Sweat Bee Research Articles

Floral biology of Nuphar pumila (Timm) DC. (Nymphaeaceae) in China

Anthesis in individual flowers of Nuphar pumila (Timm) DC. occurs for four consecutive days. First-day flowers are protogynous and functionally female. Flowers can open completely on the first day of anthesis. This contrasts with all previous reports, which state that first-day flowers of Nuphar are characterized by partial expansion of the calyx, leaving a distal small triangular opening just above the stigmatic disc. Flowers close completely on the first night of anthesis and remain partially open on the subsequent three nights. During the entire anthesis period the stigmas emit a sweet, fruity odor and the petal nectaries produce visible nectar drops. The stigma of N. pumila is secretory and unicellular-papillate. Pollen grains are monosulcate with long spines. Our observations on the mating system of N. pumila indicate that neither asexual seed production nor spontaneous self-pollination occurs. Cross-pollination of second-, third- and fourth-day flowers produced few seeds. Flowers of N. pumila were mainly pollinated by sweat bees, with flies playing a minor pollination role. No beetle visits were observed. Our insect-pollination observations substantiate the view that the relative contribution of flies, bees, and beetles to pollination in a single Nuphar population depends on two factors: the relative abundance of the insects, and presence of alternative food sources.

Ocellar optics in nocturnal and diurnal bees and wasps

Nocturnal bees, wasps and ants have considerably larger ocelli than their diurnal relatives, suggesting an active role in vision at night. In a first step to understanding what this role might be, the morphology and physiological optics of ocelli were investigated in three tropical rainforest species – the nocturnal sweat bee Megalopta genalis, the nocturnal paper wasp Apoica pallens and the diurnal paper wasp Polistes occidentalis – using hanging-drop techniques and standard histological methods. Ocellar image quality, in addition to lens focal length and back focal distance, was determined in all three species. During flight, the ocellar receptive fields of both nocturnal species are centred very dorsally, possibly in order to maximise sensitivity to the narrow dorsal field of light that enters through gaps in the rainforest canopy. Since all ocelli investigated had a slightly oval shape, images were found to be astigmatic: images formed by the major axis of the ocellus were located further from the proximal surface of the lens than images formed by the minor axis. Despite being astigmatic, images formed at either focal plane were reasonably sharp in all ocelli investigated. When compared to the position of the retina below the lens, measurements of back focal distance reveal that the ocelli of Megalopta are highly underfocused and unable to resolve spatial detail. This together with their very large and tightly packed rhabdoms suggests a role in making sensitive measurements of ambient light intensity. In contrast, the ocelli of the two wasps form images near the proximal boundary of the retina, suggesting the potential for modest resolving power. In light of these results, possible roles for ocelli in nocturnal bees and wasps are discussed, including the hypothesis that they might be involved in nocturnal homing and navigation, using two main cues: the spatial pattern of bright patches of daylight visible through the rainforest canopy, and compass information obtained from polarised skylight (from the setting sun or the moon) that penetrates these patches.

Is big the best? Queen size, usurpation and nest closure in a primitively eusocial sweat bee (Lasioglossum malachurum)

For primitively eusocial insects in which a single foundress establishes a nest at the start of the colony cycle, the solitary provisioning phase before first worker emergence represents a risky period when other, nestless foundresses may attempt to usurp the nest. In the primitively eusocial sweat bee Lasioglossum malachurum (Hymenoptera, Halictidae), spring foundresses compete for nests which are dug into hard soil. Nest-searching foundresses (‘floaters’) frequently inspected nests during this solitary phase and thereby exerted a usurpation pressure on resident queens. Usurpation has been hypothesised to increase across the solitary provisioning phase and favour closure of nests at an aggregation, marking the termination of the solitary provisioning phase by foundresses, before worker emergence. However, our experimental and observational data suggest that usurpation pressure may remain constant or even decrease across the solitary provisioning phase and therefore cannot explain nest closure before first worker emergence. Levels of aggression during encounters between residents and floaters were surprisingly low (9% of encounters across 2 years), and the outcome of confrontations was in favour of residents (resident maintains residency in 94% of encounters across 2 years). Residents were significantly larger than floaters. However, the relationship between queen size and offspring production, though positive, was not statistically significant. Size therefore seems to confer a considerable advantage to a queen during the solitary provisioning phase in terms of nest residency, but its importance in terms of worker production appears marginal. Factors other than intraspecific usurpation need to be invoked to explain the break in provisioning activity of a foundress before first worker emergence.

