Sara D. Leonhardt

The same, but different: pollen foraging in honeybee and bumblebee colonies

Like many other bees worldwide, honeybees and bumblebees are threatened by human-induced disturbances. Yet some species decline and others thrive, likely due to different foraging strategies. As little is known about how resource intake translates into nutrients available to colonies, our study aimed to better understand how differences in foraging strategies may affect colony health by relating differences in pollen spectra collected to differences in nutrient composition. The Apis and Bombus colonies studied were all located at the same site, but nevertheless differed in the spectra of plant species visited for pollen collection and the quality of pollen collected. Bumblebees generally collected pollen with significantly higher pollen protein content and more essential amino acids. Unlike honeybees that tend to exploit large resource patches, bumblebees thus seem to select the “better” pollen and to focus on quality instead of quantity.

Ecology and Evolution of Communication in Social Insects

Insect life strategies comprise all levels of sociality from solitary to eusocial, in which individuals form persistent groups and divide labor. With increasing social complexity, the need to communicate a greater diversity of messages arose to coordinate division of labor, group cohesion, and concerted actions. Here we summarize the knowledge on prominent messages in social insects that inform about reproduction, group membership, resource locations, and threats and discuss potential evolutionary trajectories of each message in the context of social complexity.

Smelling like resin: terpenoids account for species-specific cuticular profiles in Southeast-Asian stingless bees.

Insects may be unique in having a cuticle with a species-specific chemical profile. In social insects, colony survival depends not only on species-specific but also on colony-specific cuticular compounds with hydrocarbons playing an important role in the communication systems of ants, termites, wasps and bees. We investigated inter- and intraspecific differences in the composition of compounds found on the body surface of seven paleotropical stingless bee species (Apidae: Meliponini) at two different sites in Borneo (Sabah, Malaysia). Besides hydrocarbons, the body surface of all seven stingless bee species comprised terpenoid compounds, a substance class that has not been reported for chemical profiles of any social insect so far. Moreover, the chemical profile of some species differed fundamentally in the composition of terpenoids with one group (e.g. sesquiterpenes) being present in one species, but missing in another. Chemical profiles of different colonies from the same species showed the same hydrocarbon- and terpenoid compounds over different regions, as tested for Tetragonilla collina and Tetragonula melanocephala. However, chemical profiles differed quantitatively between the different colonies especially in T. melanocephala. It is likely that the terpenoids are derived from plant resins because stingless bees are known to collect and use large amounts of resins for nest construction and defence, suggesting an environmental origin of the terpenoids in the chemical profile of paleotropical stingless bees.

How to know which food is good for you: bumblebees use taste to discriminate between different concentrations of food differing in nutrient content

ABSTRACT In view of the ongoing pollinator decline, the role of nutrition in bee health has received increasing attention. Bees obtain fat, carbohydrates and protein from pollen and nectar. As both excessive and deficient amounts of these macronutrients are detrimental, bees would benefit from assessing food quality to guarantee an optimal nutrient supply. While bees can detect sucrose and use it to assess nectar quality, it is unknown whether they can assess the macronutrient content of pollen. Previous studies have shown that bees preferentially collect pollen of higher protein content, suggesting that differences in pollen quality can be detected either by individual bees or via feedback from larvae. In this study, we examined whether and, if so, how individuals of the buff-tailed bumblebee (Bombus terrestris) discriminate between different concentrations of pollen and casein mixtures and thus nutrients. Bumblebees were trained using absolute and differential conditioning of the proboscis extension response (PER). As cues related to nutrient concentration could theoretically be perceived by either smell or taste, bees were tested on both olfactory and, for the first time, chemotactile perception. Using olfactory cues, bumblebees learned and discriminated between different pollen types and casein, but were unable to discriminate between different concentrations of these substances. However, when they touched the substances with their antennae, using chemotactile cues, they could also discriminate between different concentrations. Bumblebees are therefore able to discriminate between foods of different concentrations using contact chemosensory perception (taste). This ability may enable them to individually regulate the nutrient intake of their colonies. Highlighted Article: Bumblebee workers are able to discriminate different concentrations of a food mixture and hence nutrients by using their sense of taste, which may enable them to individually regulate food intake.

