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Pollinators are thought to be the main drivers of floral evolution. Flowers are also colonized by abundant communities of microbes that can affect the interaction between plants and their pollinators. Very little is known, however, about how flower-colonizing microbes influence floral evolution. Here we performed a 6-generation experimental evolution study using fast-cycling Brassica rapa, in which we factorially manipulated the presence of pollinators and flower microbes to determine how pollinators and microbes interact in driving floral evolution. We measured the evolution of 6 morphological traits, as well as the plant mating system and flower attractiveness. Only one of the 6 traits (flower number) evolved in response to pollinators, while microbes did not drive the evolution of any trait, nor did they interact with pollinators in driving the evolution of morphological traits. Moreover, we did not find evidence that pollinators or microbes affected the evolution of flower attractiveness to pollinators. However, we found an interactive effect of pollinators and microbes on the evolution of autonomous selfing, a trait that is expected to evolve in response to pollinator limitations. Overall, we found only weak evidence that microbes mediate floral evolution. However, our ability to detect an interactive effect of pollinators and microbes might have been limited by weak pollinator-mediated selection in our experimental setting. Our results contrast with previous (similar) experimental evolution studies, highlighting the susceptibility of such experiments to drift and to experimental artefacts., (© The Author(s) 2024. Published by Oxford University Press on behalf of the European Society of Evolutionary Biology.)

Plants produce an array of defensive compounds with toxic or deterrent effects on insect herbivores. Pollen can contain relatively high concentrations of such defense compounds, but the causes and consequences of this enigmatic phenomenon remain mostly unknown. These compounds could potentially protect pollen against antagonists but could also reduce flower attractiveness to pollinators. We combined field observations of the pollen-rewarding Lupinus argenteus with chemical analysis and laboratory assays to test three hypotheses for the presence of pollen defense compounds: (1) these compounds are the result of spillover from adjacent tissues, (2) they protect against pollen thieves, and (3) they act as antimicrobial compounds. We also tested whether pollen defense compounds affect pollinator behavior. We found a positive relationship between alkaloid concentrations in pollen and petals, supporting the idea that pollen defense compounds partly originate from spillover. However, pollen and petals exhibited quantitatively (but not qualitatively) distinct alkaloid profiles, suggesting that plants can adjust pollen alkaloid composition independently from that of adjacent tissues. We found no relationship between pollen alkaloid concentration and the abundance of pollen thieves in Lupinus flowers. However, pollen alkaloids were negatively associated with bacterial abundance. Finally, plants with more alkaloids in their pollen received more pollinator visits, but these visits were shorter, resulting in no change in the overall number of flowers visited. We propose that pollen defense compounds are partly the result of spillover from other tissues, while they also play an antimicrobial role. The absence of negative effects of these compounds on pollinator visitation likely allows their maintenance in pollen at relatively high concentrations. Taken together, our results suggest that pollen alkaloids affect and are mediated by the interplay of multiple interactions., (© 2024 The Authors. Ecology published by Wiley Periodicals LLC on behalf of The Ecological Society of America.)

Background and Aims: In sub-alpine habitats, patchiness in snowpack produces marked, small-scale variation in flowering phenology. Plants in early- and late-melting patches are therefore likely to experience very different conditions during their flowering periods. Mertensia fusiformis is an early-flowering perennial that varies conspicuously in style length within and among populations. The hypothesis that style length represents an adaptation to local flowering time was tested. Specifically, it was hypothesized that lower air temperatures and higher frost risk would favour short-styled plants (with stigmas more shielded by corollas) in early-flowering patches, but that the pollen-collecting behaviour of flower visitors in late-flowering patches would favour long-styled plants., Methods: Floral morphology was measured, temperatures were monitored and pollinators were observed in several matched pairs of early and late populations. To evaluate effects of cold temperatures on plants of different style lengths, experimental pollinations were conducted during mornings (warm) and evenings (cool), and on flowers that either had or had not experienced a prior frost. The effectiveness of different pollinators was quantified as seed set following single visits to plants with relatively short or long styles., Key Results: Late-flowering populations experienced warmer temperatures than early-flowering populations and a different suite of pollinators. Nectar-foraging bumble-bee queens and male solitary bees predominated in early populations, whereas pollen-collecting female solitary bees were more numerous in later sites. Pollinators differed significantly in their abilities to transfer pollen to stigmas at different heights, in accordance with our prediction. However, temperature and frost sensitivity did not differ between long- and short-styled plants. Although plants in late-flowering patches tended to have longer styles than those in early patches, this difference was not consistent., Conclusions: Seasonal change in pollinator-mediated selection on style length may help maintain variation in this trait in M. fusiformis, but adaptation to local flowering time is not apparent. The prevalence of short styles in these populations requires further explanation.