Characterization of microsatellite loci from the solitary sweat bees Lasioglossum leucozonium and Lasioglossum oenotherae (Hymenoptera, Halictidae)

AbstractMicrosatellite loci were isolated from two solitary sweat bees: the polylectic Lasioglossum leucozonium (10 loci) and the oligolectic Lasioglossum oenotherae (9 loci) (Hymenoptera, Halictidae). All loci were polymorphic with high observed heterozygosities (0.07–0.75 for L. leucozonium; 0.06–0.92 for L. oenotherae). These loci will be used to study the consequences of diet specialization on the population and conservation genetics of bees.

The evolution of activity breaks in the nest cycle of annual eusocial bees: a model of delayed exponential growth.

BackgroundSocial insects show considerable variability not only in social organisation but also in the temporal pattern of nest cycles. In annual eusocial sweat bees, nest cycles typically consist of a sequence of distinct phases of activity (queen or workers collect food, construct, and provision brood cells) and inactivity (nest is closed). Since the flight season is limited to the time of the year with sufficiently high temperatures and resource availability, every break reduces the potential for foraging and, thus, the productivity of a colony. This apparent waste of time has not gained much attention.ResultsWe present a model that explains the evolution of activity breaks by assuming differential mortality during active and inactive phases and a limited rate of development of larvae, both reasonable assumptions. The model predicts a systematic temporal structure of breaks at certain times in the season which increase the fitness of a colony. The predicted pattern of these breaks is in excellent accordance with field data on the nest cycle of the halictid Lasioglossum malachurum.ConclusionActivity breaks are a counter-intuitive outcome of varying mortality rates that maximise the reproductive output of primitively eusocial nests.

Social and ecological contexts of trophallaxis in facultatively social sweat bees, Megalopta genalis and M. ecuadoria (Hymenoptera, Halictidae)

Exchange of liquid food among adults (trophallaxis) is documented for the first time in New World sweat bees (Halictinae). Megalopta genalis and M. ecuadoria are facultatively social, and in social groups foragers regularly give food to the oldest resident female bee, which dominates social interactions. In turn, the oldest resident sometimes re-distributes this food, and shares it with younger foragers. Food is sometimes offered freely, but often the dominant bee exhibits escalating aggressive behavior until she is fed, whereupon she immediately ceases to be aggressive. The occurrence of trophallaxis in a species with mass-provisioned larvae provides an opportunity to examine the ritualization of social behavior. Trophallaxis also increases survivorship of males and females by almost 50% under experimental conditions, suggesting the behavior is also important in ecological contexts.

Visual summation in night-flying sweat bees: A theoretical study

Bees are predominantly diurnal; only a few groups fly at night. An evolutionary limitation that bees must overcome to inhabit dim environments is their eye type: bees possess apposition compound eyes, which are poorly suited to vision in dim light. Here, we theoretically examine how nocturnal bees Megalopta genalis fly at light levels usually reserved for insects bearing more sensitive superposition eyes. We find that neural summation should greatly increase M. genalis’s visual reliability. Predicted spatial summation closely matches the morphology of laminal neurons believed to mediate such summation. Improved reliability costs acuity, but dark adapted bees already suffer optical blurring, and summation further degrades vision only slightly.

