Pollen chemical and mechanical defences restrict host-plant use by bees.

Plants produce an array of chemical and mechanical defences that provide protection against many herbivores and pathogens. Putatively defensive compounds and structures can even occur in floral rewards: for example, the pollen of some plant taxa contains toxic compounds or possesses conspicuous spines. Yet little is known about whether pollen defences restrict host-plant use by bees. In other words, do bees, like other insect herbivores, tolerate the defences of their specific host plants while being harmed by non-host defences? To answer this question, we compared the effects of a chemical defence from Lupinus (Fabaceae) pollen and a putative mechanical defence (pollen spines) from Asteraceae pollen on larval survival of nine bee species in the tribe Osmiini (Megachilidae) varying in their pollen-host use. We found that both types of pollen defences reduce larval survival rate in some bee species. These detrimental effects were, however, mediated by host-plant associations, with bees being more tolerant of the pollen defences of their hosts, relative to the defences of plant taxa exploited by other species. This pattern strongly suggests that bees are adapted to the pollen defences of their hosts, and that host-plant use by bees is constrained by their ability to tolerate such defences.

Exposure to a fungicide for a field-realistic duration does not alter bumble bee fecal microbiota structure.

Social bees are frequently exposed to pesticides when foraging on nectar and pollen. Recent research has shown that pesticide exposure not only impacts social bee host health but can also alter the community structure of social bee gut microbiotas. However, most research on pesticide-bee gut microbiota interactions has been conducted in honey bees; bumble bees, native North American pollinators, have received less attention and, due to differences in their ecology, may be exposed to certain pesticides for shorter durations than honey bees. Here, we examine how exposure to the fungicide chlorothalonil for a short, field-realistic duration alters bumble bee fecal microbiotas (used as a proxy for gut microbiotas) and host performance. We expose small groups of Bombus impatiens workers (microcolonies) to field-realistic chlorothalonil concentrations for 5 days, track changes in fecal microbiotas during the exposure period and a recovery period, and compare microcolony offspring production between treatments at the end of the experiment. We also assess the use of fecal microbiotas as a gut microbiota proxy by comparing community structures of fecal and gut microbiotas. We find that chlorothalonil exposure for a short duration does not alter bumble bee fecal microbiota structure or affect microcolony production at any concentration but that fecal and gut microbiotas differ significantly in community structure. Our results show that, at least when exposure durations are brief and unaccompanied by other stressors, bumble bee microbiotas are resilient to fungicide exposure. Additionally, our work highlights the importance of sampling gut microbiotas directly, when possible.IMPORTANCEWith global pesticide use expected to increase in the coming decades, studies on how pesticides affect the health and performance of animals, including and perhaps especially pollinators, will be crucial to minimize negative environmental impacts of pesticides in agriculture. Here, we find no effect of exposure to chlorothalonil for a short, field-realistic period on bumble bee fecal microbiota community structure or microcolony production regardless of pesticide concentration. Our results can help inform pesticide use practices to minimize negative environmental impacts on the health and fitness of bumble bees, which are key native, commercial pollinators in North America. We also find that concurrently sampled bumble bee fecal and gut microbiotas contain similar microbes but differ from one another in community structure and consequently suggest that using fecal microbiotas as a proxy for gut microbiotas be done cautiously; this result contributes to our understanding of proxy use in gut microbiota research.

Changes in bumblebee queen gut microbiotas during and after overwintering diapause.

Bumblebees are key pollinators with gut microbiotas that support host health. After bumblebee queens undergo winter diapause, which occurs before spring colony establishment, their gut microbiotas are disturbed, but little is known about community dynamics during diapause itself. Queen gut microbiotas also help seed worker microbiotas, so it is important that they recover post-diapause to a typical community structure, a process that may be impeded by pesticide exposure. We examined how bumblebee queen gut microbiota community structure and metabolic potential shift during and after winter diapause, and whether post-diapause recovery is affected by pesticide exposure. To do so, we placed commercial Bombus impatiens queens into diapause, euthanizing them at 0, 2 and 4 months of diapause. Additionally, we allowed some queens to recover from diapause for 1 week before euthanasia, exposing half to the common herbicide glyphosate. Using whole-community, shotgun metagenomic sequencing, we found that core bee gut phylotypes dominated queen gut microbiotas before, during and after diapause, but that two phylotypes, Schmidhempelia and Snodgrassella, ceased to be detected during late diapause and recovery. Despite fluctuations in taxonomic community structure, metabolic potential remained constant through diapause and recovery. Also, glyphosate exposure did not affect post-diapause microbiota recovery. However, metagenomic assembly quality and our ability to detect microbial taxa and metabolic pathways declined alongside microbial abundance, which was substantially reduced during diapause. Our study offers new insights into how bumblebee queen gut microbiotas change taxonomically and functionally during a key life stage and provides guidance for future microbiota studies in diapausing bumblebees.

