Pollinator effectiveness in the mixed-pollination system of a Neotropical Proteaceae, Oreocallis grandiflora

In pollination systems with a diverse community of floral visitors, qualitative and quantitative variations in pollination effectiveness can lead to a system in which higher effectiveness results from the synergetic contribution of multiple pollinators. By employing a series of field and laboratory experiments in the south Andes of Ecuador, we compared the effectiveness of hummingbirds and nocturnal mammals visiting Oreocallis grandiflora, an Andean member of the Proteaceae. Pollinator effectiveness was measured with a quantitative component that assessed visitation rates to inflorescences and number of contacted stigmas per visit, and with a qualitative component that counted the number of deposited pollen grains in stigmas. Effectiveness estimates were complemented with controlled pollination experiments that contrasted fruit and viable seed set among flowers exposed to either diurnal or nocturnal pollination, self-pollinated flowers, and a control group with flowers exposed to naturally occurring pollination. Four species of hummingbirds and two species of nocturnal mammals visited the flowers of O. grandiflora. Hummingbirds outperformed nocturnal mammals in the quantitative components of pollination, while mammals were more effective than hummingbirds in the qualitative component. Pollination success was higher for the control group than for the other treatments, while hummingbirds and mammals performed similarly. Our findings suggest a case of functional complementarity in pollination: hummingbirds’ quantitative effectiveness complements nocturnal mammals’ qualitative effectiveness. Furthermore, our study demonstrates that pollination success in plants can be greater when flowers are visited by the whole diversity of pollinators.

Pollinator effectiveness of a specialist bee exploiting a generalist plant—tracking pollen transfer by Heriades truncorum with quantum dots

Heriades truncorum (Megachilidae) is a specialist bee that forages on Asteraceae and collects pollen by tapping its abdomen on pollen-presenting florets which places the grains directly in the ventral scopa. We tracked pollen transfer by female H. truncorum between conspecific inflorescences of Inula ensifolia and Pulicaria dysenterica by labelling pollen with quantum dots. On average, bees transferred 31.14 (I. ensifolia) and 9.96 (P. dysenterica) pollen grains from the last visited inflorescence, 39% and 45% of which were placed on receptive styles. Pollen germination ratio is significantly lower for inflorescences of P. dysenterica visited by one H. truncorum (0.13%) compared with open control inflorescences (0.51%), which suggests that the bees mainly transfer self-pollen of these self-incompatible plants. Thus, a single visit by H. truncorum does not grant the plant high reproductive success, but the bees’ abundance and flower constancy might reduce this disadvantage.

Pollinator effectiveness and importance between female and male mining bee ( Andrena )

Bees are often considered to be effective pollinators in both agricultural and natural ecosystems but could be ineffective pollinators in that they collect large quantities of pollen for food provision but deliver little to stigmas. Male bees do not collect pollen to feed larvae, and their pollination role has been underappreciated. Here we compare pollination effectiveness, visit frequency and pollen foraging behaviour between female and male individuals of a mining bee, Andrena emeishanica , visiting a nectariferous spring flower ( Epimedium pubescens ). Female bees were observed to forage for both pollen and nectar, but male bees foraged only for nectar. Female bees had large hairy hind tibiae with conspicuous scopae, and nearly 90% of the pollen grains they collected went onto the hind legs. Male bees removed less pollen from anthers than female bees but deposited more pollen on stigmas per visit. The higher pollen transfer efficiency of male bees was due to 48.4% of pollen grains remaining ungroomed on the thorax and abdomen, available for stigma contact, but their visitation rate to flowers was much lower. Our results indicate that male solitary bees could transfer more pollen on the stigma per visit but were less important (transferred less pollen in total, because they made fewer visits per unit time) than females.

Native pollinators alone provide full pollination on small-scale commercial cranberry (Ericaceae) farms

Cranberry (Vaccinium macrocarpon Aiton (Ericaceae)) requires insect pollen vectors to maximise fruit yield. In many areas, commercial producers use managed bees (Hymenoptera: Apidae) to supplement native pollinators. On the island of Newfoundland, Newfoundland and Labrador, Canada, due to the small number of available honey bee hives and import restrictions on commercially reared bumble bees, the use of supplemental pollinators is rare. Four farms were studied for two years to identify key pollinators and determine the relationship between fruit yield and bee abundance. The most commonly collected bees were species of Bombus Latreille (Hymenoptera: Apidae), which buzz-pollinate and are likely the primary pollinator on these farms; thus, fruit yield was examined with respect to total Bombus abundance. Stigma loading was also used as a measure of pollinator effectiveness. Contrary to expectation, there was no relationship between Bombus abundance or stigma loading and either fruit set or weight, but there was significant year-to-year variation. Other factors were likely more important in determining yield, and further research is needed to identify those. Under current conditions, native bees provide ample pollination services for maximal yield.