It's good to be queen: classically eusocial colony structure and low worker fitness in an obligately social sweat bee

Lasioglossum malachurum, a bee species common across much of Europe, is obligately eusocial across its range but exhibits clear geographic variation in demography and social behaviour. This variation suggests that social interactions between queens and workers, opportunities for worker oviposition, and patterns of relatedness among nest mates may vary considerably, both within and among regions. In this study, we used three microsatellite loci with 12-18 alleles each to examine the sociogenetic structure of colonies from a population at Agios Nikolaos Monemvasias in southern Greece. These analyses reveal that the majority of colonies exhibit classical eusocial colony structure in which a single queen mated to a single male monopolizes oviposition. Nevertheless, we also detect low rates of multiqueen nest founding, occasional caste switching by worker-destined females, and worker oviposition of both gyne and male-producing eggs in the final brood. Previous evidence that the majority of workers show some ovarian development and a minority (17%) have at least one large oocyte contrasts with the observation that only 2-3% of gynes and males (the so-called reproductive brood) are produced by workers. An evaluation of the parameters of Hamilton's Rule suggests that queens benefit greatly from the help provided by workers but that workers achieve greater fitness by provisioning and laying their own eggs rather than by tending to the queen's eggs. This conflict of interest between the queen and her workers suggests that the discrepancy between potential and achieved worker oviposition is due to queen interference. Comparison of relatedness and maternity patterns in the Agios Nikolaos Monemvasias population with those from a northern population near Tübingen, Germany, points to a north-south cline of increasingly effective queen control of worker behaviour.

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Spatio-temporal variation of colony structure and eusociality level of the Japanese sweat bee Lasioglossum (Evylaeus) duplex (Hymenoptera: Halictidae)

Colony structure and eusociality level of the sweat bee Lasioglossum (Evylaeus) duplex were studied in 2001 in Sapporo and Assabu, Hokkaido, northern Japan. Sakagami and his colleagues had also studied this species in Sapporo in 1957–1968. Brood size, sex ratio and queen–worker size dimorphism were geographically and temporally variable, indicating spatio-temporal variation at the eusociality level. Inseminated workers constituted only 7.9% of the populations in 1957–1968 Sapporo but about 60% in 2001 in Sapporo and Assabu. A few of the inseminated workers were believed to leave natal nests for independent colony founding. Thus, partial bivoltinism is likely in this sweat bee species. The presence of workers with developed ovaries and/or corpora lutea suggests the occurrence of worker oviposition.

The evolution of nocturnal behaviour in sweat bees, Megalopta genalis and M. ecuadoria (Hymenoptera: Halictidae): an escape from competitors and enemies?

Evolutionary transitions to dim-light foraging (predawn matinal, crepuscular, nocturnal) have occurred repeatedly in bees, and may be associated with an escape from enemies or competitors. To date, however, little information has been available to test these hypotheses. Here we provide the first detailed information on the nesting behaviour of two species of Neotropical, nocturnal sweat bees, Megalopta genalis and M. ecuadoria (Hymenoptera: Halictidae). Females are facultatively social or solitary, and construct nests in dead wood. Nocturnal foraging behaviour is bimodal. Bees began foraging after sunset (∼18:30 h) and ceased foraging approximately 1 h later even though nocturnal flowers with pollen were still abundant; a second foraging bout occurred in the predawn morning, which began at ∼04:45 h and ended around sunrise (∼06:15 h) when diurnal-blooming flowers were abundant. Bees are capable of controlled flight in full light. They utilized pollen from both canopy and understory plant species, which have diurnal or nocturnal pollen anthesis. Megalopta nests are attacked by generalist predators such as ants, as well as the endoparasitic fly Melaloncha sp. nov. (Phoridae), the beetle Macrosaigon gracilis (Rhipophoridae), the parasitic wasp Lophostigma cincta (Mutillidae), and the brood parasite Megalopta byroni (Halictidae). Overall nest survivorship rates were comparable to those for diurnal relatives, but rates of cell parasitism for Megalopta (< < 5%) were substantially lower than they are for day-flying relatives, offering some support for the hypothesis that the evolution of nocturnal behaviour enables escape from natural enemies. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 83, 377–387.

