Spectral purity, intensity and dominant wavelength: Disparate colour preferences of two Brazilian stingless bee species

Environmental Sources of Possible Associated Pathogens and Contaminants of Stingless Bees in the Neotropics

Stingless bees are crucial for pollination and support diverse ecological relationships, offering economic benefits and contributing to enhanced crop yields. Their tropical pollinator status makes them highly sensitive to environmental changes and disruptions, which could affect their survival, as well as to pathogens that threaten their health. The lack of comprehensive research and the scattering of reports make it difficult to identify pathogens and contaminants. This review aims to provide an overview of diseases in stingless bees, examine chemical contaminants in their products, and explore threatened sources. Using the PRISMA flowchart, a total of 30 articles from 2009 to 2024 concerning pathogens and contaminants in stingless bees were retrieved. A total of 15 pathogens and 26 pollutants affect life expectancy and survival rate of stingless bees (mainly the genera Melipona and Tetragonisca) were identified in five major areas of the Neotropics, including Brazil, Mexico, Costa Rica, Australia, and Asia. Studies indicated that the bacterial genera Pseudomonas, Melissococcus, and Lysinibacillus are affecting the survival of stingless bees, particularly their brood, and contributing to annual colony deaths. Heavy metals, polycyclic aromatic hydrocarbons (PAHs), and microplastics have been detected in by-products of stingless bees, especially honey. Epidemiological research is crucial, including studies on pathogens associated with diseases, the effects of contaminants on bees, and the development of quality guidelines for stingless-bee products.

Flower colour contrast, 'spectral purity' and a red herring

Nature offers a bewildering diversity of flower colours. Understanding the ecology and evolution of this fantastic floral diversity requires knowledge about the visual systems of their natural observers, such as insect pollinators. The key question is how flower colour and pattern can be measured and represented to characterise the signals that are relevant to pollinators. A common way to interpret flower colours is using animal vision models that incorporate the spectral sensitivity of a focal observer (e.g. bees). These vision models provide a measure of colour contrast, which represents the perceived chromatic difference between two objects, such as a yellow flower against green leaves. Colour contrast is a behaviourally and physiologically validated proxy for relative conspicuousness of a stimulus. A growing number of studies attempt to interpret flower colouration through parameters that are grafted on to principles of human colour perception. A perpetuating measure to describe floral colours is via saturation, which is a metric in human perception describing a certain aspect of colourfulness and is, in pollination literature, often referred to as 'spectral purity'. We caution against the concept, calculation and biological interpretation of 'spectral purity' and similar measures that rest on an anthropocentric view, because it does not represent the diversity and complexity of animal visual systems that are the natural observers of flowers. We here discuss the strengths and weaknesses of common ways to interpret flower colouration and provide concrete suggestions for future colourful research.

Innate floral object identification in a solitary pollinator employs a combination of both visual and olfactory cues

Object identification is fundamental to animal behaviour and identifying nutritive objects is key for survival. Solitary insects emerging far from food sources and without access to social learning must therefore employ innate identification of food cues to locate relevant nutritive objects from a distance. Such innate preferences for food cues should be both specific enough to allow discrimination between food and non-food objects and general enough to allow for the variety of food objects relevant to the insect species. Here, we examined innate floral object identification behaviour in solitary generalist insect pollinator Eristalinus aeneus using an artificial floral object with both visual and olfactory cues previously found to be attractive to several hoverfly species across multiple environments. We used a subtractive two choice assay to present flower-naïve E. aeneus with a choice between this object and a similar object that differed in either a single visual or olfactory cue. We found that innate floral choices of the hoverfly E. aeneus are a product of broad, plant-based olfactory cues and visual cues, where a combination of radial symmetry and reflectance in the 300-400 nm and 500-700 nm wavelength range was particularly important for innate floral object preference. Our study, therefore, shows how solitary animals without prior experience can efficiently employ multimodal cues to identify multiple relevant nutritive objects.