Diversity matters: how bees benefit from different resin sources

Biodiverse environments provide a variety of resources that can be exploited by consumers. While many studies revealed a positive correlation between biodiversity and consumer biomass and richness, only few studies have investigated how resource diversity affects single consumers. To better understand whether a single consumer species benefits from diverse resources, we tested how the protective function of a defensive plant resource (i.e. resin exploited by social bees) varied among different sources and target organisms (predators, parasites and pathogens). To assess synergistic effects, resins from different plant genera were tested separately and in combination. We found that resin diversity is beneficial for bees, with its functional properties depending on the target organisms, type and composition of resin. Different resins showed different effects, and mixtures were more effective than some of the single resins (functional complementarity). We conclude that resins of different plant species target different organisms and act synergistically where combined. Bees that rely on resin for protection benefit more when they have access to diverse resin sources. Loss of biodiversity may in turn destabilize consumer populations due to restricted access to a variety of resources.

Foraging loads of stingless bees and utilisation of stored nectar for pollen harvesting

We compared nectar, pollen and resin loads of individual workers among colonies from six Trigona species in Sabah, Borneo. Individual bees rarely collected large amounts of both nectar and pollen during the same foraging trip. Instead, comparison of crop contents across departing, flower-visiting, and returning bees suggests that pollen-collecting workers often carried highly concentrated nectar in their crop upon nest departure. During their foraging trip, this crop nectar volume decreased progressively until crops were largely empty when they returned to their nest. Individually marked pollen foragers carried highly concentrated nectar when they left their nest, while crops and corbiculae from marked nectar foragers were empty upon departure. We suggest that a large proportion of previously stored and highly concentrated nectar may be required for pollen adhesion to corbiculae and/or serve as fuel during foraging on nectar-poor flowers.ZusammenfassungIn dieser Studie wurde das individuelle Sammelverhalten von Arbeiterinnen bei sechs Arten der Gattung Trigona (Apidae, Meliponinae) in einem Tieflandregenwald Borneos (Sabah, Malaysia) untersucht. Von Honigbienen und Hummeln ist bekannt, dass Nektar und Pollen teilweise während desselben individuellen Sammelflugs geerntet werden. Das Vorhandensein einer solchen „Mischsammelstrategie” wurde jedoch bei Stachellosen Bienen bislang nicht genauer untersucht. Daher wurden quantitative Daten zu Nektar-, Pollen- und Harzmengen aufgenommen und verglichen zwischen aus dem Nest ausfliegenden Arbeiterinnen, solchen an Blüten, und bei ins Nest zurückkehrenden Arbeiterinnen. Alle untersuchten Kolonien trugen Nektar, Pollen und Harz ein (Abb. 1a). Ein Viertel der Arbeiterinnen, die Pollen eintrugen, wiesen kleine Nektarmengen in ihren Honigmägen auf. Bienen mit großen Nektarmengen trugen dagegen keinen oder nur sehr geringe Mengen Pollen ein. Folglich gibt es nur einen geringen Anteil potentieller „Mischsammlerinnen“. Der Nektar in den Honigmägen von Pollensammlerinnen wird vermutlich selten während desselben Ausflugs an Blüten gesammelt, sondern stellt einen Vorrat dar, der bereits aus dem Nest mitgenommen wurde. Bei den sechs Arten hatten zwischen 10 % und 70 % der ausfliegenden Bienen Nektar im Honigmagen (Abb. 1b). Diese Nektarfüllung war signifikant kleiner als bei heimkehrenden Nestgenossen, die allein Nektar eintrugen (Tab. II). Bei den meisten Pflanzenarten konnte Trigona lediglich beim Sammeln von Pollen, nicht aber Nektar beobachtet werden (11 von 14 Pflanzenarten, Tab. I). Jedoch konnte bei diesen Arbeiterinnen bis zu 1 μL hoch konzentrierten Nektars im Honigmagen nachgewiesen werden. Das Volumen dieses Nektarvorrats verringert sich offenbar während des Sammelflugs mit zunehmender Pollenladung (Abb. 2). Um die individuellen Strategien bei Trigona melanocephala genauer zu untersuchen, markierten wir einzelne zum Nest heimkehrende Pollen- und Nektarsammlerinnen und fingen diese beim Verlassen des Nests wieder ab. Alle vorherigen Pollensammlerinnen trugen hochkonzentrierten Nektar in ihren Honigmägen, jedoch keine der Nektarsammlerinnen. Diese Befunde stützen daher die Hypothese, dass Nektar aus dem Nest gezielt für das Pollensammeln verwendet wird. Nektar wird entweder für metabolische Kosten beim Pollensammeln oder zur Befestigung der Pollenladungen an den Körbchen verwendet. Arbeiterinnen Stachelloser Bienen spezialisierten sich häufig auf den Eintrag von Pollen oder Nektar. Mischsammelstrategien sind dagegen die Ausnahme. Unsere Untersuchung zeigt, dass Rückschlüsse über Mischsammelstrategien problematisch sind, die allein auf Beobachtungen heimkehrender oder fouragierender Bienen basieren, und ein quantitativer Vergleich mit ausfliegenden Bienen notwendig ist.

