Floral resource wastage: Most nectar produced by the mass-flowering crop oilseed rape (Brassica napus) is uncollected by flower-visiting insects

Cultivation of the mass-flowering crop oilseed rape (OSR), Brassica napus, can provide insects with super-abundant nectar and pollen while in bloom. Several authors have suggested breeding cultivars to produce more abundant nectar and pollen to help mitigate insect decline. However, in Britain most, 95%, OSR blooms in spring (March-May), which has been suggested to be a period of nectar surplus and reduced exploitative competition. Therefore, a large proportion of floral resources produced by OSR during this period may be uncollected. Although there has been extensive work examining OSR nectar and pollen production, no study, to our knowledge, has measured this in relation to the demand by the flower-visiting insects. Here we quantified the percentage of nectar produced by spring blooming OSR which was uncollected in four OSR fields per year over 2 years. This was achieved by measuring the nectar in both insect accessible and inaccessible (i.e. mesh-covered) flowers. We also quantified uncollected pollen in flowers at the beginning and the end of anthesis using a haemocytometer. Most of the nectar (69%) and a fifth of pollen (19%) was uncollected in spring blooming OSR. Based on the estimates of nectar production and observed number of insects, nectar supply per insect was estimated at 2204 μL nectar insect-1 h-1, which exceeds potential collection rates by flower-visiting insects. Given the majority of B. napus is spring blooming, breeding cultivars of OSR which produce more nectar, while not being detrimental to flower-visiting insects, may be of little conservation benefit.

Why Petals? Naïve, but Not Experienced Bees, Preferentially Visit Flowers with Larger Visual Signals

Flower evolution includes a range of questions concerning the function of showy morphological features such as petals. Despite extensive research on the role of petals in attracting pollinators, there has been little experimental testing of their importance in attracting naïve versus experienced flower-visitors. In an exploratory field study, we manipulated the ray petals of inflorescences of two garden flowers, Rudbeckia hirta and Helenium autumnale, to test the hypothesis that these showy structures primarily function to attract first-time, naïve, visitors. On their first inflorescence visit to both species, naïve honey bees and bumble bees were more likely to visit intact inflorescences, than those with ray petals removed. However, by the tenth consecutive inflorescence on the same visit to the flower patch, test insects showed no preference. A positive correlation was observed between the visitation of inflorescences with zero petals and inflorescence number on both study plants, for both bees. These results suggest that a key function of showy petals is to attract naïve, first-time visitors. Similar to how a restaurant attracts diners with a large sign, showy signals may be vital to enticing first-time visitors when competing with other establishments or plants for customers or pollinators. We hope the findings of this exploratory study will stimulate further work in this area.

DoPI: The Database of Pollinator Interactions

Despite the importance of pollinating insects to natural environments and agriculture, there have been few attempts to unite the existing plant-pollinator interaction datasets into a single depository using a common format. Accordingly, we have created one of the world's first online, open-access, and searchable pollinator-plant interaction databases. DoPI (The Database of Pollinator Interactions) was built from a systematic review of the scientific literature and unpublished datasets requested from researchers and organizations. We collated records of interactions between British plant and insect flower-visitor species (or genera), together with associated metadata (date, location, habitat, source publication) when available. The dataset currently (December 2021) contains 101,539 records, detailing over 320,000 interactions. The number of interactions (i.e., the number of times a pairwise species interaction was recorded per occasion) varies considerably among records, averaging 3.6. These include records from 1888 pollinator species and 1241 plant species, totaling >17,000 pairwise species interactions. By combining a large volume of information in a single repository, DoPI can be used to answer fundamental ecological questions on the dynamics of pollination interactions in space and time, as well as applied questions in conservation practice. We hope this dynamic database will be a useful tool not only for researchers, but also for conservationists, funding agencies, governmental departments, beekeepers, agronomists, and gardeners. We request that this paper is cited when using the data in publications and individual studies when appropriate. Researchers and organizations are encouraged to add further data in the future. The database can be accessed at: https://www.dopi.org.uk/.

Plants and pollinators: Will natural selection cause an imbalance between nectar supply and demand?

Pollination is an important ecological process. However, plant and pollinator needs are not always met. Commonly, pollen limitation reduces seed set or bees experience nectar dearth. Using a cost-benefit approach, we show that natural selection will lead to lower nectar production when pollinators are abundant, and vice-versa. At the community level, competition among plants for pollinators causes positive feedback that exacerbates pre-existing seasonal imbalances between nectar supply and demand. When pollinators are scarce, plants will be selected to produce more nectar to outcompete other plants in attracting pollinators, and when pollinators are abundant, plants will be selected to produce less nectar. We suggest ways to test this positive feedback hypothesis and note that evidence for seasonal variation in nectar availability provides preliminary empirical support. If correct, our hypothesis indicates that pollination faces a particular challenge in balancing nectar supply with pollinator demand and is a further example of the underappreciated role of positive feedback in ecology and evolution.