Do flower-colonizing microbes influence floral evolution? A test with fast-cycling Brassica.

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 earlier the better? Nesting timing and reproductive success in subalpine cavity-nesting bees.

Reproductive timing can affect an organism's production of offspring and its offspring's success, both of which contribute to its overall fitness. In seasonal environments, the timing of reproductive activity may be restricted to short periods of the year owing to numerous potential selective pressures such as variation in daylength, weather, food availability, predation or competition. We documented the relationships between reproductive timing and individual reproductive success (total reproductive output and offspring success) in subalpine populations of five cavity-nesting solitary bee species. We also examined the relationships between bee reproductive success and environmental variables that are likely ultimate drivers of bee phenology in subalpine environments (i.e. seasonality of floral resource abundance and temperature). Over 6 years, we recorded solitary bee nesting timing, egg production and offspring success using artificial nesting structures ('trap-nests') established at multiple study sites. We also quantified floral resources and recorded temperature throughout growing seasons. Bees nesting earlier in the season exhibited greater reproductive success. Reproductive output generally increased with floral abundance, although this relationship was weak and only significant for some bee species. Elevated temperatures were associated with increased nest construction rate, but not with greater reproductive output. These contrasting effects of temperature may have been driven by the negative relationship between temperature and bee longevity. Bees who nested for shorter durations of time (a proxy for longevity) produced fewer offspring, and individuals exhibiting the shortest nesting durations were also those that began nesting late in the season. Overall, bees who initiated nesting early and sustained activity for a long duration had the highest reproductive output. This work documents the relationship between reproductive phenology and fitness in wild insect populations and highlights the ways in which organisms can cope with the challenges of living in seasonal and highly variable environments.

Understanding pollen specialization in mason bees: a case study of six species.

Many bee species are dietary specialists and restrict their pollen foraging to a subset of the available flowers. However, the reasons for specialization-and the reasons certain plant taxa support numerous specialists-are often unclear. Many bees specialize on the plant family Asteraceae, despite evidence its pollen is a poor food for non-specialists. Here, we studied six mason bee (Osmia) species, including three Asteraceae specialists, to test whether observed pollen-usage patterns reflect larval nutritional requirements, to investigate what aspects of Asteraceae pollen make it unsuitable for non-specialists, and to understand how Asteraceae specialists tolerate their seemingly low-quality diet. We reared larval bees on host and nonhost pollen and found that Asteraceae specialists could develop on nonhost provisions, but that other bees could not survive on Asteraceae provisions. These effects did not seem related to nutritional deficiencies, since Asteraceae provisions were not amino acid deficient, and we found no consistent differences in digestive efficiency among pollen types. However, Asteraceae specialists completed more foraging flights per larva, generally collected relatively larger provisions, and produced more frass (waste) than the other species, suggesting quantitative compensation for low food quality. Toxins, deficiencies in unmeasured nutrients, or aspects of pollen grain structure might explain poor survival of non-specialists on Asteraceae provisions. Our results suggest that floral host selection by specialist bees is not related to optimizing larval nutrition. We recommend further investigation of host-selection behaviour in adult bees and of pollen digestion in larvae to better understand the evolution of bee-flower associations.

Defence compounds in pollen: why do they occur and how do they affect the ecology and evolution of bees?

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.

Spring wildflower phenology and pollinator activity respond similarly to climatic variation in an eastern hardwood forest.

