Solitary Bees Research Articles

Single and combined exposure to 'bee safe' pesticides alter behaviour and offspring production in a ground-nesting solitary bee (Xenoglossa pruinosa).

Mounting evidence supporting the negative impacts of exposure to neonicotinoids on bees has prompted the registration of novel 'bee-friendly' insecticides for agricultural use. Flupyradifurone (FPF) is a butenolide insecticide that shares the same mode of action as neonicotinoids and has been assessed to be 'practically non-toxic to adult honeybees' using current risk assessment procedures. However, these assessments overlook some routes of exposure specific to wild bees, such as contact with residues in soil for ground-nesters. Co-exposure with other pesticides may also lead to detrimental synergistic effects. In a fully crossed experiment, we assessed the possible lethal and sublethal effects of chronic exposure to two pesticides used on Cucurbita crops, the insecticide Sivanto Prime (FPF) and the fungicide Quadris Top (azoxystrobin and difenoconazole), alone or combined, on solitary ground-nesting squash bees (Xenoglossa pruinosa). Squash bees exposed to Quadris Top collected less pollen per flower visit, while Sivanto-exposed bees produced larger offspring. Pesticide co-exposure induced hyperactivity in female squash bees relative to both the control and single pesticide exposure, and reduced the number of emerging offspring per nest compared to individual pesticide treatments. This study demonstrates that 'low-toxicity' pesticides can adversely affect squash bees under field-realistic exposure, alone or in combination.

The Efficacy of Social Media Communication in Engaging Citizen Scientists: Insights From the Jozi Bee Hotel Project

The Jozi Bee Hotel Project recruited Johannesburg residents to help gather data on solitary bee abundance. They were motivated and guided by a strategic communications campaign. This study explores social media’s impact on the citizen scientists’ project involvement, particularly their interaction with visual elements of the communications campaign. Analysis of engagement metrics reveals that social media played a crucial role in sustaining volunteer participation. This demonstrates that a well-executed, ongoing social media campaign has the power to maintain participant engagement throughout the duration of a research project. It suggests that a consistent, long-term social media strategy can drive citizen science participation.

Impact Of Beekeeping On The Wild Bee Diversity In Northern Cote D'Ivoire (West Africa)

In Cote d’Ivoire, beekeeping takes an important place among the major economic activities. It contributes to improve the population livelihoods thanks to its derived products which are honey, propolis, wax and royal jelly. However, the installation of honey bee hives could put pressure on wild bee species which are live often alone (solitary bees). However, these wild bees are excellent pollinators of cultivated and wild plants. Our study aimed to assess the effect of honey bee hives on the diversity of wild bee. The methodology approach consisted to capture bees in three different plots within the forest fragment of the university Peleforo Gon Coulibaly. The first plot was located near the apiary containing ten hives. The two other plots, far to the apiary, were located to 200 m and 400 m from the first plot, respectively. Bees were captured once a month for three months using pan traps (UV - blue, yellow and white). A total of 17 bee species belonging to three families (Apidae, Megachilidae and Halictidae) were identified. The furthest plot from the apiary was the most diverse (H’ = 2.49) and that near the apiary was the least diverse (H’ = 0.11). Only, two wild bee species, Hypotrigona sp and Thrinchostoma petersi, persisted inside the plot near the apiary where honey bee was most abundant. The furthest plots from the apiary seems to have the highest diversity of wild bees. These findings are very relevant because they can play an essential role in the policies of wild bee communities’ conservation and in the beekeeping activities management in our regions.

Bionomical observations of Small Carpenter Bee, Ceratina smaragdula Fibricius (Hymenoptera: Apidae)

The small carpenter bee, Ceratina smaragdula Fibricius is a non-Apis pollinator of many economically important crops. The nesting biology, nesting preferences, architects, foraging, hibernation, and mating behaviour of this solitary bee were examined in this study. Fifteen nests of C. smaragdula were collected and examined for the targeted attributes. C. smaragdula highly preferred (>80%) the area of field sides in Ravenna grass (Saccharum ravennae) for making their nests, at a preferred height of 82.74 cm from the soil surface. The nesting of C. smaragdula along roadsides and houses was 30-60% and <30%, respectively. C. smaragdula hibernated during early October to February, broke hibernation in the first week of March and initiated mating immediately. The main reproductive period was from March to August and the first generation emerged from late April to early May. The peak insect population was recorded from June to August. The mean nest length, nest diameter, and nest entrance diameter were 11.15 cm, 0.79 cm and 0.53 mm, respectively. This bee preferred a single gallery, having 3-5 cells per gallery. The gallery contents are eggs, larvae, pupae and adults. The generation development period was from 4-5 weeks and 3-4 generations per year were existed. C. smaragdula started foraging early in the morning (7.27 am) and ceased in the afternoon at (4.51 pm). The average time of foraging per flower was 21.67 seconds, and the bees returned to the nest after 7.48 min of their first forage. These findings could be helpful in designing artificial nesting required for the preservation of C. smaragdula.