Energetic efficiency of foraging mediates bee niche partitioning

Revitalizing our understanding of species distributions and assembly in community ecology requires greater use of functional (physiological) approaches based on quantifiable factors such as energetics. Here, we explore niche partitioning between bumble and honey bees by comparing a measure of within-patch foraging efficiency, the ratio of flower visitation rate (proportional to energy gain) to body mass (energy cost). This explained a remarkable 74% of the variation in the proportions of bumble to honey bees across 22 plant species and was confirmed using detailed energy calculations. Bumble bees visited flowers at a greater rate (realizing greater energy benefits) than honey bees, but were heavier (incurring greater energy costs) and predominated only on plant species where their benefit : cost ratio was higher than for honey bees. Importantly, the competition between honey bees and bumble bees had no consistent winner, thus highlighting the importance of plant diversity to the coexistence of competing bees. By contrast, tongue : corolla-tube-length ratio explained only 7% of the variation (non-significant). Our results confirm the importance of energetics in understanding community ecology and bee foraging niche and highlight the energetic tightrope navigated by foraging bees, since approximately half the nectar energy gained was expended in its collection.

Exploitative competition and displacement mediated by eusocial bees: experimental evidence in a wild pollinator community

Abstract

Eusocial bees are likely to be ecologically important competitors for floral resources, although competitive effects can be difficult to quantify in wild pollinator communities. To investigate this, we excluded honeybees (HBE treatment), bumblebees (BBE) or both (HB&BBE) from wild-growing patches of bramble, Rubus fruticosus L. agg., flowers in two eight-day field trials at separate locations, with complementary mapping of per-site local floral resource availability. Exclusions increased per-flower volume of nectar and visitation rates of non-excluded bees, compared to control patches with no bee exclusions (CON). There was a large increase in average nectar standing crop volume both at Site 1 (+ 172%) and Site 2 (+ 137%) in HB&BBE patch flowers, and no significant change in HBE or BBE, compared to CON patches. Foraging bee responses to exclusion treatments were more pronounced at Site 2, which may be due to lower local floral resource availability, since this is likely to increase the degree of exploitative competition present. Notably, at Site 2, there was a 447% increase in larger-bodied solitary (non-Apis/Bombus) bees visiting HB&BBE patches, suggesting ecological release from competition. Hoverflies showed no response to bee removals. Numbers of other non-bee insect groups were very small and also showed no clear response to exclusions. Our findings reveal patterns of competitive exclusion between pollinator groups, mediated by resource depletion by eusocial bees. Possible long-term implications of displacement from preferred flowers, particularly where alternative forage is reduced, are discussed.

Significance statement

Understanding patterns of exploitative competition and displacement is necessary for pollinator conservation, particularly for vulnerable or threatened species. In this research, experimental methods reveal underlying patterns of resource competition exerted by eusocial bees in a wild pollinator community. We show that honeybees and bumblebees competitively displace each other and particularly solitary (non-Apis/Bombus) bees from bramble, an important native nectar and pollen source. Effects were stronger where local floral resource availability was identified to be limited. Notably, following experimental exclusion of both honey- and bumblebees from flowers, visitation by solitary bees increased by up to 447%, strongly suggesting ecological release from competition. These results highlight the need for informed landscape management for pollinator wellbeing, including appropriate honeybee stocking densities and improved floral resource availability.

Gut microbiota composition is associated with environmental landscape in honey bees

There is growing recognition that the gut microbial community regulates a wide variety of important functions in its animal hosts, including host health. However, the complex interactions between gut microbes and environment are still unclear. Honey bees are ecologically and economically important pollinators that host a core gut microbial community that is thought to be constant across populations. Here, we examined whether the composition of the gut microbial community of honey bees is affected by the environmental landscape the bees are exposed to. We placed honey bee colonies reared under identical conditions in two main landscape types for 6 weeks: either oilseed rape farmland or agricultural farmland distant to fields of flowering oilseed rape. The gut bacterial communities of adult bees from the colonies were then characterized and compared based on amplicon sequencing of the 16S rRNA gene. While previous studies have delineated a characteristic core set of bacteria inhabiting the honey bee gut, our results suggest that the broad environment that bees are exposed to has some influence on the relative abundance of some members of that microbial community. This includes known dominant taxa thought to have functions in nutrition and health. Our results provide evidence for an influence of landscape exposure on honey bee microbial community and highlight the potential effect of exposure to different environmental parameters, such as forage type and neonicotinoid pesticides, on key honey bee gut bacteria. This work emphasizes the complexity of the relationship between the host, its gut bacteria, and the environment and identifies target microbial taxa for functional analyses.