Recently, the Yukon has seen a large growth in agricultural activity. Crops of commercial interest for local consumption and the export market include domestic berries, especially haskap (Lonicera caerulea L.). However, information on the pollination of these crops in our northern climate is lacking. To begin addressing this knowledge gap, we characterized foraging habits of on-farm bees in southwest Yukon by: 1) identifying pollen collected by bees occupying solitary bee houses; and 2) identifying and counting insect visitors to haskap flowers. Results show that cavity-nesting bees collect a wide variety of pollen including pollen from haskap, and that bumble bees (Bombus spp.) were much more common on haskap flowers than domestic honey bees (Apis mellifera L.), other bee species, syrphid flies, and butterflies. The number of bumble bees per haskap flower was also higher than reported elsewhere in Canada. The ability of bumble bees to be active in cool temperatures and the proximity of the study farms to natural ecosystems likely explain the prevalence of bumble bees in this study. In Yukon, it is still possible to support insect pollinators by maintaining natural areas among agricultural lands. Such undeveloped lands are, at present, typical of agricultural landscapes in subarctic Canada.

Identifying the resources that limit bee populations is essential for both bee conservation and pollination management in agroecosystems. Land‐use change typically leads to decreased habitat availability for wild pollinators, including loss of nesting habitat, which is an essential but often‐overlooked resource for wild bees. Cavity‐nesting bees, such as many Osmia spp. (Hymenoptera: Megachilidae), occupy holes in wood or reeds to build their nests; due to their nesting habits, they are frequently scarce in agricultural settings. Nevertheless, under the right circumstances, these bees can be ideal pollinators of apple and other orchard crops. Artificial nesting structures ('bee hotels' or 'trap‐nests') are often used to study cavity‐nesting bees and have been proposed as tools for bee conservation.To evaluate the effects of additional nesting habitat on the local abundance of pollinators, we selected 24 sites in apple orchards in eastern Canada in 2021 and 2022. Each site comprised two study plots: one in which we installed artificial nesting structures for cavity‐nesting bees, and one without added nesting habitat (control).Pollinator surveys were conducted in both plot types to measure pollinator visits to apple blossoms and, after apple bloom, to flowers in the undergrowth and/or in shrubs. Numbers and size of developing fruit were also recorded. Both Osmia bees and overall pollinator numbers were significantly higher in the treatment with trap‐nests (respectively, 33% and 22% higher during the 2021 growing season, and 113% and 30% higher during apple bloom across the 2 years of the study); however, there was no consistent difference in apple size or fruit set between the two treatments. Thus, trap‐nests locally increased the numbers of cavity‐nesting bees as well as total pollinator numbers, but they had little effect on apple yield, likely because apple production was not pollinator limited in the years of this study.Synthesis and applications. These findings suggest that bee populations in apple orchards can be limited by nesting resources such that addition of constructed nesting habitat could be an effective means of increasing pollinator numbers in orchards, and potentially, supporting bee conservation. [ABSTRACT FROM AUTHOR]

Summary: Pollen plays two important roles in angiosperm reproduction, serving as a vehicle for the plant's male gametes, but also, in many species, as a lure for pollen‐feeding animals. Despite being an important food source for many pollinators, pollen often contains compounds with known deterrent or toxic properties, as documented in a growing number of studies. Here we review these studies and discuss the role of pollen defensive compounds in the coevolutionary relationship between plants and bees, the preeminent consumers of pollen. Next, we evaluate three hypotheses that may explain the existence of defensive compounds in pollen. The pleiotropy hypothesis, which proposes that defensive compounds in pollen merely reflect physiological spillover from other plant tissues, is contradicted by evidence from several species. Although plants may experience selection to defend pollen against poor‐quality pollinators, we also find only partial support for the protection‐against‐pollen‐collection‐hypothesis. Finally, pollen defences might protect pollen from colonisation by antagonistic microorganisms (antimicrobial hypothesis), although data to evaluate this idea are scarce. Further research on the effects of pollen defensive compounds on pollinators, pollen thieves, and pollen‐colonising microbes will be needed to understand why many plants have chemically defended pollen, and the consequences of those defences for pollen consumers. [ABSTRACT FROM AUTHOR]