Climate warming could disrupt species interactions if organisms' phenologies respond to climate change at different rates. Phenologies of plants and insects can be sensitive to temperature and timing of snowmelt; however, many important pollinators including ground-nesting bees have been little studied in this context. Without knowledge of the environmental cues affecting phenologies of co-occurring species, we have little ability to predict how species assemblages, and species interactions, will be affected by climate change. Here, we studied a hardwood forest understory over six years, to determine how spring temperatures, snowmelt timing, and photoperiod influence the phenology of two spring wildflowers (Anemone spp. and Trillium grandiflorum), activity of ground-nesting bees, and their temporal overlap. Surface degree-day accumulation was a better predictor of phenology for Anemone spp. (plant) and Nomada (bees) than were day of year (a proxy for photoperiod) or snowmelt date, whereas Trillium flowering appeared most sensitive to photoperiodic cues. Activity periods of Andrena and Lasioglossum bees were equally well described by degree-day accumulation and day of year. No taxon's phenology was best predicted by snowmelt date. Despite these differences among taxa in their phenological responses, timing of bee activity and flowering responded similarly to variation in snowmelt date and early spring temperatures. Furthermore, temporal overlap between flowering and bee activity was similar over the years of this study and was unaffected by variability in snowmelt date or temperature. Nevertheless, the differences among some taxa in their phenological responses suggests that diverging temporal shifts are a possibility for the future.

Two-Year Bee, or Not Two-Year Bee? How Voltinism Is Affected by Temperature and Season Length in a High-Elevation Solitary Bee.

Organisms must often make developmental decisions without complete information about future conditions. This uncertainty-for example, about the duration of conditions favorable for growth-can favor bet-hedging strategies. Here, we investigated the causes of life cycle variation in Osmia iridis, a bee exhibiting a possible bet-hedging strategy with co-occurring 1- and 2-year life cycles. One-year bees reach adulthood quickly but die if they fail to complete pupation before winter; 2-year bees adopt a low-risk, low-reward strategy of postponing pupation until the second summer. We reared larval bees in incubators in various experimental conditions and found that warmer-but not longer-summers and early birthdates increased the frequency of 1-year life cycles. Using in situ temperature measurements and developmental trajectories of laboratory- and field-reared bees, we estimated degree-days required to reach adulthood in a single year. Local long-term (1950-2015) climate records reveal that this heat requirement is met in only ∼7% of summers, suggesting that the observed distribution of life cycles is adaptive. Warming summers will likely decrease average generation times in these populations. Nevertheless, survival of bees attempting 1-year life cycles-particularly those developing from late-laid eggs-will be <100%; consequently, we expect the life cycle polymorphism to persist.

On the ecological significance of pollen color: a case study in American trout lily (Erythronium americanum).

Evolutionary ecologists seek to explain the processes that maintain variation within populations. In plants, petal color variation can affect pollinator visitation, environmental tolerance, and herbivore deterrence. Variation in sexual organs may similarly affect plant performance. Within-population variation in pollen color, as occurs in the eastern North American spring ephemeral Erythronium americanum, provides an excellent opportunity to investigate the maintenance of variation in this trait. Although the red/yellow pollen-color polymorphism of E. americanum is widely recognized, it has been poorly documented. Our goals were thus (1) to determine the geographic distribution of the color morphs and (2) to test the effects of pollen color on components of pollen performance. Data provided by citizen scientists indicated that populations range from monomorphic red, to polymorphic, to monomorphic yellow, but there was no detectable geographic pattern in morph distribution, suggesting morph occurrence cannot be explained by a broad-scale ecological cline. In field experiments, we found no effect of pollen color on the probability of predation by the pollen-feeding beetle Asclera ruficollis, on the ability of pollen to tolerate UV-B radiation, or on siring success (as measured by the fruit set of hand-pollinated flowers). Pollinators, however, exhibited site-specific pollen-color preferences, suggesting they may act as agents of selection on this trait, and, depending on the constancy of their preferences, could contribute to the maintenance of variation. Collectively, our results eliminate some hypothesized ecological effects of pollen color in E. americanum, and identify effects of pollen color on pollinator attraction as a promising direction for future investigation.

Explaining the apparent paradox of persistent selection for early flowering.