Annual and Social Variation in Foraging Effort of the Obligately Eusocial Sweat Bee, Halictus ligatus (Hymenoptera: Halictidae)

Abstract Previous studies of the obligately eusocial sweat bee Halictus ligatus Say, at a nesting aggregation in Victoria, southern Ontario, Canada, showed how changes in local environmental conditions led to marked variation in demography, body size, and social patterns. Under the harsher weather conditions of 1990, mortality was higher, colony sizes were lower, brood body sizes were smaller, and surviving colonies were more strongly eusocial than under the milder conditions of 1991. These patterns suggested that the link between environmental conditions and colony social organization might be mediated by foraging effort, especially by queens during late spring and early summer when colonies were most susceptible to environmental constraints. Social parameters such as differences in reproductive effort between queens and workers or between haplometrotic and pleometrotic colonies, might also have a basis in differences in foraging effort. In this paper, I examine annual, seasonal, and social sources of va...

Pollination effects of the sweat bee Lasioglossum villosulum trichopse (Hymenoptera: Halictidae) on genic male-sterile lettuce

For development of an F1 hybrid of lettuce cultivars, an efficient pollinator must be found. To evaluate pollination effects of a candidate pollinator, Lasioglossum villosulum trichopse, the foraging behavior and pollination ability of this sweat bee were investigated on the flowers of male-fertile (MF) and genic male-sterile (GMS) lettuce. Females of the sweet bee visited three lettuce cultivars for a short time corresponding to each cultivar's full blooming time and showed unique foraging behavior on the MF lettuce flower. The flower heads thus visited by a single pollen-gathering female sweat bee indicated a seed set as high as that of self-pollinated flower heads on a fine day. On the GMS lettuce, the rate of seed set was significantly lower in the bee-pollinated flower heads than that of hand-pollinated flower heads which bloomed on fine days. The low seed set was considered to be associated with weather conditions during the experimental period. This is the first report of successful F1 hybrid seed production of lettuce using a pollinator insect.

Social polymorphism in the sweat bee Lasioglossum (Evylaeus) baleicum (Cockerell) (Hymenoptera, Halictidae) in Hokkaido, northern Japan

The life cycle and social behaviour of the sweat bee Lasioglossum (Evylaeus) baleicum (Cockerell) was investigated in two geographically separate populations in Hokkaido, northern Japan. Colonies were excavated throughout the brood rearing season from an aggregation in Nishioka forest park, Sapporo, and near Kawakita in eastern Hokkaido during 2000 and 2001. The Nishioka population produced two discrete broods during the year and was weakly eusocial; 57% of workers were mated and 28% exhibited some ovarian development, 12–16% of the first brood was male, and workers were on average 4.5% smaller than their respective queen. In contrast, the population at Kawakita was solitary, and produced a single brood per year with an unbiased sex ratio. In addition however, there were some solitary colonies in the Nishioka population and evidence of a partial second brood in some colonies at Kawakita, suggesting differences between the populations are not fixed and that this species is truly socially polymorphic. L. (E.) baleicum is a member of the fulvicorne species group, which includes other examples of social polymorphic species as well as solitary and eusocial species, though this is the only species of this group so far known to exhibit a solitary/non-delayed eusocial polymorphism. Recent studies suggest that social polymorphism has both genetic and environmental influences, raising questions as to the relative import of each.

Assured fitness returns favor sociality in a mass-provisioning sweat bee, Megalopta genalis (Hymenoptera: Halictidae)

Assured fitness returns models for the evolution of sociality emphasize the selective value of ensuring that offspring receive adequate parental care to reach maturity. If a member of a social group dies, it can accrue returns on investment in offspring through the efforts of surviving social partners. We provide evidence that in the mass-provisioning, facultatively social sweat bee Megalopta genalis, adult presence in the nest throughout brood development provides protection from ant predation. Nests with adults present were well protected, and brood in nests with adults removed suffered higher predation. Females in observation nests showed effective defensive behavior against experimentally introduced ants, and bees in natural nests repulsed naturally occurring ant raids. Megalopta nest architecture and behavior are such that the brood of several cooperating females can be defended with little additional cost relative to solitary nesting. The benefits of cooperative defense may favor group living in mass provisioning bees. Our observations and experiments suggest that parental care throughout brood development can be adaptive in mass provisioning species, supporting the predictions of assured fitness returns models.