Home-and-away comparisons of life history traits indicate enemy release and founder effects of the solitary bee, Megachile sculpturalis

Occurrences of introduced and invasive pollinators are increasing worldwide. To predict the potential impacts of exotic bees on native ecosystems we need to understand their ecological interactions. Life history traits are fundamental for understanding ecological interactions and often help to explain the spread of exotic species. We conducted home-and-away comparisons of life history traits of the first invasive bee in Europe, the sculptured resin bee Megachile sculpturalis Smith 1853. We compiled information on nest architecture, offspring, natural enemies, body size and phenology using published literature, museum specimen, data from citizen science initiatives, field observations and reared specimen. Megachile sculpturalis uses a broad variety of nesting materials for brood cell construction, including even plastic at the exotic range. Body size at warm temperate climate was similar in the native and exotic ranges, but phenology shifted forward by about one month (28.9 ± 3.3 SE days) in the exotic ranges. The abundance of natural enemies was similar between native and exotic ranges but specialist enemies were missing in the exotic ranges. These trait shifts may be explained by founder effects or ecological filtering. The comparison of life history traits in native and exotic ranges sheds light on the ecological-evolutionary process of this quickly spreading species and provides a better understanding of invasion processes in solitary bees.

Pesticide hazard, floral resource availability and natural enemies interactively drive the fitness of bee species depending on their crop fidelity

Pesticides, loss of floral resources and parasitism are considered drivers of wild bee decline in agroecosystems, but their interactions are poorly understood under field conditions. Negative impacts are predicted to vary across species of distinct crop fidelity because of the associated consequences for pesticide exposure and dependence on wild flowers. We experimentally established populations of two solitary bee species in apple orchards with distinct pesticide hazard and flower availability in surrounding landscapes. The number of nesting females, their reproduction and offspring survival and parasitism rate were tracked throughout apple bloom. We show that in the solitary bee Osmia cornuta, which showed a strong preference to forage in apple orchards, the number of females remaining at nesting sites towards the end of apple bloom and the overall reproductive success of the released bees declined with pesticide hazard. In contrast, the closely related Osmia bicornis, which showed a lower preference for apple, was not affected by pesticide use in orchards. Floral resource distribution furthermore modulated parasitism rates in both species: while large amounts of locally available resources reduced offspring parasitism, it increased with relatively higher food availability in the landscapes surrounding focal orchards, probably due to the bees foraging on a larger scale and thus longer periods during which nests remained unattended. Our findings provide novel insights into the complex interplay of pesticide exposure, flower availability and parasitism risk driving population dynamics of bees in agroecosystems. They show that in particular the most reliable crop pollinator species are especially harmed by high pesticide use in crops. Our study underpins the key role of reducing pesticide risks for pollinators through effective regulation of potentially hazardous agrochemicals and the promotion of alternative pest regulation approaches along with conserving and restoring flower-rich habitats to sustain pollinator populations in agroecosystems.

Heatwaves increase larval mortality and delay development of a solitary bee

Abstract Heatwaves are expected to increase in frequency, intensity and duration due to climate change. For organisms like insects with discrete development, sensitivity may differ among life stages. Thermal sensitivity is of particular concern for species like bees that provide critical ecosystem services. Although social bees moderate nest temperatures through worker behaviour, solitary bees do not thermoregulate their nests, making immobile developing offspring especially vulnerable to such extreme events. We studied the effects of heatwaves on larval development in the solitary bee, Osmia lignaria, an important orchard pollinator and model species for solitary bee biology. We used a factorial design to assess the impacts of heatwave temperature and duration on larval mortality and development rate. Larvae were exposed to heatwaves under realistic diel temperature regimes, with daytime maxima of 31 or 37°C for 4 or 7 days at the beginning of development. Heatwave temperature strongly affected larval mortality. Exposure to 37°C heatwaves increased larval mortality by 130%, but the cooler 31°C heatwaves did not significantly impact mortality. Heatwave duration did not impact larval mortality. Larval development time also was affected by heatwave exposure. Compared with the no‐heatwave‐control, bees in the 31°C heatwave developed faster, and bees in the 37°C heatwave developed slower. Our study reveals the importance of stage‐specific effects of extreme events and suggests that the timing and maximum temperature of projected heatwaves may be more detrimental to populations than heatwave duration.

