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 utilised 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 specialised 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.
Highlights
Western honey bees (Apis mellifera) are the single most important pollinator of most crop monocultures worldwide (Delaplane and Mayer, 2000)
There were no differences in proportion of pollen collected from native plants between social and solitary species for April (W = 136, p = 0.680), May (W = 646, p = 0.87), June (W = 667, p = 0.078), or July (W = 581, p = 0.457), but by the end of the summer, solitary bees collected a significantly greater proportion of their pollen from native plants compared to social bees in August (71.3, 45.0%, W = 200, p = 0.027) and September (97.4, 60.7%, W = 2, p = 0.008)
These results suggest that, over evolutionary time, the solitary bee community has responded to the summer flora through increased specialization, and that there are a narrower range of native resources for both native social bees and introduced honey bees to exploit at this time
Summary
Western honey bees (Apis mellifera) are the single most important pollinator of most crop monocultures worldwide (Delaplane and Mayer, 2000). Honey bees live in colonies and can store food to consume during periods of resource scarcity. Comparative studies have shown the abundance of summer resources is the principle factor determining honey bee colony survival and honey production (Gallant et al, 2014; Requier et al, 2015; Smart et al, 2016; Alaux et al, 2017). Floral resource scarcity for honey bees and for wild bees has been reported predominantly from the Northern Hemisphere summer months of July and August (Inouye, 1978; Wetherwax, 1986; Couvillion et al, 2014a,b; Scheper et al, 2014), but mostly for nectar, with relatively little known about whether there is a shortage of pollen sources at this time as well
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