Decades of observation in natural plant populations have revealed pervasive phenotypic selection for early flowering onset. This consistent pattern seems at odds with life-history theory, which predicts stabilizing selection on age and size at reproduction. Why is selection for later flowering rare? Moreover, extensive evidence demonstrates that flowering time can and does evolve. What maintains ongoing directional selection for early flowering? Several non-mutually exclusive processes can help to reconcile the apparent paradox of selection for early flowering. We outline four: selection through other fitness components may counter observed fecundity selection for early flowering; asymmetry in the flowering-time-fitness function may make selection for later flowering hard to detect; flowering time and fitness may be condition-dependent; and selection on flowering duration is largely unaccounted for. In this Viewpoint, we develop these four mechanisms, and highlight areas where further study will improve our understanding of flowering-time evolution.

Direct benefits and indirect costs of warm temperatures for high-elevation populations of a solitary bee.

Warm temperatures are required for insect flight. Consequently, warming could benefit many high-latitude and high-altitude insects by increasing opportunities for foraging or oviposition. However, warming can also alter species interactions, including interactions with natural enemies, making the net effect of rising temperatures on population growth rate difficult to predict. We investigated the temperature-dependence of nesting activity and lifetime reproductive output over 3 yr in subalpine populations of a pollen-specialist bee, Osmia iridis. Rates of nest provisioning increased with ambient temperatures and with availability of floral resources, as expected. However, warmer conditions did not increase lifetime reproductive output. Lifetime offspring production was best explained by rates of brood parasitism (by the wasp Sapyga), which increased with temperature. Direct observations of bee and parasite activity suggest that although activity of both species is favored by warmer temperatures, bees can be active at lower ambient temperatures, while wasps are active only at higher temperatures. Thus, direct benefits to the bees of warmer temperatures were nullified by indirect costs associated with increased parasite activity. To date, most studies of climate-change effects on pollinators have focused on changing interactions between pollinators and their floral host-plants (i.e., bottom-up processes). Our results suggest that natural enemies (i.e., top-down forces) can play a key role in pollinator population regulation and should not be overlooked in forecasts of pollinator responses to climate change.

Asteraceae Pollen Provisions Protect Osmia Mason Bees (Hymenoptera: Megachilidae) from Brood Parasitism.

Many specialist herbivores eat foods that are apparently low quality. The compensatory benefits of a poor diet may include protection from natural enemies. Several bee lineages specialize on pollen of the plant family Asteraceae, which is known to be a poor-quality food. Here we tested the hypothesis that specialization on Asteraceae pollen protects bees from parasitism. We compared rates of brood parasitism by Sapyga wasps on Asteraceae-specialist, Fabeae-specialist, and other species of Osmia bees in the field over several years and sites and found that Asteraceae-specialist species were parasitized significantly less frequently than other species. We then tested the effect of Asteraceae pollen on parasites by raising Sapyga larvae on three pollen mixtures: Asteraceae, Fabeae, and generalist (a mix of primarily non-Asteraceae pollens). Survival of parasite larvae was significantly reduced on Asteraceae provisions. Our results suggest that specialization on low-quality pollen may evolve because it helps protect bees from natural enemies.

Plant size, sexual selection, and the evolution of protandry in dioecious plants.

It is frequently observed that males of dioecious plant species flower earlier in the season than females, although the generality of this pattern has not been quantified. One hypothesis for earlier male flowering is that females require more time for resource acquisition before reproduction; another is that selection for access to unfertilized ovules favors early-flowering males. Here I show that protandry is indeed the usual pattern in dioecious plants--males typically initiate flowering before females--and I propose a new hypothesis to explain this pattern. In many natural plant populations, individuals that begin flowering early are larger and--in the case of females or hermaphrodites--therefore more fecund. When this population-level seasonal decline in size is included in simulations of flowering time evolution in a dioecious plant, males evolve earlier flowering onset than females. Correlations between size (or condition) and reproductive phenology are widespread and likely contribute to the prevalence of protandry in both plants and animals, but their importance seems to have been overlooked by botanists. I suggest that sexual selection (specifically, male-male competition for access to high-quality ovules) may play a more important role in the evolution of flowering phenology than has previously been recognized.

Consequences of pollen defense compounds for pollinators and antagonists in a pollen-rewarding plant.

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.

