Jordi Bosch

Developing and establishing bee species as crop pollinators: the example of Osmia spp. (Hymenoptera: Megachilidae) and fruit trees

The development of a bee species as a new crop pollinator starts with the identification of a pollination-limited crop production deficit and the selection of one or more candidate pollinator species. The process continues with a series of studies on the developmental biology, pollinating efficacy, nesting behaviour, preference for different nesting substrates, and population dynamics of the candidate pollinator. Parallel studies investigate the biology of parasites, predators and pathogens. The information gained in these studies is combined with information on the reproductive biology of the crop to design a management system. Complete management systems should provide guidelines on rearing and releasing methods, bee densities required for adequate pollination, nesting materials, and control against parasites, predators and pathogens. Management systems should also provide methods to ensure a reliable pollinator supply. Pilot tests on a commercial scale are then conducted to test and eventually refine the management system. The process culminates with the delivery of a viable system to manage and sustain the new pollinator on a commercial scale. The process is illustrated by the development of three mason bees, Osmia cornifrons (Radoszkowski), O. lignaria Say and O. cornuta (Latreille) as orchard pollinators in Japan, the USA and Europe, respectively.

Plant-pollinator networks: adding the pollinator's perspective.

Pollination network studies are based on pollinator surveys conducted on focal plants. This plant-centred approach provides insufficient information on flower visitation habits of rare pollinator species, which are the majority in pollinator communities. As a result, pollination networks contain very high proportions of pollinator species linked to a single plant species (extreme specialists), a pattern that contrasts with the widely accepted view that plant-pollinator interactions are mostly generalized. In this study of a Mediterranean scrubland community in NE Spain we supplement data from an intensive field survey with the analysis of pollen loads carried by pollinators. We observed 4265 contacts corresponding to 19 plant and 122 pollinator species. The addition of pollen data unveiled a very significant number of interactions, resulting in important network structural changes. Connectance increased 1.43-fold, mean plant connectivity went from 18.5 to 26.4, and mean pollinator connectivity from 2.9 to 4.1. Extreme specialist pollinator species decreased 0.6-fold, suggesting that ecological specialization is often overestimated in plant-pollinator networks. We expected a greater connectivity increase in rare species, and consequently a decrease in the level of asymmetric specialization. However, new links preferentially attached to already highly connected nodes and, as a result, both nestedness and centralization increased. The addition of pollen data revealed the existence of four clearly defined modules that were not apparent when only field survey data were used. Three of these modules had a strong phenological component. In comparison to other pollination webs, our network had a high proportion of connector links and species. That is, although significant, the four modules were far from isolated.

Weather-Dependent Pollinator Activity in an Apple Orchard, with Special Reference to Osmia cornuta and Apis mellifera (Hymenoptera: Megachilidae and Apidae)

Abstract The foraging activity of pollinator insects in relation to weather factors (ambient temperature, solar radiation, relative humidity, and wind speed) was studied in an apple orchard with special reference to two managed bee species, Osmia cornuta (Latreille) and Apis mellifera L. Over the range of observed weather values, A. mellifera activity was significantly dependent on temperature, solar radiation, and wind speed; O. cornuta activity was dependent on solar radiation and wind speed. These results were confirmed through video recordings at one O. cornuta nesting shelter and one A. mellifera hive. For both species, daily activity started at lower temperatures than it ceased, whereas solar radiation did not differ between these two events. In general, O. cornuta was active from 10 to 12°C and 200 w/m2, and A. mellifera from 12 to 14°C and 300 w/m2. O. cornuta was the only bee species seen visiting apple flowers under strong wind or light rain. Because of its greater tolerance to inclement weather, O. cornuta pollinated apple flowers for longer periods (both daily and seasonally) than other flower visitors.

