Across urban environments, vegetated habitats provide refuge for biodiversity. Gardens (designed for food crop production) and nurseries (designed for ornamental plant production) are both urban agricultural habitats characterized by high plant species richness but may vary in their ability to support wild pollinators, particularly bees. In gardens, pollinators are valued for crop production. In nurseries, ornamental plants rarely require pollination; thus, the potential of nurseries to support pollinators has not been examined. We asked how these habitats vary in their ability to support wild bees, and what habitat features relate to this variability. In 19 gardens and 11 nurseries in California, USA, we compared how local habitat and landscape features affected wild bee species abundance and richness. To assess local features, we estimated floral richness and measured ground cover as proxies for food and nesting resources, respectively. To assess landscape features, we measured impervious land cover surrounding each site. Our analyses showed that differences in floral richness, local habitat size, and the amount of urban land cover impacted garden wild bee species richness. In nurseries, floral richness and the proportion of native plant species impacted wild bee abundance and richness. We suggest management guidelines for supporting wild pollinators in both habitats.

A key conservation goal in agroecosystems is to understand how management practices may affect beneficial species, such as pollinators. Currently, broad gaps exist in our knowledge as to how horticultural management practices, such as irrigation level, might influence bee reproduction, particularly for solitary bees. Despite the extensive use of ornamental plants by bees, especially little is known about how irrigation level may interact with insecticides, like water-soluble neonicotinoids, to influence floral rewards and bee reproduction. We designed a two-factor field cage experiment in which we reared Megachile rotundata (Fabricius) (Hymenoptera: Megachilidae) on containerized ornamental plants grown under two different irrigation levels and imidacloprid treatments (30% label rate dosage of a nursery formulation or an untreated control). Lower irrigation was associated with modest decreases in nectar volume and floral abundance in untreated plants, whereas irrigation did not affect plants treated with imidacloprid. Furthermore, higher irrigation decreased the amount of imidacloprid entering nectar. Imidacloprid application strongly reduced bee foraging activity and reproduction, and higher irrigation did not offset any negative effects on bees. Our study emphasizes the impact of a nursery neonicotinoid formulation on solitary bee foraging and reproduction, while highlighting interactions between irrigation level and neonicotinoid application in containerized plants themselves.

1. An ongoing challenge in ecology is predicting how characteristics of communities correspond to habitat features. Examining variation in functional traits across species may reveal patterns not discernible from measures of mere abundance or richness. For beneficial insects like wild bees, functional trait-based approaches are often used to characterise communities in different agricultural habitats.2. However, no such approach has yet been applied in horticultural plant nurseries, which represent intensively managed artificial flowering plant assemblages. Certain nurseries mostly cultivate regionally native flowering plants, allowing one to test how differences between local plant assemblages may correlate with bee functional traits.3. We surveyed bee assemblages at native and conventional plant nurseries in southern California from spring through autumn over 2 years, while also documenting the native status of blooming plants in sampling plots. Bees were classified into different functional categories based on their diet breadth, nesting location, and social organisation.4. At native plant nurseries, we netted proportionally more specialist bee species and fewer generalist species than at conventional nurseries. Nesting location and social organisation of bee samples did not differ between nursery types. Meanwhile, landscape-level features were not associated with any observed functional trait metrics of bee collections. Furthermore, network-level specialisation of bee-plant interactions was higher at conventional nurseries.5. Our results suggest that a horticultural cultivation practice is quantifiably correlated with the functional distribution of resident bee assemblages. These results are important and encouraging to pollinator conservation efforts in nursery systems and other human-modified landscapes dominated by ornamental plants.

Citation: Cecala, J. M., and E. E. Wilson Rankin. 2020. Mark-recapture experiments reveal foraging behavior and plant fidelity of native bees in plant nurseries. Ecology 101(6):e03021.

Bees are economically critical pollinators, but are declining broadly due to several stressors, including nontarget exposure to insecticides and deficiencies in nutrition. Understanding the simultaneous impact of stressors, particularly interactions between them, is critical to effectively conserving bees. Although behavioral effects of pesticides like neonicotinoids have received some attention in solitary bees, our understanding of how they are modulated by diet quality is limited. Furthermore, scarce data exist on what concentrations of orally ingested neonicotinoids elicit mortality in solitary bees. In a controlled exposure laboratory experiment, we investigated how diet quality, as sugar concentration, and chronic oral exposure to imidacloprid affect adult alfalfa leafcutting bees, Megachile rotundata (Fabricius). We provided individuals ad libitum with either 20 or 50% (m/m) sucrose syrups containing either 0, 30, or 300 ppb imidacloprid (measuring 0, 27, and 209 ppb via an ELISA assay). Over 5 wk, we tracked behavior and survivorship of individuals. Imidacloprid decreased survivorship in a dose-dependent fashion, but sucrose content did not affect survivorship, even in bees not fed imidacloprid. In the high imidacloprid treatment, 45% of bees were observed in a motionless supine position while still alive, with this effect appearing to be buffered against by the higher sucrose diet. Our results suggest that diets higher in sugar concentration may prevent an intermediate stage of poisoning, but do not ultimately extend longevity. In devising risk assessments for bees, it is important to consider that interactions between stressors may occur in the stages leading up to death even if survivorship is unaffected.

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1. Measurements of pollinator performance are crucial to pollination studies, enabling researchers to quantify the relative value of different pollinator species to plant reproduction. One of the most widely employed measures of pollinator performance is single‐visit pollen deposition, the number of conspecific pollen grains deposited to a stigma after one pollinator visit. To ensure a pollen‐free stigma, experimenters must first bag flowers before exposing them to a pollinator. 2. Bagging flowers, however, may unintentionally manipulate floral characteristics to which pollinators respond. In this study, we quantified the effect of bagging on nectar volume in watermelon (Citrullus lanatus) flowers, and how this affects pollinator performance and behaviour. 3. Experimental bagging resulted in roughly 30‐fold increases in nectar volume relative to unmanipulated, open‐pollinated field flowers after only a few hours. Honey bees, but not native bees, consistently displayed elevated handling times and single‐visit pollen deposition on unmanipulated bagged flowers relative to those from which we removed nectar to mimic volumes in open‐pollinated flowers. 4. Furthermore, we identify specific bee foraging behaviours during a floral visit that account for differences in pollen deposition, and how these differ between honey bees and native bees. 5. Our findings suggest that experimental bagging of flowers, without accounting for artificially accumulated nectar, can lead to biased estimates of pollinator performance in pollinator taxa that respond strongly to nectar volume. We advise that pollination studies be attentive to nectar secretion dynamics in their focal plant species to ensure unbiased estimates of pollinator performance across multiple pollinator species.

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