Narcís Vicens

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.

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.

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.

Sex allocation in the solitary bee Osmia cornuta: do females behave in agreement with Fisher's theory?

Fishers theory of sex allocation predicts that, in a panmictic population, parental investment will be equally distributed between male and female progeny. Most studies on parental investment in nesting solitary bees and wasps use offspring or provision weight as estimators of parental investment and do not corroborate Fishers theory. The measurement of parental investment may be confounded by several factors. First, the use of offspring or provision size does not account for seasonal variation in foraging costs associated with aging of nesting females. Second, provision or offspring size do not reflect parental investment associated with nest construction. In this two-year study we measured parental investment in a solitary bee. We calculated sex allocation using both provision weight and foraging time as parental investment estimators. Investment in pollen-nectar provisions decreased, while investment in mud structures (nest construction) increased, as the nesting period progressed. Overall investment in provisions per nest was ∼25 times higher than investment in mud. Pollen-nectar foraging trips became longer as the season progressed, but mud trip duration did not vary. Due to weather differences between years, more offspring per female were produced in the first year, but progeny sex ratio and mean offspring size of both sexes were similar between years. Mortality did not differ between sexes. As predicted by Fishers theory, production cost ratios did not differ from 1 in either year, irrespective of the currency used to estimate parental investment (provision weight or foraging time). Our results strongly support Fishers theory.

Nest Site Orientation and Relocation of Populations of the Orchard Pollinator Osmia cornuta (Hymenoptera: Megachilidae)

Abstract Moving actively nesting populations of the orchard pollinator Osmia cornuta (Latreille) from a pollinated crop to one beginning to bloom could increase both the number of flowers pollinated and the number of bee progeny produced. This study investigates the possibility of moving actively nesting O. cornuta populations, and evaluates the effects of moving distance and nest site artificial visual landmarks on orientation and establishment of moved females. Nesting shelters with known numbers of nesting females were nocturnally relocated for distances ranging from 0 to 240 km, and the numbers of nesting females were counted after each move. Nearly all females continued to nest at shelters that were moved short (0–3 m) distances. Moves of intermediate distances (135–235 m) resulted in considerable (30–76%) bee loss (dispersal), and many females flew back to the original nesting site. At long distances (3–240 km), dispersal was higher (63–78%) and no bees were seen hovering at the original site. Moving distance was correlated positively with dispersal and with orientation (zigzagging) flights made by females when they first exited their nests after the moves. The addition or removal of supplementary visual landmarks (2 by 1.5-m banner with colored patterns) at the nesting shelter did not affect dispersal rates. Bees appeared to respond to supplementary landmarks but used other signals as primary orientation cues. An additional experiment showed that O. cornuta populations can be removed and stored at low temperature for up to 4 d (e.g., to avoid pesticide treatments) and then reinstalled at their nesting site, without affecting female orientation or nesting.

Biology and population structure of Osmia tricornis Latreille (Hym., Megachilidae)

Osmia tricornis Latreille is a univoltine species that in NE Spain flies from April to June. In 1990 and 1991 two populations were reared in the laboratory. Bees were incubated at 25–30 °C during the summer and overwintered in refrigerators at 3–7 °C for 112 and 167 days respectively. In spring bees were released in the field and provided with nesting materials with holes of different diameters. Bees that had been exposed to longer over‐wintering periods emerged sooner and expressed lower overwintering mortality than bees exposed to shorter over‐wintering periods. Females preferred to nest in holes of 7 mm i.d. The nesting season lasted from 2 to 2,5 months. O. tricornis is a highly polylectic species and collected pollen mainly from flowers producing little or no nectar (Quercus, Cistus, Papaver, Ranunculus, etc.). Several parasites and predators were found in the nests: Chaetodactylus osmiae (Dujardin), Monodontomerus obsoletus Fabricius, Anthrax anthrax Schrank, Trichodes alvearius Fabricius, Melittobia sp. Westwood, Cacoxenus sp. Loew, Sapyga quinquepunctata (Fabricius), Stelis phaeoptera (Kirby) and Ptinus sexpunctatus Panzer. In both years, populations obtained at the end of the flying season were larger than populations initially released. O. tricornis showed very similar biological features to those of other Osmia (s. str.) Panzer species currently being used as crop pollinators.

Seasonal dynamics in a cavity-nesting bee-wasp community: Shifts in composition, functional diversity and host-parasitoid network structure

Ecological communities are composed of species that interact with each other forming complex interaction networks. Although interaction networks have been usually treated as static entities, interactions show high levels of temporal variation, mainly due to temporal species turnover. Changes in taxonomic composition are likely to bring about changes in functional trait composition. Because functional traits influence the likelihood that two species interact, temporal changes in functional composition and structure may ultimately affect interaction network structure. Here, we study the seasonality (spring vs. summer) in a community of cavity-nesting solitary bees and wasps (‘hosts’) and their nest associates (‘parasitoids’). We analyze seasonal changes in taxonomic compostion and structure, as well as in functional traits, of the host and parasitoid communities. We also analyze whether these changes result in changes in percent parasitism and interaction network structure. Our host and parasitoid communities are strongly seasonal. Host species richness increases from spring to summer. This results in important seasonal changes in functional composition of the host community. The spring community (almost exclusively composed of bees) is characterized by large, univoltine, adult-wintering host species. The summer community (composed of both bees and wasps) is dominated by smaller, bivoltine, prepupa-wintering species. Host functional diversity is higher in summer than in spring. Importantly, these functional changes are not only explained by the addition of wasp species in summer. Functional changes in the parasitoid community are much less pronounced, probably due to the lower parasitoid species turnover. Despite these important taxonomic and functional changes, levels of parasitism did not change across seasons. Two network metrics (generality and interaction evenness) increased from spring to summer. These changes can be explained by the seasonal increase in species richness (and therefore network size). The seasonal shift from a bee-dominated community in spring to a wasp-dominated community in summer suggests a change in ecosystem function, with emphasis on pollination in spring to emphasis on predation in summer.