Regional plant abundance explains patterns of host use by pollen-specialist bees in eastern North America.

Specialist insect herbivores make up a substantial fraction of Earth's biodiversity; however, they exploit a minority of plant lineages. For instance, in the eastern United States and Canada, ~25% of bee species are pollen specialists, but they are hosted by a small fraction of the native, animal-pollinated angiosperms in the region: Only 6% of plant genera and 3% of families support pollen-specialist bees. It is unclear why some plant lineages host specialist bees while others do not. We know that at least some specialist bees use plant taxa that are avoided by generalists, suggesting that specialist bees favor plants with low-quality pollen, potentially as a strategy to escape competition or obtain protection from natural enemies. There is also evidence that specialist bees prefer superabundant host plants. Here we investigate whether pollen quality and plant abundance predict patterns of host use by specialist bees in eastern North America. Through field observations, we find that plants hosting specialist bees are frequent sources of pollen for generalists, suggesting that their pollen is not generally avoided by bees due to poor pollen quality. In addition, our analysis of a large citizen-science data set shows that regional abundance strongly predicts which plant genera in the eastern United States host pollen-specialist bees. Our results show that bees specialize on regionally abundant-but not necessarily low-quality-plant lineages. These plant lineages may provide more opportunities for the evolution of specialists and lower likelihood of specialist extinction.

Pesticide-induced disturbances of bee gut microbiotas.

Social bee gut microbiotas play key roles in host health and performance. Worryingly, a growing body of literature shows that pesticide exposure can disturb these microbiotas. Most studies examine changes in taxonomic composition in Western honey bee (Apis mellifera) gut microbiotas caused by insecticide exposure. Core bee gut microbiota taxa shift in abundance after exposure but are rarely eliminated, with declines in Bifidobacteriales and Lactobacillus near melliventris abundance being the most common shifts. Pesticide concentration, exposure duration, season and concurrent stressors all influence whether and how bee gut microbiotas are disturbed. Also, the mechanism of disturbance-i.e. whether a pesticide directly affects microbial growth or indirectly affects the microbiota by altering host health-likely affects disturbance consistency. Despite growing interest in this topic, important questions remain unanswered. Specifically, metabolic shifts in bee gut microbiotas remain largely uninvestigated, as do effects of pesticide-disturbed gut microbiotas on bee host performance. Furthermore, few bee species have been studied other than A. mellifera, and few herbicides and fungicides have been examined. We call for these knowledge gaps to be addressed so that we may obtain a comprehensive picture of how pesticides alter bee gut microbiotas, and of the functional consequences of these changes.

Seasonal change in a pollinator community and the maintenance of style length variation in Mertensia fusiformis (Boraginaceae).

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.

Seasonality of floral resources in relation to bee activity in agroecosystems.

The contribution of wild insects to crop pollination is becoming increasingly important as global demand for crops dependent on animal pollination increases. If wild insect populations are to persist in agricultural landscapes, there must be sufficient resources over time and space. The temporal, within-season component of floral resource availability has rarely been investigated, despite growing recognition of its likely importance for pollinator populations. Here, we examined the visitation rates of common bee genera and the spatiotemporal availability of floral resources in agroecosystems over one season to determine whether local wild bee activity was limited by landscape floral resource abundance, and if so, whether it was limited by the present or past abundance of landscape floral resources. Visitation rates and landscape floral resources were measured in 27 agricultural sites in Ontario and Québec, Canada, across four time periods and three spatial scales. Floral resources were determined based on species-specific floral volume measurements, which we found to be highly correlated with published measurements of nectar sugar mass and pollen volume. Total floral volume at varying spatial scales predicted visits for commonly observed bee genera. We found Lasioglossum and Halictus visits were highest in landscapes that provided either a stable or increasing amount of floral resources over the season. Andrena visits were highest in landscapes with high floral resources at the start of the season, and Bombus visits appeared to be positively related to greater cumulative seasonal abundance of floral resources. These findings together suggest the importance of early-season floral resources to bees. Megachile visits were negatively associated with the present abundance of floral resources, perhaps reflecting pollinator movement or dilution. Our research provides insight into how seasonal fluctuations in floral resources affect bee activity and how life history traits of bee genera influence their responses to food availability within agroecosystems.