Flowering phenology, floral traits and pollinator composition in a herbaceous Mediterranean plant community

Abstract The relationships between flowering plants and their insect visitors were studied in a Mediterranean grassland in north-east Spain. Floral traits (size, shape, symmetry, and colour), floral rewards (pollen and nectar), flowering period, and floral visitors were recorded for the 17 most abundant plants in the community. Flowering was year-round, but most species flowered in spring. The three species that flowered after spring had small flowers, but the distribution of floral features (including rewards offered) did not show a strong seasonality. Ants contributed 58.5% to the flower visits recorded. Other frequent visitors were beetles (12%), flies (9.5%), honey bees (6.4%), wild bees (6.4%), and wasps (5.2%). Honey bees were most abundant in April, wild bees from April to July, beetles from May to July, and ants from May to September. The lack of tight plant-insect associations was the rule, with most plant species visited by a rather diverse array of insects representing two or more orders. The plant species having narrower spectra of visitors either had flower rewards exposed or attracted mostly illegitimate visitors. By means of correspondence analysis four categories of plants were defined according to their main groups of visitors: (1) honey bees and large wild bees; (2) large wild bees; (3) ants and beetles; and (4) beetles and small-sized bees. The Mantel test was used to calculate correlations among four matrices representing similarities in visitors attracted, floral morphological traits, pollen-nectar rewards, and blooming time, respectively. In spite of seasonality shown by the different insect groups, results indicate that the observed patterns of visitor distribution among plants were most affected by pollen-nectar rewards.

A geographic selection mosaic in a generalized plant–pollinator–herbivore system

The concept of Selection Mosaic is central to the Geographic Mosaic Theory of Coevolution. Most information on coevolving interactions, however, comes from specialized organisms. In contrast, an accurate understanding of the effect of geographically varying evolutionary dynamics on the evolution of generalist organisms is lacking, although these kinds of organisms are the most frequent in nature. In flowering plants, pollinators and herbivores are important selective agents. In this study we investigate whether a geographic selection mosaic for floral traits in a generalist plant, Erysimum mediohispanicum (Brassicaceae), can be mediated by the interplay of mutualistic and antagonistic interacting organisms. In eight populations we quantified the selection exerted by these organisms on several plant traits. We found significant spatial variation in pollinator assemblage. In different populations, the main pollinators belonged to different functional groups (beeflies, large bees, small bees, and beetles). Damage by ungulates also varied among populations. Consequently, we found that different populations were under different selective regimes, and the traits affected by selection depended on the local interaction intensity with pollinators and mammal herbivores. Some traits, such as flower number and stalk height, were selected similarly in most populations. Other traits, such as corolla diameter and tube length, were selected only in some populations. Finally, we found divergent selection for some traits, such as corolla tube width and corolla shape, which were selected in contrasting directions in different localities. This spatial variation in selective scenarios results in populations with strong selective regimes (hot spots) intermingled with populations with weak selective regimes (cold spots). Four important outcomes emerge from the E. mediohispanicum selection mosaic. (1) Interactions with generalist organisms may produce strong selection. (2) Spatial changes in main pollinators result in divergent selection across populations. (3) Geographic mosaics depend on a balance between mutualistic and antagonistic selection. (4) Selection mosaics operate at fairly small spatial scales. These findings will surely contribute to expanding the conceptual framework of the Geographic Mosaic Theory of Coevolution.

Pollinating Efficacy of Osmia cornuta and Apis mellifera (Hymenoptera: Megachilidae, Apidae) on ‘Red Delicious’ Apple

Abstract The foraging behavior and pollinating efficacy of Osmia cornuta (Latreille) and Apis mellifera L. were studied in an orchard of ‘Delicious’ apple, Malus domestica Borkh, in northeastern Spain. Yields after one single visit were more than five times higher in flowers visited by O. cornuta than in those visited by A. mellifera nectar gatherers. This is attributed to the lower rate of stigma contact in A. mellifera visits, rather than to insufficient deposition of compatible pollen when the stigmas are contacted. A. mellifera pollen collectors had very high rates of stigma contact, but they were very scarce (3%) on ‘Delicious’ flowers despite the presence of abundant brood in their hives. One single visit per flower by O. cornuta produced commercial fruit set (27.4%) and fruit size (>70 mm diameter). Based on cell production, average number of trips required to provision a male and a female cell, and flower visiting rates, it is estimated that a mean of 22,252 apple flower visits per female O. cornuta were made during the 15-d flowering period. This result indicates that 530 nesting O. cornuta females per hectare are enough to provide adequate apple pollination.