Standardising bee sampling: A systematic review of pan trapping and associated floral surveys

The use of coloured pan traps (bee bowls, Moericke traps) for sampling bees (and other pollinators) has continuously increased over the last two decades. Although a number of methodological studies and conceptual frameworks offer guidance on standardised sampling, pan trap setups vary widely in characteristics even when optimised for capturing bees. Moreover, some uncertainty persists as to how local flower abundance and diversity influence sampling. We systematically reviewed peer-reviewed studies that used pan traps for bee collection and that were listed in the Web of Science core collection. To gauge methodological variation, we identified a set of relevant methodological criteria and assessed the studies accordingly. For obtaining evidence that pan trap samples and floral environment around traps are correlated, we screened the relevant studies for such correlations. While some aspects of pan trapping (e.g., trap coloration and elevation) were similar in the majority of studies, other aspects varied considerably (e.g., trap volume/diameter and sampling duration). Few studies used floral abundance and/or diversity as an explanatory variable in their analyses of bee samples. Among these studies, we found a considerable variation in key aspects of floral survey methods, such as time and space between vegetation surveys and pan trap sampling, abundance measures (quantitative, semi-quantitative and presence-absence), and processing of raw data prior to analysis. Often studies did not find any correlation between the floral environment and bee samples. Reported correlations varied markedly across studies, even within groups of studies applying a similar method or analysing a similar group of bees. Our synthesis helps to identify key issues of further standardisation of pan trap methodology and of associated floral surveys. In addition to the few aspects that have been standardised over the past decades, we suggest methodological direction for future research using pan traps as a better standardised method for the collection of wild bees. We encourage further studies to illuminate if and how varying floral resources around traps bias bee samples from pan traps. More generally, our synthesis shows that trapping methodologies should be reviewed regularly when their use increases to ensure standardisation.

Can trap color affect arthropod community attraction in agroecosystems? A test using yellow vane and colorless traps

Vane trapping is one of the most effective methods for sampling flower-visiting arthropods. Despite its importance in pollinator studies, the effects of trap color on the abundance and richness of pollinators are less understood. To test this, we conducted a 3-season field experiment over 2 years with two types of vane traps: yellow and colorless. We set up twelve traps each in three field sites within the Lower Rio Grande Valley in south Texas, planted with Vigna unguiculata, Crotalaria juncea, Raphanus raphanistrum, and Sorghum drummondii. At each site, six colorless vane and six yellow vane traps were placed equidistant from each other. The experiment was replicated three times across three seasons, first during the pre-flowering season, when the crops were in full bloom, and when there was no crop on the field. In total, we collected 1912 insects, out of which 76.7% were pollinators. Generalized Linear Regression analyses showed that yellow traps consistently attracted significantly more arthropods and pollinators, but these differences were also season dependent. Furthermore, we noticed that Hymenoptera, followed by Coleoptera, were the most prevalent orders in both the yellow vane and colorless vane traps. Interestingly, although there was no significant difference in species richness of the arthropods in the yellow and colorless vane traps, our results suggest that trap color plays a significant role in capturing pollinators, including non-target arthropods. Our data add another line of evidence suggesting that trap color should be accounted for designing experiments that estimate pollinator and arthropod community diversity.

Caution with colour calculations: spectral purity is a poor descriptor of flower colour visibility

BACKGROUND

The colours of flowers are of key interest to plant and pollination biologists. An increasing number of studies have investigated the importance of saturation of flower colours (often called 'spectral purity' or 'chroma') for visibility to pollinators, but the conceptual, physiological and behavioural foundations for these metrics as well as the calculations used rest on slender foundations.

METHODS

We discuss the caveats of colour attributes that are derived from human perception, and in particular spectral purity and chroma, as variables in flower colour analysis. We re-analysed seven published datasets encompassing 774 measured reflectance spectra to test for correlations between colour contrast, spectral purity and chroma.

MAIN FINDINGS AND CONCLUSIONS

We identify several concerns with common calculation procedures in animal colour spaces. Studies on animal colour vision provide no ground to assume that any pollinator perceives (or responds to) saturation, chroma or spectral purity in the way humans do. A re-analysis of published datasets revealed that values for colour contrast between flowers and their background are highly correlated with measures for spectral purity and chroma, which invalidates treating these factors as independent variables as is currently commonplace. Strikingly, spectral purity and chroma - both of which are metrics for saturation and are often used synonymously - are not correlated at all. We conclude that alternative, behaviourally validated metrics for the visibility of flowers to pollinators, such as colour contrast and achromatic contrast, are better in understanding the role of flower colour in plant-pollinator signalling.