Integrating data from different taxonomic resolutions to better estimate community alpha diversity

Integrated distribution models (IDMs), in which datasets with different properties are analysed together, are becoming widely used to model species distributions and abundance in space and time. To date, the IDM literature has focused on technical and statistical issues, such as the precision of parameter estimates and mitigation of biases arising from unstructured data sources. However, IDMs have an unrealised potential to estimate ecological properties that could not be properly derived from the source datasets if analysed separately. We present a model that estimates community alpha diversity metrics by integrating one species‐level dataset of presence–absence records with a co‐located dataset of group‐level counts (i.e. lacking information about species identity). We illustrate the ability of community IDMs to capture the true alpha diversity through simulation studies and apply the model to data from the UK Pollinator Monitoring Scheme, to describe spatial variation in the diversity of solitary bees, bumblebees and hoverflies. The simulation and case studies showed that the proposed IDM produced more precise estimates of the community diversity than the single models, and the analysis of the real dataset further showed that the alpha diversity estimates from the IDM were averages of the single models. Our findings also revealed that IDMs had a higher prediction accuracy for all the insect groups in most cases, with this performance linked to the information provided by a data source into the IDM.

Fungicide ingestion reduces net energy gain and microbiome diversity of the solitary mason bee

Fungicides are frequently used during tree fruit bloom and can threaten insect pollinators. However, little is known about how non-honey bee pollinators such as the solitary bee, Osmia cornifrons, respond to contact and systemic fungicides commonly used in apple production during bloom. This knowledge gap limits regulatory decisions that determine safe concentrations and timing for fungicide spraying. We evaluated the effects of two contact fungicides (captan and mancozeb) and four translaminar/plant systemic fungicides (cyprodinil, myclobutanil, penthiopyrad, and trifloxystrobin) on larval weight gain, survival, sex ratio, and bacterial diversity. This assessment was carried out using chronic oral ingestion bioassays where pollen provisions were treated with three doses based on the currently recommended field use dose (1X), half dose (0.5X), and low dose (0.1X). Mancozeb and penthiopyrad significantly reduced larval weight and survival at all doses. We then sequenced the 16S gene to characterize the larvae bacteriome of mancozeb, the fungicide that caused the highest mortality. We found that larvae fed on mancozeb-treated pollen carried significantly lower bacterial diversity and abundance. Our laboratory results suggest that some of these fungicides can be particularly harmful to the health of O. cornifrons when sprayed during bloom. This information is relevant for future management decisions about the sustainable use of fruit tree crop protection products and informing regulatory processes that aim to protect pollinators.

Body mass, temperature, and pathogen intensity differentially affect critical thermal maxima and their population-level variation in a solitary bee.

Climate change presents a major threat to species distribution and persistence. Understanding what abiotic or biotic factors influence the thermal tolerances of natural populations is critical to assessing their vulnerability under rapidly changing thermal regimes. This study evaluates how body mass, local climate, and pathogen intensity influence heat tolerance and its population-level variation (SD) among individuals of the solitary bee Xenoglossa pruinosa. We assess the sex-specific relationships between these factors and heat tolerance given the differences in size between sexes and the ground-nesting behavior of the females. We collected X. pruinosa individuals from 14 sites across Pennsylvania, USA, that varied in mean temperature, precipitation, and soil texture. We measured the critical thermal maxima (CTmax) of X. pruinosa individuals as our proxy for heat tolerance and used quantitative PCR to determine relative intensities of three parasite groups-trypanosomes, Spiroplasma apis (mollicute bacteria), and Vairimorpha apis (microsporidian). While there was no difference in CTmax between the sexes, we found that CTmax increased significantly with body mass and that this relationship was stronger for males than for females. Air temperature, precipitation, and soil texture did not predict mean CTmax for either sex. However, population-level variation in CTmax was strongly and negatively correlated with air temperature, which suggests that temperature is acting as an environmental filter. Of the parasites screened, only trypanosome intensity correlated with heat tolerance. Specifically, trypanosome intensity negatively correlated with the CTmax of female X. pruinosa but not males. Our results highlight the importance of considering size, sex, and infection status when evaluating thermal tolerance traits. Importantly, this study reveals the need to evaluate trends in the variation of heat tolerance within and between populations and consider implications of reduced variation in heat tolerance for the persistence of ectotherms in future climate conditions.