Landscape Scale Study of the Net Effect of Proximity to a Neonicotinoid-Treated Crop on Bee Colony Health

Since 2013, the European Commission has restricted the use of three neonicotinoid insecticides as seed dressings on bee-attractive crops. Such crops represent an important source of forage for bees, which is often scarce in agro-ecosystems. However, this benefit has often been overlooked in the design of previous field studies, leaving the net impact of neonicotinoid treated crops on bees relatively unknown. Here, we determine the combined benefit (forage) and cost (insecticide) of oilseed rape grown from thiamethoxam-treated seeds on Bombus terrestris and Apis mellifera colonies. In April 2014, 36 colonies per species were located adjacent to three large oilseed rape fields (12 colonies per field). Another 36 were in three nearby locations in the same agro-ecosystem, but several kilometers distant from any oilseed rape fields. We found that Bombus colony growth and reproduction were unaffected by location (distant versus adjacent) following the two month flowering period. Apis colony and queen survival were unaffected. However, there was a small, but significant, negative relationship between honey and pollen neonicotinoid contamination and Apis colony weight gain. We hypothesize that any sublethal effects of neonicotinoid seed dressings on Bombus colonies are potentially offset by the additional foraging resources provided. A better understanding of the ecological and agronomic factors underlying neonicotinoid residues is needed to inform evidence-based policy.

Quality versus quantity: Foraging decisions in the honeybee (Apis mellifera scutellata) feeding on wildflower nectar and fruit juice

Foraging animals must often decide among resources which vary in quality and quantity. Nectar is a resource that exists along a continuum of quality in terms of sugar concentration and is the primary energy source for bees. Alternative sugar sources exist, including fruit juice, which generally has lower energetic value than nectar. We observed many honeybees (Apis mellifera scutellata) foraging on juice from fallen guava (Psidium guajava) fruit near others foraging on nectar. To investigate whether fruit and nectar offered contrasting benefits of quality and quantity, we compared honeybee foraging performance on P. guajava fruit versus two wildflowers growing within 50 m, Richardia brasiliensis and Tridax procumbens. Bees gained weight significantly faster on fruit, 2.72 mg/min, than on either flower (0.17 and 0.12 mg/min, respectively). However, the crop sugar concentration of fruit foragers was significantly lower than for either flower (12.4% vs. 37.0% and 22.7%, respectively). Fruit foragers also spent the most time handling and the least time flying, suggesting that fruit juice was energetically inexpensive to collect. We interpret honeybee foraging decisions in the context of existing foraging models and consider how nest-patch distance may be a key factor for central place foragers choosing between resources of contrasting quality and quantity. We also discuss how dilute solutions, such as fruit juice, can help maintain colony sugar-water balance. These results show the benefits of feeding on resources with contrasting quality and quantity and that even low-quality resources have value.

Appetite for self-destruction: suicidal biting as a nest defense strategy in Trigona stingless bees

Self-sacrificial behavior represents an extreme and relatively uncommon form of altruism in worker insects. It can occur, however, when inclusive fitness benefits are high, such as when defending the nest. We studied nest defense behaviors in stingless bees, which live in eusocial colonies subject to predation. We introduced a target flag to nest entrances to elicit defensive responses and quantified four measures of defensivity in 12 stingless bee species in São Paulo State, Brazil. These included three Trigona species, which are locally known for their aggression. Species varied significantly in their attack probability (cross species range = 0-1, P < 0.001), attack latency (7.0-23.5 s, P = 0.002), biting duration of individual bees (3.5-508.7 s, P < 0.001), and number of attackers (1.0-10.8, P < 0.001). A "suicide" bioassay on the six most aggressive species determined the proportion of workers willing to suffer fatal damage rather than disengage from an intruder. All six species had at least some suicidal individuals (7-83 %, P < 0.001), reaching 83 % in Trigona hyalinata. Biting pain was positively correlated with an index of overall aggression (P = 0.002). Microscopic examination revealed that all three Trigona species had five sharp teeth per mandible, a possible defensive adaptation and cause of increased pain. Suicidal defense via biting is a new example of self-sacrificial altruism and has both parallels and differences with other self-sacrificial worker insects, such as the honey bee. Our results indicate that suicidal biting may be a widespread defense strategy in stingless bees, but it is not universal.