Sight or scent: lemur sensory reliance in detecting food quality varies with feeding ecology.

Visual and olfactory cues provide important information to foragers, yet we know little about species differences in sensory reliance during food selection. In a series of experimental foraging studies, we examined the relative reliance on vision versus olfaction in three diurnal, primate species with diverse feeding ecologies, including folivorous Coquerels sifakas (Propithecus coquereli), frugivorous ruffed lemurs (Varecia variegata spp), and generalist ring-tailed lemurs (Lemur catta). We used animals with known color-vision status and foods for which different maturation stages (and hence quality) produce distinct visual and olfactory cues (the latter determined chemically). We first showed that lemurs preferentially selected high-quality foods over low-quality foods when visual and olfactory cues were simultaneously available for both food types. Next, using a novel apparatus in a series of discrimination trials, we either manipulated food quality (while holding sensory cues constant) or manipulated sensory cues (while holding food quality constant). Among our study subjects that showed relatively strong preferences for high-quality foods, folivores required both sensory cues combined to reliably identify their preferred foods, whereas generalists could identify their preferred foods using either cue alone, and frugivores could identify their preferred foods using olfactory, but not visual, cues alone. Moreover, when only high-quality foods were available, folivores and generalists used visual rather than olfactory cues to select food, whereas frugivores used both cue types equally. Lastly, individuals in all three of the study species predominantly relied on sight when choosing between low-quality foods, but species differed in the strength of their sensory biases. Our results generally emphasize visual over olfactory reliance in foraging lemurs, but we suggest that the relative sensory reliance of animals may vary with their feeding ecology.

Plant diversity and composition compensate for negative effects of urbanization on foraging bumble bees

Bumble bees play an important role as pollinators of many crop plants and wild flowers. As in many wild bees, their abundance and diversity have declined in recent years, which may threaten the stability of pollination services. The observed decline is often linked with the loss or alteration of natural habitat, e.g., through urbanization, the conversion of natural habitat into largely sealed areas (concrete) inhabited by humans. The effects of urbanization on bumble bees remain as yet controversial with both positive and negative effects reported. We investigated how habitat isolation through increasing areas of concrete, as well as the diversity, abundance, and community composition of floral resources, determine bumble bee abundance and diversity in cities. We found plant species diversity and abundance to be more important than the amount of concrete in driving the abundance and species richness of common bumble bees in a German city. Moreover, plant species composition, i.e., the presence of specific plant species and families (e.g., Fabaceae), played a prominent role. In particular, flower-rich parks and gardens can offer a continuous food supply for bumble bees and attract bumble bee foragers even to isolated patches in the city center.

Stingless bees use terpenes as olfactory cues to find resin sources.

Insects largely rely on olfactory cues when seeking and judging information on nests, partners, or resources. Bees are known to use volatile compounds-besides visual cues-to find flowers suitable for pollen and nectar collection. Tropical stingless bees additionally collect large amounts of plant resins for nest construction, nest maintenance, nest defense, and to derive chemical constituents for their cuticular profiles. We here demonstrate that stingless bees of Borneo also use olfactory cues to find tree resins. They rely on volatile mono- and sesquiterpenes to locate or recognize known resin sources. Moreover, by modifying resin extracts, we found that stingless bees do not use the entire resin bouquet but relative proportions of several terpenes. In doing so, the bees are able to learn specific tree resin profiles and distinguish between tree species and partly even tree individuals.

Terpenoids tame aggressors: role of chemicals in stingless bee communal nesting

Social insects aggressively defend their nest and surrounding against non-nestmates, which they recognize by an unfamiliar profile of aliphatic hydrocarbons on the cuticle. Prominent exceptions are communal nest aggregations of stingless bees. Stingless bees (Apidae: Meliponini) are also unique in possessing cuticular terpenes which are derived from tree resins and have not yet been reported for any other insect. We showed experimentally that sesquiterpenes from the body surface of the communal nesting bee Tetragonilla collina reduced aggression in otherwise aggressive bees which did not have sesquiterpenes themselves. In the field, bee species nesting in aggregations with T. collina often lack sesquiterpenes in their own cuticular profiles. These species show little aggression towards T. collina, whereas it can be heavily attacked by non-aggregated species that also possess cuticular sesquiterpenes. We conclude that appeasement by sesquiterpenes represents a novel mechanism to achieve interspecific tolerance in social insects.