A diverse array of wild bee species may provide more effective pollination than the widely employed European honey bee (Apis mellifera L.). High species richness within crop pollinator assemblages has been linked to enhanced fruit and seed yields, but species richness is often confounded with abundance in studies of pollinator communities.We investigated the effects of bee diversity and species identity on pollen deposition and crop yield in the strawberry (Fragaria × ananassa) variety Jewel through a field experiment that independently manipulated the species richness and abundance of flower visitors. We used a new pollen deposition measurement technique to determine the pollen contribution of individual bees in an assemblage of flower visits. We compared the performance of wild bee species and managed honey bees, as pollinators of strawberry. We also calculated the influence of species richness, visit frequency and visitor identity on fruit mass, using the fruit that developed from each sampled flower.Species richness of flower visitors did not influence floral pollen loads or strawberry mass. Honey bees and wild bees deposited the same amount of pollen per visited flower. However, strawberries that developed from flowers visited by wild bees were heavier than flowers visited by honey bees. In addition, flowers visited by a combination of wild and honey bees produced strawberries that weighed less than flowers receiving purely WB visits.Synthesis and applications. Our findings show that honey bee pollination results in lower yields than wild bee pollination in a strawberry crop. Consequently, if managed honey bees in strawberry fields displace wild pollinators, growers may obtain suboptimal yields. Management efforts aimed at the maintenance or enhancement of wild pollinator populations may therefore be a cost‐effective way to increase both crop yield and biodiversity on strawberry farms. Our findings show that honey bee pollination results in lower yields than wild bee pollination in a strawberry crop. Consequently, if managed honey bees in strawberry fields displace wild pollinators, growers may obtain suboptimal yields. Management efforts aimed at the maintenance or enhancement of wild pollinator populations may therefore be a cost‐effective way to increase both crop yield and biodiversity on strawberry farms. [ABSTRACT FROM AUTHOR]

Climate change can affect plant - pollinator interactions in a variety of ways, but much of the research attention has focused on whether independent shifts in phenology will alter temporal overlap between plants and pollinators. Here I review the research on plant - pollinator mismatch, assessing the potential for observational and experimental approaches to address particular aspects of the problem. Recent, primarily observational studies suggest that phenologies of co-occurring plants and pollinators tend to respond similarly to environmental cues, but that nevertheless, certain pairs of interacting species are showing independent shifts in phenology. Only in a few cases, however, have these independent shifts been shown to affect population vital rates (specifically, seed production by plants) - but this largely reflects a lack of research. Compared to the few long-term studies of pollination in natural plant populations, experimental manipulations of phenology have yielded relatively optimistic conclusions about effects of phenological shifts on plant reproduction, and I discuss how issues of scale and frequency-dependence in pollinator behaviour affect the interpretation of these ' temporal transplant ' experiments. Comparable research on the impacts of mismatch on pollinator populations is so far lacking, but both observational studies and focused experiments have the potential to improve our forecasts of pollinator responses to changing phenologies. Finally, while there is now evidence that plant - pollinator mismatch can affect seed production by plants, it is still unclear whether this phenological impact will be the primary way in which climate change affects plant - pollinator interactions. It would be useful to test the direct effects of changing climate on pollinator population persistence, and to compare the importance of phenological mismatch with other threats to pollination. [ABSTRACT FROM AUTHOR]