Optimizing coffee production: Increased floral visitation and bean quality at plantation edges with wild pollinators and natural vegetation

Abstract Animal pollination is important for more than 75% of agricultural crops, including coffee, whose productivity can increase with adequate pollination. Bees, including many solitary species, are diverse pollinators, with around 85% of them considered more effective than honeybees in pollen transfer. We assessed the coffee plantation and its surrounding vegetation for solitary bee nesting throughout the coffee flowering season and measured their impact on coffee productivity. We installed collection stations with trap nests inside a coffee plantation, on the border and inside the native vegetation in a farm in Diamantina, MG, Brazil. We used 10 weekly monitored replicates at least 1 km apart. We evaluated fruiting by autogamy in relation to natural pollination and used the increase in fruit set from pollinators to calculate the farmer's monetary gain. We recorded bee visits to the exposed flowers during coffee flowering considering both on the edge and inside the coffee plantation. Ripe fruits were dried, counted and weighed. We discovered 132 solitary bee nests outside the plantation, with 54% containing coffee pollen grains, indicating coffee as an essential resource for bees even outside the crop area. More bee visits occurred at the coffee plantation's edge, resulting in increased fruit production, denser fruits, and rounder fruits in that area. Bagged flowers produced consistent seeds in all locations. The farmer could earn an extra US$1736.37 per hectare if the entire area received the same level of pollination contribution from bees as observed at the coffee border. Synthesis and applications. Our study emphasises the key role of pollinators in coffee production and their impact on fruit and seed characteristics. Bee visits were more frequent on border areas, emphasising their reliance on natural nesting sites. Bee‐mediated pollination positively affected fruit traits and self‐pollinated fruits in plantation borders had reduced mass. Solitary bee nesting was primarily observed in native vegetation, underlining its importance for bee populations. Pollen composition in nests varied with proximity to coffee plantations, indicating landscape vegetation influences pollinator foraging. These findings support optimising coffee plantation design by preserving native vegetation to increase coffee yields and conserve biodiversity.

Pollinator intraspecific body size variation and sociality influence their interactions with plants

Abstract Species morphological and behavioural traits are key determinants of which pollinator species interact with which plant species. However, individuals within species are not identical in their traits and this diversity could help us understand plant–pollinator interaction patterns. Using three independent data sets, we assessed whether bee intraspecific body size variation (ITV) and sociality influenced pollinator interaction specialisation, intraspecific niche partitioning, centrality in the interaction network and phylogenetic diversity of the plants visited. We found that solitary pollinators were more specialised in their interactions with plants and had lower intraspecific niche partitioning compared to social pollinators. Furthermore, solitary pollinators with higher ITV had higher centrality in the network and visited a higher phylogenetic diversity of plants compared to solitary species with lower ITV, whereas the opposite pattern emerged for social pollinators. Pollinator ITV did not differ between social and solitary bee species. Our findings show that the effect of pollinator body size variation on plant–pollinator interactions depends on pollinator species sociality. Specifically, solitary pollinators with higher ITV and social pollinators with lower ITV seem to be the most important contributors to maintaining the evolutionary diversity of the plant community, and also the species with the largest potential to affect (via cascade effects) the entire plant–pollinator network. Read the free Plain Language Summary for this article on the Journal blog.

Effect of landscape complexity, nesting substrate, and nest orientation on cavity-nesting solitary bees in southern Punjab, Pakistan

Effect of landscape complexity, nesting substrate, and nest orientation on cavity-nesting solitary bees in southern Punjab, Pakistan

Metabolic and transcriptomic characterization of summer and winter dormancy in the solitary bee, Osmia lignaria

