Wild Bee Pollen Diets Reveal Patterns of Seasonal Foraging Resources for Honey Bees

Abstract

Western honey bees (Apis mellifera) are dominant crop pollinators, and access to summer forage is a critical factor influencing colony health in agricultural landscapes. In many temperate agricultural regions, honey bees forage extensively from non-native plants during the summer, but it is unclear whether the use of these species is due to honey bee preference for these plants or is a result of their relative abundance. The foraging choices made by native bees that have evolved with native plants can reveal the seasonal availability of native plant pollens, and so we quantified the pollen collected by 181 wild bee species native to Michigan. Pollen was also trapped from honey bee colonies during the summer to confirm the peak period of non-native pollen collection in this region. Across the state, the generic richness of native pollens collected by wild bees peaked in May before linearly declining into September. Wild social and solitary bees collected a similar proportion of their pollen from non-native plants from April to July, but during August and September social bees collected a significantly greater proportion from non-natives. At a local scale, honey bees collected the majority of their pollen from non-native plants between 4 July and 21 August, with the same trend seen in both social and solitary bees. Across the region, a significantly greater proportion of the solitary bee species that peak during this time are specialists, most of which collect from native plant species that are little utilized by social bees for pollen, such as Dasiphora, Helianthus, Physalis, and Vernonia. Our results suggest that Michigan has relatively few native flowering resources during the height of the summer, and that many of those which flower during this time are used primarily by specialized solitary bee species rather than the social bee community, including honey bees. As a result, non-native plant species with a late summer flowering phenology fill a forage gap and thus can contribute to the diet of both honey bees and generalist wild bees during this time, despite the well-documented negative impacts of these species on native plant communities.

Do honey bee (Apis mellifera) foragers recruit their nestmates to native forbs in reconstructed prairie habitats?

“…The average distances that we found are within the range of previous studies [36,43,71,75]. However, the decrease in distances advertised is surprising because multiple studies have indicated that honey bee colonies in the Upper Midwest and surrounding states face a dearth period when there is a lower diversity of blooming flowers at the end of the summer and into the fall (North Dakota: [76]; Ohio: [75]; Michigan: [77]; Iowa: [78]). The period from the beginning of July to early August is generally the time of year when honey bee colonies in Minnesota gain the most weight and produce the most honey [79,80].…”

Narrow pollen diets are associated with declining Midwestern bumble bee species

“…This trend is seen in the solitary bee community in the Midwest, where the summer‐flying community contains a greater proportion of dietary specialists compared to the spring‐flying community (Graenicher , Wood et al. ). Based on pollen‐foraging phenology as determined in this study, the two Michigan bumble bee species showing the greatest declines, B. affinis and B. pensylvanicus , are those that peak latest in the year.…”

“…In Michigan, the diversity of pollen types collected by the wild bee community peaks in May, before linearly declining into September (Wood et al. ). Relative to early‐emerging species, late‐emerging bumble bee species may consequently have a narrower diet simply due to a lower diversity of plants flowering at this time, necessitating greater specialization.…”

Towards ecological intensification : the relative importance of wild pollinators as an agricultural input in seed production

“…In contrast to the crop pollinators, hardly any of the species that relied solely on semi-natural habitats were observed in large numbers. Potential reasons for this could be that non-crop pollinators may have restricted or specialised pollen diet requirements (Wood et al 2018b), or that in semi-natural habitats resource availability was too low and scattered to maintain species with larger populations for the duration of their activity period.…”

“…Instead, more effective pollination-enhancing measures on the long term are probably permanent flower-rich linear landscape features (Morandin and Kremen 2013, Dainese et al 2017). Many of pollinator species that provide crop-pollination services collect surprising amounts of pollen from woody plants (Wood et al 2018a, Wood et al 2018b, Hass et al 2019, with up to 80% of the pollen collected from woody plants in spring (Kamper et al 2016). It may therefore be not surprising that restoration of woody elements has been shown to enhance crop pollination services (Morandin et al 2016, Dainese et al 2017), but the effects of restoration of woody elements on crop yield have not yet been studied.…”