Comparative psychophysics of Western honey bee (Apis mellifera) and stingless bee (Tetragonula carbonaria) colour purity and intensity perception

Bees play a vital role as pollinators worldwide and have influenced how flower colour signals have evolved. The Western honey bee, Apis mellifera (Apini), and the Buff-tailed bumble bee, Bombus terrestris (Bombini) are well-studied model species with regard to their sensory physiology and pollination capacity, although currently far less is known about stingless bees (Meliponini) that are common in pantropical regions. We conducted comparative experiments with two highly eusocial bee species, the Western honey bee, A. mellifera, and the Australian stingless bee, Tetragonula carbonaria, to understand their colour preferences considering fine-scaled stimuli specifically designed for testing bee colour vision. We employed stimuli made of pigment powders to allow manipulation of single colour parameters including spectral purity (saturation) or colour intensity (brightness) of a blue colour (hue) for which both species have previously shown innate preferences. Both A. mellifera and T. carbonaria demonstrated a significant preference for spectrally purer colour stimuli, although this preference is more pronounced in honey bees than in stingless bees. When all other colour cues were tightly controlled, honey bees receiving absolute conditioning demonstrated a capacity to learn a high-intensity stimulus significant from chance expectation demonstrating some capacity of plasticity for this dimension of colour perception. However, honey bees failed to learn low-intensity stimuli, and T. carbonaria was insensitive to stimulus intensity as a cue. These comparative findings suggest that there may be some common roots underpinning colour perception in bee pollinators and how they interact with flowers, although species-specific differences do exist.

Sublethal agrochemical exposures can alter honey bees' and Neotropical stingless bees' color preferences, respiration rates, and locomotory responses

Stingless bees such as Partamona helleri Friese play important roles in pollination of native plants and agricultural crops in the Neotropics. Global concerns about declining bee populations due to agrochemical pollutants have, however, been biased towards the honey bee, Apis mellifera Linnaeus. Here, we analysed the unintended effects of commercial formulations of a neonicotinoid insecticide, imidacloprid, and a fungicide mixture of thiophanate-methyl and chlorothalonil on color preference, respiration rates and group locomotory activities of both P. helleri and A. mellifera. Our results revealed that P. helleri foragers that were not exposed to pesticides changed their color preference during the course of a year. By contrast, we found that pesticide exposure altered the color preference of stingless bees in a concentration-dependent manner. In addition, imidacloprid decreased the overall locomotion of both bee species, whereas the fungicide mixture increased locomotion of only stingless bees. The fungicide mixture also reduced respiration rates of forager bees of both species. Forager bees of both species altered their color preference, but not their locomotory and respiration rates, when exposed to commercial formulations of each fungicidal mixture component (i.e., chlorothalonil and thiophanate-methyl). Our findings emphasize the importance of P. helleri as a model for Neotropical wild pollinator species in pesticide risk assessments, and also the critical importance of including groups of agrochemicals that are often considered to have minimal impact on pollinators, such as fungicides.

Color signals of bee-pollinated flowers: the significance of natural leaf background

PREMISE

Flower color is a primary pollinator attractant and generally adjusted to the cognitive system of the pollinators. The perception of flower color depends on the visual system of pollinators and also on environmental factors such as light conditions and the background against which flowers are displayed.

METHODS

Using bee-pollinated Fabaceae species as a model, we analyzed flower color diversity and compared flower color signals considering both the standard green and the natural leaf background of two tropical seasonally dry vegetations-a mountain rupestrian grassland (campo rupestre) and a woody savanna (cerrado)-compared to a nontropical Mediterranean shrubland.

RESULTS

By using natural background, bees discriminated color for 58% of the flowers in the campo rupestre and for only 43% in cerrado. Both vegetations were surpassed by 75% of bee color discrimination in Mediterranean vegetation. Chromatic contrast and purity were similar among the three vegetation types. Green contrast and brightness were similar between the tropical vegetations but differed from the Mediterranean shrubland. Green contrast differences were lost when using a standard green background, and most variables (purity, green contrast, and brightness) differed according to the background (natural or standard green) in all vegetations.

CONCLUSIONS

The natural background influenced bee perception of flower color regardless of vegetation. The background of the campo rupestre promoted green contrast for flowers, ensuring flower detection by pollinators and, along with bees, may also act as a selective pressure driving the diversity of flower colors in Fabaceae species. We highlight the importance of considering the natural background coloration when analyzing flower color signals.

