Self-incompatibility of U.S. Great Basin Sphaeralcea (Malvaceae) and regional prevalence, pollination efficacy, and floral infidelity of their oligolectic Diadasia bees

Diadasia bee on Sphaeralcea flower

By Cane & Love

Globemallows (Sphaeralcea species) are widely adapted in rangeland restoration across basins of the western United States, yet little was known about how breeding biology and pollination affect seed production in the genus. We studied three common species to determine their reliance on outcrossing, document the prevalence of different bees that visit them across sagebrush‑steppe habitats, and evaluate the pollination performance of the specialist bee, Diadasia diminuta.

All three globemallows produced substantially more capsules and seeds when flowers received pollen from other plants rather than self‑pollen. Field surveys showed that a diverse assemblage of solitary, ground‑nesting bees visit globemallow flowers, with D. diminuta consistently the most abundant species. Single visits by female D. diminuta deposited enough pollen to achieve full seed set. The bees readily established in newly planted restoration plots, even where wild globemallow populations were distant. Although D. diminuta specializes on Sphaeralcea, foraging females frequently moved between species in mixed plantings, behavior consistent with the natural hybridization common in this genus.

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Who visits forest bramble flowers, and when? What camera traps reveal about pollinators in the woods

A bumblebee on a bramble flower

by Bonneau-Chloup et al.

Forest understories are the scene of everyday interactions between plants and insects, yet we know surprisingly little about who visits forest flowers, when they do it, and if they are sensitive to the very local microclimate. Most pollinator research focuses on open meadows or crops, leaving forest flower-visitor communities largely overlooked, even though forests host some specialized insects and may represent a refuge for more generalist wild pollinators in increasingly farmed or urbanised landscapes.

To fill this gap, we set up a non-invasive video monitoring system on bramble flowers (Rubus fruticosus agg., a common forest-edge shrub) across four patches of the Fontainebleau forest, near Paris. Over 369 hours of footage, we recorded every insect that visited the flowers, day and night, without ever touching or trapping a single one.

We counted 3,636 visits from 17 different types of visitors. Ants turned out to be the most frequent guests, accounting for nearly half of all visits, followed by ordinary flies, then bees, hoverflies, and moths/butterflies together. Each group had its own daily rhythm: most insects were busiest from late morning to late afternoon, but ants were more active at dusk and dawn, and some flies and hoverflies showed two separate activity peaks in a single day. A few visitors, including some beetles and grasshoppers, lingered on a single flower for over an hour, while bumblebees and hoverflies typically stayed just a few seconds. Interestingly, sunlight and temperature had only a limited effect on which insects showed up, flies and ants were less active in bright light, and flies also foraged faster as it got warmer, but most other visitors carried on regardless of the microclimate.

Overall, our results suggest that forest flower visitors avoid competing with each other mainly by using the flowers at different times of day. Beyond the ecological findings, this study shows that filming brambles with cameras is a simple, low-cost, and environmental-friendly way to study forest flower visitors, one that can run continuously, day and night, and could be used to track forest pollinator communities more widely in the future.

 

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Pollination services of managed common eastern bumble bees (Bombus impatiens) in seedless watermelon (Citrullus lanatus)

Common eastern bumble bees: A) at the entrance of a commercial colony and 
B) visiting a watermelon flower.

By Mateos-Fierro et al.

Pollinator-dependent crops rely on pollinating insects for successful pollination and thereby achieving commercial yields. Seedless watermelon (Citrullus lanatus) is particularly dependent on insect pollination, as pollinators must transfer pollen from pollen-donor plants. While pollen transfer can be provided for free by wild pollinators, these gratis pollination services are typically insufficient. As a result, growers use managed pollinators. The western honey bee (Apis mellifera) has historically been the only managed species employed, but recently, growers have also supplemented with bumble bees, such as the common eastern bumble bee (Bombus impatiens). However, adding a second managed pollinator entails a greater investment, which may not pay off, particularly since the role of managed bumble bees in seedless watermelon production is uncertain.

In this study, we investigated the pollination contribution of managed bumble bees by combining field observations and experiments. Field observations were conducted in commercial seedless watermelon fields; all fields were stocked with honey bees, but only some were also supplemented with bumble bees. We recorded the number of flower visits and the behaviour of different pollinator groups. Additionally, we conducted a powder tracing experiment in which we sprayed fluorescent powder into the bumble bee colonies so that they would leave traces on flowers. Finally, we analysed pollen collected from bumble bees and their colonies to explore what percentage of the pollen belonged to watermelon.

Our field observations revealed that bumble bees were infrequent visitors to watermelon flowers. Supplementing fields with managed bumble bees did not increase watermelon visits across the entire field, but visits increased locally, within 10 m around their colonies. Additionally, managed bumble bee presence in the fields influenced other pollinators’ behaviours, but this differed across pollinator groups. Small solitary bees were, in general, more affected. The powder tracing experiment indicated little managed bumble bee activity, with only 1.5% of flowers showing powder traces, whereas the pollen analysis revealed that watermelon pollen made up only 2.9% of the pollen collected by bumble bees.

These data suggest that stocking fields with managed bumble bees may not increase visits to watermelon flowers and therefore may not be important contributors to watermelon pollination. However, their direct contribution to yield and whether an increase in stocking rate can increase visits to flowers have yet to be explored.

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