James H. Cane

It is widely documented that many crops depend on animal—and primarily insect—pollination, but the degree to which pollinators limit yield in comparison with other factors is poorly understood. Recent studies conclude that pollinator visitation rates limit yields of many crops, based upon positive correlations between these two variables. However, these studies typically suffer from incomplete data on two key variables that affect both pollinator recruitment and yield: flower density per area and yield potential per flower, both driven by maternal resource availability. Here we review the literature on animal‐pollinated crops showing that (i) yields can be positively associated with flower density, and (ii) pollinator density can also be positively correlated with flower density; the third positive association observed between pollinator density and yield may result from the two first correlations, without causal relationship. Likewise, positive associations observed between (i) the amount of maternal resources per flower and yield potential per flower, and (ii) between the amount of maternal resources per flower and pollinator density per flower can both explain the positive correlation observed between yield per flower and pollinator density per flower. We conclude with an illustration reusing data from the literature to show that, without incorporating data on flower density and yield potential per flower, measuring yields and gradients of pollinator density per area alone can overestimate the degree to which pollinator visitation rates directly limit yield. Read the free Plain Language Summary for this article on the Journal blog.

Several genera of osmiine bees (Osmia and Hoplitis) are unique among animals for building nests using leaf pulp (mastic). Only scattered anecdotes indicate their leaf choices. To learn more, a large captive nesting population of cavity-nesting Osmia bruneri was offered foliage from 46 plant species (18 families) differing in scent, pubescence, latex, and cutting resistance. The choice trials presented four species over 12 min, counting approaches by females, landings, foliage bites, and successful pulping bouts. Daily and lifetime efforts to acquire pulp balls were estimated. Leaf pulping by O. bruneri was not random. Foliage of some families was consistently rejected (e.g. Asteraceae); others were regularly pulped (e.g. Malvaceae). Anecdotal usage of some leaf species was confirmed. Bees preferred leaves bearing stellate trichomes, but dense pubescence or abundant long trichomes interfered with pulp ball formation. Scented foliage was also rejected. Leaf thickness and leaf fracture (cutting) force were not correlated with pulping preference among the more flexible leaves tested. Pulp balls were acquired with ∼150 bites in 40–60 s. Over a lifetime, >100 pulp balls were acquired through >15 000 bites; mandibles were only moderately worn. Some preferred leaf species are readily grown in greenhouses or outside for leaf-pulping Osmia used in commercial pollination.

All solitary and primitively eusocial bees of the temperate zones survive winter in a dormant diapausing state. Weight loss of Osmia and Megachile bees has been solely attributed to metabolism of stored triglycerides, as evidenced by loss of fat body. I hypothesized that uptake or loss of water vapor may represent another contributor to wintering weight change. Using a range of atmospheric humidities (0–88%), it is shown that wintering cocooned O. californica (adults) and M. rotundata (prepupae) gained weight in humid atmospheres and lost weight in drier air, the inflection point being between 35 and 47% humidity. Most of this weight change occurred in early winter. No humidity treatment enhanced mortality. Winter storage humidity is an available management variable for these bees when kept for commercial pollination.

SUMMARY: Successful fertilization of insect‐pollinated crops hinges on a delicate interplay of olfactory and visual signals of pollinator attraction, the chemical complexity of nectar and pollen rewards, and the physical interaction between insects and flower anatomy for efficient pollen transfer. These traits, which are controlled genetically and exhibit phenotypic variance even within species, present opportunities for breeding technologies to map and select genotypes with floral traits that actively guide pollinator preferences. Recent technological advancements and automation have enabled high‐throughput metabolic phenotyping of floral chemical traits of pollinator attraction and rewards. These measurements, when integrated with computed tomography (CT) scans of flower shape analysis and video tracking of pollinator behavior, can guide the selection of genotypes with enhanced insect visitation rates and effective cross‐pollination. In this perspective article, we highlight the potential of this strategy for blueberry (Vaccinium corymbosum L.), a crop heavily reliant on bee pollination for fruit production and with flowers that display considerable variance of chemical and morphological traits and pollinator visitation rates. Leveraging blueberry's genetic diversity can address pollination issues exacerbated by global warming and declining health of managed bees, thus contributing to a more sustainable agricultural production.

Among the ground-nesting bees are several proven crop pollinators, but only the alkali bee ( Nomia melanderi) has been successfully managed. In <80 years, it has become the world's most intensely studied ground-nesting solitary bee. In many ways, the bee seems paradoxical. It nests during the torrid, parched midsummer amid arid valleys and basins of the western United States, yet it wants damp nesting soil. In these basins, extensive monocultures of an irrigated Eurasian crop plant, alfalfa (lucerne), subsidize millions of alkali bees. Elsewhere, its polylectic habits and long foraging range allow it to stray into neighboring crops contaminated with insecticides. Primary wild floral hosts are either non-native or poorly known. Kleptoparasitic bees plague most ground nesters, but not alkali bees, which do, however, host other well-studied parasitoids. Building effective nesting beds requires understanding the hydraulic conductivity of silty nesting soils and its important interplay with specific soil mineral salts. Surprisingly, some isolated populations endure inhospitably cold climates by nesting amid hot springs. Despite the peculiarities and challenges associated with its management, the alkali bee remains the second most valuable managed solitary bee for US agriculture and perhaps the world.