Multiple DNA elements determine basal and thyroid hormone regulated expression of the human glycoprotein alpha subunit gene in pituitary cells

Basal and thyroid hormone-regulated expression of constructs containing varying lengths of 5' flanking sequence of the human glycoprotein hormone alpha subunit gene ligated to the reporter gene encoding chloramphenicol acetyl transferase (CAT) were examined in transfected pituitary GH3 cells. CAT gene expression was quantified by measurement of CAT enzyme activity. In addition, an RNase protection assay was used to confirm changes in CAT mRNA levels and to verify regulation of transcription from the authentic alpha gene start site. The pattern of basal alpha promoter activity indicated the presence of inhibitory DNA elements between -98 and -172 and upstream of -846 relative to the transcriptional start site, as well as a stimulatory element between -346 and -846. Each of the transfected constructs exhibited tri-iodothyronine inhibition of alpha promoter activity, suggesting the presence of an inhibitory thyroid hormone response element within 98 bp of the alpha gene transcriptional start site.

Effect of triiodothyronine treatment on thyrotrophin beta- and alpha-messenger RNAs in the pituitary of the euthyroid rat

We have examined the effect of triiodothyronine (T3) treatment of the euthyroid rat on serum thyrotrophin (TSH) concentration, pituitary content of TSH and on pituitary cytoplasmic levels of TSH beta- and alpha-subunit messenger (m)RNAs, measured using the technique of dot hybridization to radiolabelled complementary (c)DNA probes. Serum concentration of TSH fell rapidly after T3 treatment of euthyroid rats, but no effect of T3 treatment was evident on pituitary TSH content. In contrast, marked effects of T3 on cytoplasmic levels of both TSH beta- and alpha-subunit mRNAs were demonstrated, with reductions in beta- and alpha-mRNAs at both early (6 h) and late (72 h) time points after administration of T3. These observations, which have not been documented previously in the euthyroid state, contrast with earlier findings in hypothyroidism in which stimulatory influences of T3 on TSH mRNAs are evident at 6 h.

Modulation by oestrogen of thyroid hormone effects on thyrotrophin gene expression

The role of oestrogen in the regulation of TSH gene expression is unclear. We have examined the effect of administration of oestrogen in the rat on serum TSH, pituitary TSH content and pituitary cytoplasmic concentrations of mRNA encoding the TSH beta and alpha subunits, thus deriving measures of hormone release and synthesis. In addition, we have examined the effect of oestrogen on the binding of tri-iodothyronine (T3) to nuclear receptors in the anterior pituitary. Administration of oestrogen did not affect serum concentrations of TSH in euthyroid or untreated hypothyroid rats, but did augment the effects of T3 (1 and 2 micrograms) on serum TSH in hypothyroid animals 6 h after injection of T3. No influence of oestrogen or of thyroid status on pituitary content of TSH was seen. A marked increase in the concentrations of TSH beta and alpha mRNA in pituitary cytoplasm was found in hypothyroidism, compared with those in the euthyroid state. No effect of oestrogen on TSH mRNA was seen in euthyroid animals but concentrations of TSH beta and alpha mRNA were lower in hypothyroid animals than in vehicle-treated controls. A stimulatory influence of T3 on TSH mRNA was seen 6 h after injection of T3; this stimulation was absent in oestrogen-treated rats. No effect of oestrogen on the action of T3 was evident 72 h after beginning treatment with T3. In addition to effects on serum TSH and TSH mRNA, an increase in the number of pituitary nuclear receptors for T3 was seen after oestrogen treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of hypothyroidism and thyroid hormone replacement in vivo on pituitary cytoplasmic concentrations of thyrotropin-beta and alpha-subunit messenger ribonucleic acids

We have studied the influence of hypothyroidism and thyroid hormone replacement in vivo on rat pituitary cytoplasmic concentrations of TSH beta and alpha-subunit mRNA, measured by cytoplasmic dot hybridization, as well as circulating TSH and pituitary TSH content. Cytoplasmic concentrations of GH, PRL, and LH beta-subunit mRNA were measured in parallel. Marked increases in serum TSH and TSH beta and alpha mRNA were found in hypothyroidism; these changes reversed 72 h after beginning T3 replacement. A more marked effect of hypothyroidism on TSH beta mRNA than on alpha mRNA was evident, suggesting differential control of regulation of the TSH genes. In contrast to a rapid fall in serum TSH after T3 administration, an increase in TSH beta and alpha mRNA was evident at 1 and 6 h, followed by a later fall in TSH mRNA to euthyroid values. Pituitary cytoplasmic concentrations of GH and PRL mRNA were reduced in hypothyroidism and increased after administration of T3; LH beta mRNA was unaffected by hypothyroidism or T3 replacement. The differential effects of thyroid status on TSH, GH, PRL, and LH mRNA indicate specificity of regulation of these anterior pituitary hormone genes.