Pollinator diversity affects plant reproduction and recruitment: the tradeoffs of generalization

One outstanding and unsolved challenge in ecology and conservation biology is to understand how pollinator diversity affects plant performance. Here, we provide evidence of the functional role of pollination diversity in a plant species, Erysimum mediohispanicum (Brassicaceae). Pollinator abundance, richness and diversity as well as plant reproduction and recruitment were determined in eight plant populations. We found that E. mediohispanicum was generalized both at the regional and local (population) scale, since its flowers were visited by more than 100 species of insects with very different morphology, size and behaviour. However, populations differed in the degree of generalization. Generalization correlated with pollinator abundance and plant population size, but not with habitat, ungulate damage intensity, altitude or spatial location. More importantly, the degree of generalization had significant consequences for plant reproduction and recruitment. Plants from populations with intermediate generalization produced more seeds than plants from populations with low or high degrees of generalization. These differences were not the result of differences in number of flowers produced per plant. In addition, seedling emergence in a common garden was highest in plants from populations with intermediate degree of generalization. This outcome suggests the existence of an optimal level of generalizations even for generalized plant species.

Body size as an estimator of production costs in a solitary bee

Abstract 1. Body weight is often used as an estimator of production costs in aculeate Hymenoptera; however, due to differences between sexes in metabolic rates and water content, conversion of provision weight to body weight may differ between males and females. As a result, the cost of producing female progeny may often have been overestimated.

Development and Emergence of the Orchard Pollinator Osmia lignaria (Hymenoptera: Megachilidae)

Abstract The solitary bee Osmia lignaria Say has been developed as an orchard pollinator in the western United States. Immatures develop through the spring and summer. By late summer, bees become adults and remain in this stage inside their cocoons throughout the winter. In this study, we reared O. lignaria at various temperature regimes in the laboratory and outdoors. Developmental rates increased with temperature: bees reared at 18°C took >120 d to complete development, whereas bees reared at 29°C took half that long. Bees reared outdoors under fluctuating ambient conditions took ≈95 d. At 18°C, some bees were unable to complete prepupal dormancy. Different developmental stages responded differently to the various temperature regimes. Fluctuating temperatures averaging 22°C significantly shortened the dormant prepupal stage, and, as a result, bees developed faster than at the equivalent constant temperatures. Bees that developed faster (29°C and fluctuating temperatures) could be wintered as early as August and incubated for emergence in March, 1 mo ahead of bees exposed to natural conditions. These results can be applied to field populations for pollination of early-blooming crops such as almonds, Prunus amygdalus Batsch.

Bee population returns and cherry yields in an orchard pollinated with Osmia lignaria (Hymenoptera: Megachilidae)

During 1998-2003, we used populations of the solitary bee Osmia lignaria Say to pollinate a commercial sweet cherry orchard in northern Utah. Bee densities released each year ranged from 1290 to 1857 females/ha, with approximately twice as many males. Female progeny produced each year were greater than parental populations released, except in 2003, when nesting was poor due to bird predation. Despite poor weather during bloom, and in contrast to most other local producers, the study orchard produced harvestable crops in 1999 (2,964 kg/ha) and 2001 (3,154 kg/ha). In 1998 and 2000, record yields were obtained (10,625 and 12,096 kg/ha, respectively). Including only those years with harvestable crops, average production was 2.2 times higher in 1998-2003 (when O. lignaria populations were used) compared with 1992-1997 (when 10 Apis mellifera hives were used). This is the first study reporting multiyear cherry yields in an orchard pollinated with O. lignaria in North America.