One possible effect of climate change is the generation of a mismatch in the seasonal timing of interacting organisms, owing to species-specific shifts in phenology. Despite concerns that plants and pollinators might be at risk of such decoupling, there have been few attempts to test this hypothesis using detailed phenological data on insect emergence and flowering at the same localities. In particular, there are few data sets on pollinator flight seasons that are independent of flowering phenology, because pollinators are typically collected at flowers. To address this problem, we established standardized nesting habitat (trap nests) for solitary bees and wasps at sites along an elevational gradient in the Rocky Mountains, and monitored emergence during three growing seasons. We also recorded airtemperatures and flowering phenology at each site. Using a reciprocal transplant experiment with nesting bees, we confirmed that local environmental conditions are the primary determinants of emergence phenology. We were then able to develop phenology models to describe timing of pollinator emergence or flowering, across all sites and years, as a function of accumulated degree-days. Although phenology of both plants and insects is well described by thermal models, the best models for insects suggest generally higher threshold temperatures for development or diapause termination than those required for plants. In addition, degreeday requirements for most species, both plants and insects, were lower in locations with longer winters, indicating either a chilling or vernalization requirement that is more completely fulfilled at colder sites, or a critical photoperiod before which degree-day accumulation does not contribute to development. Overall, these results suggest that phenology of plants and trap-nesting bees and wasps is regulated in similar ways by temperature, but that plants are more likely than insects to advance phenology in response to springtime warming. We discuss the implications of these results for plants and pollinators, and suggest that phenological decoupling alone is unlikely to threaten population persistence for most species in our study area. [ABSTRACT FROM AUTHOR]

Climate change is expected to alter patterns of species co-occurrence, in both space and time. Species-specific shifts in reproductive phenology may alter the assemblages of plant species in flower at any given time during the growing season. Temporal overlap in the flowering periods (co-flowering) of animal-pollinated species may influence reproductive success if competitive or facilitative interactions between plant species affect pollinator services. We used a 33-year data set on flowering phenology in subalpine meadows in Colorado, USA, to determine whether interannual variation in snowmelt date, which marks the start of the growing season, affected co-flowering patterns. For two of four species considered, we found a significant relationship between snowmelt timing and composition of the assemblage of co-flowering plants. In years of early snowmelt, Lathyrus lanszwertii var. leucan thus (Fabaceae), the species we investigated in most detail, tended to overlap with earlier-flowering species and with fewer species overall. In particular, overlap with the flowering period of Lupinus polyphyllus var. prunophllus, with which Lathyrus leucan thus shares pollinators, was significantly reduced in early-snowmelt years. The observed association between timing of snowmelt and patterns of flowering overlap could not have been predicted simply by examining temporal trends in the dates of peak flowering of the dominant species in the community, as peak flowering dates have largely shifted in parallel with respect to snowmelt date. However, subtle interspecific differences in responsiveness of flowering time, duration, and intensity to interannual climate variation have likely contributed to the observed relationship. Although much of the year-to-year variation in flowering overlap remains unexplained by snowmelt date, our finding of a measurable signal of climate variation suggests that future climate change may lead to altered competitive environments for these wildflower species. [ABSTRACT FROM AUTHOR]

• Row configuration affects fruit yield in a strawberry crop with poor selfing ability. • Wild bees yield larger berries than honey bees in multi-variety strawberry fields. • The effectiveness of wild bees is likely due to their frequent between-row movement. The distance travelled by pollinators between successive flower visits can affect the quality of pollen transferred among plants. In cropping environments, especially monoculture systems, pollinators that travel between plants or rows may increase cross-pollination and consequently crop yield. However, the most commonly utilized crop pollinator, Apis mellifera L., tends to forage consecutively on nearest-neighbouring plants within rows. The level of cross-pollination can be further restricted in crops that are propagated by cloning. When a clonal variety is planted over large areas, the potential for outcrossed pollen deposition could be limited, regardless of pollinator flight distances. To investigate how pollinator movement and varietal diversity interact to affect crop pollination, we conducted an experiment with wild and honey bees in single- and multiple-variety strawberry fields. We hypothesized that the amount of cross-pollination provided by wild bees in multiple-variety strawberry fields would be greater than in single-variety fields, and greater than that provided by honey bees in either field type. We found that, indeed, flowers visited by wild bees produced larger strawberries than those visited by honey bees in multiple-variety plots, but only in the more self-incompatible of the two strawberry varieties tested. Strawberries resulting from honey bee pollination were of similar size regardless of the number of varieties planted in the field. Our results show that certain multiple-variety strawberry fields can benefit from the irregular foraging patterns of some solitary bee species, leading to increased cross-pollination and crop yield. Strawberry growers could take advantage of this effect by planting multiple varieties in close proximity and by supporting wild bee populations on farms. [ABSTRACT FROM AUTHOR]