The potential management of the drone fly (Eristalis tenax) as a crop pollinator in New Zealand

The drone fly (Eristalis tenax) pollinates many crops and is found almost worldwide. Its successful management as a field-crop pollinator would provide an additional option to augment bee pollination. We reviewed literature to assess their management potential. A literature search was conducted for information on drone-fly abundance across New Zealand crops, pollinator effectiveness, lifecycle-requirements and potential for mass rearing. Relevant literature was then evaluated to assess the feasibility, benefits and limitations of their management. Eristalis tenax is a proven pollinator of pak choi (Brassica rapa spp. chinensis), kiwifruit (Actinidia deliciosa) and onion (Allium cepa), and visits the flowers of several more crops in New Zealand. It readily completes its lifecycle under laboratory conditions when reared on various organic materials. No reviewed studies were identified that showed successful management of populations for the purpose of field-crop pollination. Key challenges for their management as field-crop pollinators include: being able to mass rear them at an appropriate scale; retaining numbers within targeted areas in the field; and ensuring their use does not significantly impact on non-target species and land-user interests.

Visiting insect behaviour and pollen transport for a generalist oak-savannah wildflower, Camassia quamash (Asparagaceae)

Many studies have investigated plant-pollinator interactions using visit records of insects contacting floral reproductive organs. However, these studies may not reflect the effectiveness of visits, since factors such as visitor behaviour and the composition of pollen on their bodies may influence conspecific pollen transfer required for fertilisation in plants. Here we study how pollen transport to a generalist wildflower, Camassia quamash (Pursh) Greene (Asparagaceae), is influenced by the behaviour and body pollen of five functional visitor groups (Andrena Fabricius (Hymenoptera: Andrenidae)/Halictidae (Hymenoptera), Apis mellifera Linnaeus (Hymenoptera: Apidae), Bombus Latreille (Hymenoptera: Apidae), Osmia Panzer (Hymenoptera: Megachilidae), and Syrphidae (Diptera). We found that functional visitor groups differed in their behaviour (Bombus and Osmia were legitimate visitors, contacting both anthers and stigmas) and in the amount of conspecific pollen on their bodies (A. mellifera had the highest levels and Andrena/Halictidae the lowest). Conspecific pollen receipt by C. quamash stigmas was high (>80%), and best explained by visitor behaviour rather than the proportion of visitors with high amounts of conspecific body pollen. Our findings highlight the utility of pollen analyses for understanding pollinator effectiveness.

Experimental loss of generalist plants reveals alterations in plant-pollinator interactions and a constrained flexibility of foraging

Species extinctions undermine ecosystem functioning but species do not disappear at random. Ecosystem dynamics are ruled by a subset of dominant species, but it is not clear how losing several of these key species would affect plant-pollinator interactions. We tested how the experimental loss of the most highly visited generalist plants would affect flower visitor abundances (visitation) and pollinator effectiveness (quantity of pollen-tubes within pistils) in several sites. Significant changes in the plant-pollinator communities were detected. Firstly, visitation decreased after removing 1-2 most visited plants, suggesting that these species mostly facilitate other plants by keeping high flower visitor abundances in the sites. Nevertheless, we recorded within-site variations of these trends, especially among sampled transects and among plant species, suggesting that complex facilitation-competition interactions occur among plants, but that these vary among transects within the same sites. Pollinator effectiveness fluctuated but not in a clear linear way and was not directly linked to changes of flower visitation. This suggests that fluctuations of pollinator effectiveness might be due to destabilization of the pollinators by the removal of key resources. In addition, we detected a constrained flexibility of the foraging of flower visitors because they did not switch among flower shapes and they favoured specific plant traits such as high sugar content, small inflorescences and taller plants. Indeed, these constraints would eventually limit utilisation of new resources after perturbations, possibly undermining the stability of the system. In conclusion, our work demonstrates that the loss of dominant, highly generalist plants alters plant-pollinator interactions with implications for pollination and insect foraging. Therefore, in order to preserve ecosystems, generalist plants should not get lost, because they sustain the complex pattern of interactions between plants and flower visitors.