No abstract

1. In recent decades, several North American bumble bee (Bombus spp.) species have undergone precipitous declines. It is suspected that a parasite or pathogen may be responsible, yet few studies have examined the extent of parasitism and the ecology of host-parasite relationships in U.S. bumble bee populations.2. A season-long survey of bumble bees in seven grassland meadows of the northern Shenandoah Valley and Piedmont regions in Virginia was conducted in 2011 to ascertain the local prevalence and predictors of parasitism by the internal parasites Nosema and Crithidia, and by parasitoid conopid flies.3. In total, 835 bumble bees representing six species were examined. Using visual detection methods, we determined that 25% of bees were infected with parasitoid larvae, 17.4% with Crithidia, and 7.3% with Nosema.4. Nosema infections were more prevalent and intense in locally rare than locally common species, with the two rarest bumble bees [B. fervidus (Fabricius) and B. auricomus (Robertson)], newly suspected to be in decline, having the highest frequencies of infection (11-17.8%).5. Crithidia was generally more prevalent in common bumble bee species (11-35%). With fewer than 5% of individuals infected, the two rarest species had the lowest frequencies of Crithidia. Conopid fly larvae were more prevalent in common species.6. Body size significantly influenced the probability of parasitism by conopids and Crithidia. Smaller bees were more likely to be parasitised by Crithidia. Larger bees were more likely to be parasitised by conopid flies, although the largest bee species (B. auricomus) was not infected by conopids in this study.

Author contributions. LSA, REI and PCS conceived of and designed the study. PCS and IWF conducted chemical analysis of pollen and synthesized spermidines. AEF, RLM, PRA, LMC, PMD, and SL carried out bioassay experiments. AEF analyzed data and prepared figures. LSA wrote the manuscript with substantial contributions from PCS. All co-authors read and provided feedback on the manuscript.

No abstract

Sustainable agriculture relies on pollinators, and wild bees benefit yield of multiple crops. However, the combined exposure to pesticides and loss of flower resources, driven by agricultural intensification, contribute to declining diversity and abundance of many bee taxa. Flower plantings along the margins of agricultural fields offer diverse food resources not directly treated with pesticides. To investigate the potential of flower plantings to mitigate bee pesticide exposure effects and support bee reproduction, we selected replicated sites in intensively farmed landscapes where half contained flower plantings. We assessed solitary bee Osmia lignaria and bumble bee Bombus vosnesenskii nesting and reproduction throughout the season in these landscapes. We also quantified local and landscape flower resources and used bee‐collected pollen to determine forage resource use and pesticide exposure and risk. Flower plantings, and their local flower resources, increased O. lignaria nesting probability. Bombus vosnesenskii reproduction was more strongly related to landscape than local flower resources. Bees at sites with and without flower plantings experienced similar pesticide risk, and the local flowers, alongside flowers in the landscape, were sources of pesticide exposure particularly for O. lignaria. However, local flower resources mitigated negative pesticide effects on B. vosnesenskii reproduction. Synthesis and applications. Bees in agricultural landscapes are threatened by pesticide exposure and loss of flower resources through agricultural intensification. Therefore, finding solutions to mitigate negative effects of pesticide use and flower deficiency is urgent. Our findings point towards flower plantings as a potential solution to support bee populations by mitigating pesticide exposure effects and providing key forage. Further investigation of the balance between forage benefits and added pesticide risk is needed to reveal contexts where net benefits occur.

Conditions experienced early in development can affect the future performance of individuals and populations. Demographic theories predict persistent population impacts of past resources, but few studies have experimentally tested such carry‐over effects across generations or cohorts. We used bumble bees to test whether resource timing had persistent effects on within‐colony dynamics over sequential cohorts of workers. We simulated a resource pulse for field colonies either early or late in their development and estimated colony growth rates during pulse‐ and non‐pulse periods. During periods when resources were not supplemented, early‐pulse colonies grew faster than late‐pulse colonies; early‐pulse colonies grew larger as a result. These results revealed persistent effects of past resources on current growth and support the importance of transient dynamics in natural ecological systems. Early‐pulse colonies also produced more queen offspring, highlighting the critical nature of resource timing for the population, as well as colony, dynamics of a key pollinator.

Conditions experienced early in development can affect the future performance of individuals and populations. Demographic theories predict persistent population impacts of past resources, but few studies have experimentally tested such carry-over effects across generations or cohorts. We used bumble bees to test whether resource timing had persistent effects on withincolony dynamics over sequential cohorts of workers. We simulated a resource pulse for field colonies either early or late in colony development and estimated colony growth rates during pulse-and non-pulse periods. During periods when resources were not supplemented, early-pulse colonies grew faster than late-pulse colonies; early-pulse colonies grew larger as a result. These results reveal persistent effects of past resources on current growth and support the importance of transient dynamics in natural ecological systems. Early-pulse colonies also produced more queen offspring, highlighting the critical nature of resource timing for population, as well as colony, dynamics of a key pollinator.

Endoparasitoids develop inside the body of a host organism and, if successful, eventually kill their host in order to reach maturity. Host species can vary in their suitability for a developing endoparasitoid; in particular, the host immune response, which can suppress egg hatching and larval development, has been hypothesized to be one of the most important determinants of parasitoid host range. In this study, we investigated whether three bumblebee host species (Bombus bimaculatus, Bombus griseocollis, and Bombus impatiens) varied in their suitability for the development of a shared parasitoid, the conopid fly (Conopidae, Diptera) and whether the intensity of host encapsulation response, an insect immune defense against invaders, could predict parasitoid success. When surgically implanted with a nylon filament, B. griseocollis exhibited a stronger immune response than both B. impatiens and B. bimaculatus. Similarly, B. griseocollis was more likely to melanize conopid larvae from natural infections and more likely to kill conopids prior to its own death. Our results indicate that variation in the strength of the general immune response of insects may have ecological implications for sympatric species that share parasites. We suggest that, in this system, selection for a stronger immune response may be heightened by the pattern of phenological overlap between local host species and the population peak of their most prominent parasitoid.