What pollinators see does not match what they smell: Absence of color-fragrance association in the deceptive orchid Ionopsis utricularioides

Many deceptive orchids present variation in floral color and fragrance. This might be advantageous for the plant, as it can disturb the associative avoidance learning of pollinators, promoting more visits to the flowers. Some studies have shown that color and fragrance can be correlated in polymorphic deceptive orchids, but these studies employed color traits based on the human visual system and not the visual perception of pollinators. Thus, we investigated the composition of the floral fragrance of Ionopsis utricularioides (Sw.) Lindl., a polymorphic deceptive orchid, and analyzed possible correlations with the floral color as seen by bees, Apis mellifera L. and Melipona quadrifasciata Lepeletier, using the color hexagon model. We found high color and fragrance intraspecific variation, as expected for deceptive species. However, we found no color-fragrance association in individuals, either by comparing fragrance profiles with the color variable saturation or by comparing them with the placement of individuals in the color hexagon for both bee species. This lack of correlation contradicts the biochemical pathway hypothesis, which proposes that associations between floral color and scent in polymorphic flowers arise from shared biochemical pathways. However, a complete absence of correlation between floral signals is consistent with selection arising through pollinator cognitive ecology. Lack of correlation would increase the floral variability perceived by bees, given their multimodal learning, and this variability could disrupt avoidance learning of deceptive flowers, thus enhancing the efficacy of the plant's deceptive pollination mechanism.

A Comparative Study of Food Source Selection in Stingless Bees and Honeybees: Scent Marks, Location, or Color

In social bees, the choice of food sources is based on several factors, including scent marks, color, and location of flowers. Here, we used similar setups, in which two stingless bee species, Melipona subnitida and Plebeia flavocincta, and the Western honeybee, Apis mellifera, were tested regarding the importance of chemical cues, color cues, and location-dependent cues for foraging behavior. It was determined whether workers chose food sources according to (1) scent marks deposited by conspecifics, (2) the color hue of a food source, (3) the trained location or the proximity of a food source to the hive. All three species preferred the scent-marked over an unmarked feeder that was presented simultaneously, but M. subnitida showed a weaker preference compared to the other species. When trained to blue feeders all three bee species preferred blue, but A. mellifera showed the strongest fidelity. The training to yellow feeders led to less distinct color choices. Only workers of M. subnitida mostly orientated at the training position and the close proximity to the nest. Whether the distance of a feeding site influenced the choice was dependent on the tested parameter (color or scent marks) and the species. Workers of M. subnitida preferably visited the feeder closer to the nest during the scent mark trials, but choose randomly when tested for color learning. Worker honeybees preferred the closer feeding site if trained to yellow, but not if trained to blue, and preferred the more distant feeder during the scent mark trials. Workers of P. flavocincta preferred the closer feeder if trained to blue or yellow, and preferred the more distant feeder during the scent mark trials. The disparity among the species corresponds to differences in body size. Smaller bees are known for reduced visual capabilities and might rely less on visual parameters of the target such as color hue, saturation, or brightness but use scent cues instead. Moreover, the dim-light conditions in forest habitats might reduce the reliability of visual orientation as compared to olfactory orientation. Honeybees showed the most pronounced orientation at floral color cues.

High innate preference of black substrate in the chive gnat, Bradysia odoriphaga (Diptera: Sciaridae)

The chive gnat, Bradysia odoriphaga, is a notorious pest of Allium species in China. Colour trapping is an established method for monitoring and control of Bradysia species. In order to clarify the effect of colour preference of B. odoriphaga for the perched substrate, multiple-choice tests were used to assess the response of the chive gnat to different colour hues and brightness levels under different intensities of white illumination and two spectrally different illuminations. Given the choice among four colours differing in hue under different intensities of white illumination and two spectrally different illuminations, chive gnat adults significant preferred the black substrate, a lesser preference to brown and green substrates, and the least preference to orange substrate irrespective of illumination. Given the choice among four levels of brightness under the same illumination conditions as those in the previous experiment (different intensities of white illumination and two spectrally different illuminations), chive gnats preferred black substrate over dark grey, light grey and white substrates. Meanwhile, both virgin and mated adults significantly preferred black over other colour hues and brightness. Based on our results, we conclude that the chive gnat adults significantly prefer black substrates irrespective of colour hues and brightness. This behaviour does not alter with ambient light condition changes. No difference observed between choices of female and male adults. Our results provide new insight for understanding the colour choice behaviour in chive gnat and pave a way to improve monitoring and control of chive gnats and management.