the overlapping flowering period of the two plant species, flower visitors seemed to favor high quality and easily accessible pollen of S. x multinervis species and visited V. myrtillus mainly for nectar resources.

Bumblebees depend on ericaceous species to survive in temperate heathlands

Bumblebees are the predominant wild pollinators for many plant species in temperate regions. A bumblebee colony requires pollen and nectar throughout its lifetime, but degraded and fragmented habitats may have gaps in the temporal and spatial continuity of floral resources. Heathlands are open biotopes that provide favourable habitat for bumblebees like Bombus jonellus, a declining species in Belgium. In heathlands, ericaceous species are the main plants that provide pollen and nectar for bumblebees. Although the nectar composition of ericaceous species has been previously studied, data on pollen composition remain scarce. We examined bumblebee diets (composition of their pollen loads) in Belgian heathlands over the course of a colony lifetime to assess the fidelity of bumblebees for ericaceous species. We compared nutritional values by investigating the chemical composition (amino acids, polypeptides and sterols) of the pollen of the ericaceous and dominant non‐ericaceous species present in pollen loads. No relationship was detected between the abundance of a particular plant species in bumblebee loads and its pollen composition. The successive flowering periods and the nutritional quality of pollen of ericaceous species offer valuable resources for bumblebees. Ericaceous species represent a large part of bumblebee diets in heathlands, especially in early spring and late summer when the diversity of other flowering species was low. Bumblebee pollen loads also contained non‐ericaceous flowering species that grow outside heathlands. Thus, land planning must incorporate conservation strategies for the different elements of the landscape matrix, including heathlands, peatlands, meadows and margins.

Conservation of hoverflies (Diptera, Syrphidae) requires complementary resources at the landscape and local scales

Accumulating evidence shows that landscape fragmentation drives the observed worldwide decline in populations of pollinators, particularly in species of Lepidoptera and Hymenoptera. However, Little is known about the effects of landscape fragmentation on hoverfly (Diptera, Syrphidae) communities. Hoverflies provide varied ecosystem services: larvae contribute to waste decomposition (saprophagous species) and pest control (aphidophagous species), and adults pollinate a wide range of flowers. To determine how the diversity and quantity of resources for larvae and adults affect hoverfly abundance and species richness at three spatial scales, we recorded insect visitors of five target plant species in Belgian heathlands, habitats that have decreased considerably due to human activities. Hoverflies represented the most abundant visitors on two plant species, and the second most abundant visitors (after bumblebees) on the other target plant species. A large proportion of hoverflies observed were aphidophagous species associated with coniferous and deciduous forests. Resources for the larvae and floral resources for the adults influenced interactions among hoverflies and plants, but acted at different scales: larval habitat availability (distance to larval habitat) was relevant at the landscape scale, whereas adult resource availability (floral density) was relevant at the plot scale. Hoverfly abundance and species richness decreased with distance to larval habitat but increased with floral density. Moreover, landscape structure and composition had different effects according to hoverfly ecological traits. Landscape composition influenced aphidophagous but not saprophagous hoverflies, in that their abundance and species richness decreased with distance to forests. Maintenance of the interactions between plants and their hoverfly visitors requires complementary resources at both landscape and local scales.

Spatial and temporal variations in floral resource availability affect bumblebee communities in heathlands

Modifications of landscape structure and composition can decrease the availability of floral resources, resulting in the decline of many pollinator species, including bumblebees. These declines may have significant ecological consequences, because bumblebees pollinate a large range of plant species. Our study was carried out in heathlands, open semi-natural habitats that have decreased considerably due to human activities. We analysed how floral resources affect bumblebee communities throughout the colony lifetime at three scales: plot scale, heathland patch scale, and landscape scale. Floral density at the plot scale and spruce plantations at the landscape scale influenced bumblebee communities. The abundance of bumblebees on ericaceous species was higher when the landscape included a substantial proportion of unsuitable foraging habitat (i.e., spruce plantations). Both life history traits and colony life cycle stage influenced bumblebee responses to the availability of floral resources. Bumblebees were more affected by floral resources during the colony development phase than during the nest-foundation or mating phases. Moreover, bumblebees of species that form large colonies needed larger quantities of favourable foraging habitat, compared with small-colony bees, and their proportion decreased in habitats dominated by spruce plantations. In conclusion, the conservation of plant–bumblebee interactions will require management at a larger spatial scale than the restricted protected habitats. Moreover, at the landscape scale, both quantity of favourable foraging patches and their ecological continuity are important to conserve both small- and large- colony species.

Nectar foragers contribute to the pollination of buzz-pollinated plant species

PREMISE OF THE STUDY Pollination performance may depend on the type of floral resource (pollen or nectar) foraged by visitors. In buzz-pollinated plant species, the poricidal anthers release pollen during active pollen collection that induces flower vibrations. These buzz-pollinated species generally do not produce nectar. Nevertheless, several Ericaceae are buzz-pollinated and produce nectar. We estimated the relative effectiveness of visitors according to the type of resource collected, nectar or pollen (buzzing). METHODS We compared the relative performance of pollen removal, transport, and deposition (effectiveness) of the main insect visitors on four ericaceous species: three buzz-pollinated species with different pore sizes, Erica tetralix, Vaccinium myrtillus, and V. vitis-idaea; and one non-buzz-pollinated species, Calluna vulgaris. KEY RESULTS Bumblebees were the main pollinators for the three buzz-pollinated species, whereas hoverflies were the main pollinators for the non-buzz-pollinated generalist C. vulgaris. For the studied plant species, we observed no difference in pollination effectiveness among bumblebee species. Buzzing bumblebees were the most effective visitors for pollination per flower visit for the two Vaccinium species, whereas nectar foragers were the most effective visitors for pollination of E. tetralix. In the case of Vaccinium myrtillus, nectar foragers contributed the most to pollination success because they were more abundant than pollen foragers. CONCLUSIONS We showed that consideration of the resource collected by visitors and their behavior is necessary to compare their relative performance. The combination of visitation rate and effectiveness per visit reveals that nectar foragers make a substantial contribution to pollination of the buzz-pollinated ericaceous species.

Tilia trees: toxic or valuable resources for pollinators?

To investigate whether Tilia trees are poisonous or valuable resources for bee visitors, we determined the nectar and pollen quantities and composition of the four main Tilia species planted in Western Europe (T. cordata, T. platyphyllos, T. tomentosa, and T. × europaea). We developed a new method to detect nicotine in nectars. We observed insect visitor diversity and abundance. We also assessed bumblebee death when individuals were only fed with Tilia flowers. No traces of mannose or nicotine, incriminated in the Tilia toxicity, have been detected in the nectars of the studied species. Huge numbers of insect visitors, mainly bees and syrphids, visited the trees which offer large numbers of flowers, plenty of sugar rich nectar, and protein-rich pollen. Bumblebees only fed with Tilia flowers did not present any particular mortality. We discuss the different hypotheses of the supposed toxicity and propose future research to solve this debate.