The solitary bee Osmia lignaria is a native pollinator in North America with growing economic importance. The life cycle of O. lignaria provides a unique opportunity to compare the physiological and molecular mechanisms underlying two ecologically contrasting dormancies within the same species. O. lignaria prepupae become dormant during the summer to avoid high temperatures. Shortly after adult eclosion, they enter a second dormancy and overwinter as diapausing adults. To compare these two dormancies, we measured metabolic rates and gene expression across development as bees initiate, maintain, and terminate both prepupal (summer) and adult (overwintering) dormancies. We observed a moderate temperature-independent decrease in gas exchange during both the prepupal dormancy after cocoon spinning (45 %) and during adult diapause after eclosion (60 %). We sequenced and assembled a high-quality reference genome from a single haploid male bee with a contiguous n50 of 5.5 Mbp to facilitate our transcriptomic analysis. The transcriptomes of dormant prepupae and diapausing adults clustered into distinct groups more closely associated with life stage than dormancy status. Membrane transport, membrane-bound cellular components, oxidoreductase activity, glutathione metabolism, and transcription factor activity increased during adult diapause, relative to prepupal dormancy. Further, the transcriptomes of adults in diapause clustered into two groups, supporting multiple phases of diapause during winter. Late adult diapause was associated with gene expression profiles supporting increased insulin/IGF, juvenile hormone, and ecdysone signaling.

Systemic pesticides in a solitary bee pollen food store affect larval development and increase pupal mortality

Solitary bees are often exposed to various pesticides applied for pest control on farmland while providing pollination services to food crops. Increasing evidence suggests that sublethal toxicity of agricultural pesticides affects solitary bees differently than the social bees used to determine regulatory thresholds, such as honey bees and bumblebees. Studies on solitary bees are challenging because of the difficulties in obtaining large numbers of eggs or young larvae for bioassays. Here we show the toxic and sublethal developmental effects of four widely used plant systemic pesticides on the Japanese orchard bee (Osmia cornifrons). Pollen food stores of this solitary bee were treated with different concentrations of three insecticides (acetamiprid, flonicamid, and sulfoxaflor) and a fungicide (dodine). Eggs were transplanted to the treated pollen and larvae were allowed to feed on the pollen stores after egg hatch. The effects of chronic ingestion of contaminated pollen were measured until adult eclosion. This year-long study revealed that chronic exposure to all tested pesticides delayed larval development and lowered larval and adult body weights. Additionally, exposure to the systemic fungicide resulted in abnormal larval defecation and increased mortality at the pupal stage, indicating potential risk to bees from fungicide exposure. These findings demonstrate potential threats to solitary bees from systemic insecticides and fungicides and will help in making policy decisions to mitigate these effects.

Bees go up, flowers go down: Increased resource limitation from late spring to summer in agricultural landscapes

Abstract The loss of floral resources is a leading cause of wild bee decline in agricultural landscapes, but little is known about the temporal aspects of floral resource limitation for both social and solitary bees. Understanding when floral resources are most needed is crucial for the optimal design of pollinator conservation measures. We surveyed bees and flowers in 160 semi‐natural habitat patches multiple times per year (May–July) for 5 years. We identified the seasonality of floral resources and wild bees and examined inter‐ and intra‐annual patterns of floral resource limitation at both local and landscape scales. Floral resource availability varied across years but generally peaked in late May, after which it declined and remained low through July. Bumblebee and solitary bee abundances increased across the season, leading to stronger floral resource limitation for both groups later in the season. Bumblebee abundance was marginally positively associated with the cumulative amount of landscape‐scale floral resources as well as the floral resources of the previous year. Solitary bee abundance was only predicted by local‐scale floral resources. Synthesis and applications: Our results indicate that agri‐environmental management should target the provision of summer floral resources for both social and solitary bees. Local‐scale enhancement of floral resources can likely benefit solitary bees, but bumblebees probably require the management of floral resources at the landscape scale. Increasing the floral resources and the flowering period of herbaceous habitats that cover large proportions of the landscape, such as pastures, has the greatest potential to improve summer floral resources for bees.

Harnessing community science to conserve and study ground-nesting bee aggregations

Protecting diverse solitary ground-nesting bees remains a pivotal conservation concern. Ground-nesting bees are negatively impacted by anthropogenic land use change that often removes suitable nesting habitat from the landscape. Despite their enormous ecological and agricultural contributions to pollination, solitary, ground-nesting bees are often neglected, partly due to the significant obstacle of discovering exactly where these bees establish their nests. To address this limitation, we have developed a ‘community science’ project to map aggregations of ground-nesting bees globally. In certain locations, their abundances reach astounding levels, sometimes in the millions, but are scarcely known. Utilizing the iNaturalist platform, which permits geo-referencing of site observations and bee identification, we are providing public education and seeking public engagement to document bee aggregations in order to understand the nesting requirements of diverse species and open new opportunities for their conservation. Conservation priorities may then unequivocally be directed to areas of high species richness, nest densities, and nesting sites of rare bees. Such community-led efforts are vital for successful long-term management of native bees and the biotic and abiotic landscape data from nest-site localities can allow modeling to predict nest-site suitability and to readily test such predictions on the ground. Here, we summarize the progress, current limitations, and opportunities of using a global mapping project (GNBee) to direct conservation efforts and research toward solitary ground-nesting bees.

Pollen Diet Diversity does not Affect Gut Bacterial Communities or Melanization in a Social and Solitary Bee Species.

Pollinators face many stressors, including reduced floral diversity. A low-diversity diet can impair organisms' ability to cope with additional stressors, such as pathogens, by altering the gut microbiome and/or immune function, but these effects are understudied for most pollinators. We investigated the impact of pollen diet diversity on two ecologically and economically important generalist pollinators, the social bumble bee (Bombus impatiens) and the solitary alfalfa leafcutter bee (Megachile rotundata). We experimentally tested the effect of one-, two-, or three-species pollen diets on gut bacterial communities in both species, and the melanization immune response in B. impatiens. Pollen diets included dandelion (Taraxacum officinale), staghorn sumac (Rhus typhina), and hawthorn (Crataegus sp.) alone, each pair-wise combination, or a mix of all three species. We fed bees their diet for 7 days and then dissected out guts and sequenced 16S rRNA gene amplicons to characterize gut bacterial communities. To assess melanization in B. impatiens, we inserted microfilament implants into the bee abdomen and measured melanin deposition on the implant. We found that pollen diet did not influence gut bacterial communities in M. rotundata. In B. impatiens, pollen diet composition, but not diversity, affected gut bacterial richness in older, but not newly-emerged bees. Pollen diet did not affect the melanization response in B. impatiens. Our results suggest that even a monofloral, low-quality pollen diet such as dandelion can support diverse gut bacterial communities in captive-reared adults of these bee species. These findings shed light on the effects of reduced diet diversity on bee health.

Since 2015, the first solitary bee hotels in Sicily located in the small island of Isola delle Femmine Nature Reserve, Sicily (Italy)

Natural pollinators, mainly represented by solitary apoidea, play an essential role in ecosystems and biodiversity. Their population has sharply declined recently, putting both flora and fauna biodiversity at risk. Many studies report that artificial nesting for reproducing solitary bees, also known as bee hotels, can support conservation and increase the number of pollinators. Unipa and LIPU Isola delle Femmine Nature Reserve wanted to create the first bee hotels in 2015 near the coast. In particular, the goal of this project was to protect and increase biodiversity on the islet of Isola delle Femmine, a peculiar and challenging to-balance environment where there are more than 200 botanical taxa present on the island, some of which are endemic to Sicilian north-western coast, including pollination by insects. In the current state of knowledge, it is the first case of installing bee hotels in a coastal area so close to the sea, which entailed a significant technical challenge in the construction using recycled material. The hotels built were wholly colonized in a very short period, and even when maintenance and replacement of materials were carried out in less than a week, the recolonization was rapid. Most individuals belonged to the Megachilidae family, which is characteristic of the Mediterranean. The bee hotels have also been a source of curiosity and insight for the reserve visitors. In conclusion, the bee hotel proved useful for colonizing pollinators and increasing reserve use.

Host and floral communities shape parasite prevalence and reproduction in intensively managed forests

AbstractUnderstanding how working landscapes can maintain viable wildlife populations is key to evaluating their conservation potential. We assessed the potential of intensively managed conifer forests for supporting healthy, productive bee populations in one of the major timber‐growing regions of the world, the Pacific Northwest. We examined the direct effect of the number of years post‐harvest and other forest characteristics on flowering plant and bee communities and their indirect effect on parasite prevalence (Apicystis spp., Ascosphaera spp., and Crithidia spp.) and reproduction of a native, forest‐dwelling solitary bee (Osmia lignaria). Forest characteristics, including the time elapsed since harvest, influenced floral and bee community diversity and abundance and indirectly impacted parasite prevalence and offspring production. We found that increased bee diversity was associated with reduced parasite prevalence—consistent with a dilution effect—but the strength of the relationship varied across the different parasites. Additionally, bee abundance was more consistently associated with increased parasite prevalence, providing evidence of amplification. Floral abundance was only associated with lower Apicystis spp. prevalence. Across all parasite models, however, the values were <20%, indicating that additional factors shape bee communities beyond those we examined. Offspring production was positively related to floral diversity but not to parasite prevalence. Our results suggest that managing floral diversity is critical to enhancing the value of these landscapes for wild bee communities, both directly through promoting bee diversity and reproduction, and indirectly through facilitating parasite dilution.