Flower visitors of lettuce under field and enclosure conditions

Insects visiting lettuce flowers were investigated under field and enclosure conditions. In a 4-year field survey, visitors of 11 species belonging to 3 orders were observed. Most species visiting the flowers were sweat bees (Hymenoptera; Halictidae). The frequency (total days of visitation to lettuce flower/total days of flight by the insect) of visitations by the sweat bee Lasioglossum villosulum trichopse was obviously higher at 65.5% than those of other insects. Honeybees were not observed on the lettuce flowers. In a 5-year survey under enclosure conditions, 10 of 17 bee species reared and a hoverfly were observed to visit the lettuce flowers. The highest frequency of visitation was shown by L. villosulum trichopse (59.4%), followed by Andrena knuthi (19.2%) and Osmia cornifrons (8.6%). Most visitors were one-day foragers and classified as temporary nectar foragers and continual pollen (and nectar) gatherers. Continual nectar foraging was observed in only 3 species, L. villosulum trichopse, An. knuthi and Ceratina boninensis. Daily flight activities of L. villosulum trichopse and the Lactuceae oligolectic bee An. knuthi corresponded to the morning blooming time of lettuce. The sweat bee L. villosulum trichopse may be a pollinator for hybrid seed production of lettuce.

Predator recognition and evasive behavior by sweat bees, Lasioglossum umbripenne (Hymenoptera: Halictidae), in response to predation by ants, Ectatomma ruidum (Hymenoptera: Formicidae)

Ectatomma ruidum is an abundant soil-nesting Neotropical ant, which displays extensive behavioral flexibility during foraging activities. We studied here one unusual element of their behavioral repertoire: ambush predation. A worker of E. ruidum waits near a nest of a social sweat bee, Lasioglossum umbripenne, lunging at incoming bees, or less frequently, at departing bees. However, bees detected ambushing ants and modified their behavior. Dead ants placed at bees' nest entrances significantly decreased bee activity, indicating that bees recognized dead ants as potential predators. Neither sim- ple black models (square and rectangle) nor olfactory cues had any effect on overall bee activity. A returning bee usually approached her entrance and immediately entered, but if an ant was waiting at the nest, a bee was significantly more likely to abort the first approach flight and then to re-approach the nest on the side opposite the ant's position. As models became increasingly ant-like, returning bees more frequently aborted their first ap- proach flight, expressing other behaviors before entering nests. These behaviors included withdrawal followed by an approach from a different direction; zigzagging flights, either from a distance or close to the entrance or even a close inspection; landing a short distance from the nest, then approaching on foot or waiting for several sec- onds before entering. Ants responded with effective counter-behaviors. Behavioral flexibility in nest enter- ing/exiting by L. umbripenne and in hunting strategy by E. ruidum shows the complexity of this predator-prey re- lationship, and illustrates the importance of information processing by both species involved in determining the outcome of the interspecific interaction.

Complex sociogenetic organization and reproductive skew in a primitively eusocial sweat bee, Lasioglossum malachurum, as revealed by microsatellites.

The sweat bees (Family Halictidae) are a socially diverse taxon in which eusociality has arisen independently numerous times. The obligate, primitively eusocial Lasioglossum malachurum, distributed widely throughout Europe, has been considered the zenith of sociality within halictids. A single queen heads a colony of smaller daughter workers which, by mid-summer, produce new sexuals (males and gynes), of which only the mated gynes overwinter to found new colonies the following spring. We excavated successfully 18 nests during the worker- and gyne-producing phases of the colony cycle and analysed each nest's queen and either all workers or all gynes using highly variable microsatellite loci developed specifically for this species. Three important points arise from our analyses. First, queens are facultatively polyandrous (queen effective mating frequency: range 1-3, harmonic mean 1.13). Second, queens may head colonies containing unrelated individuals (n = 6 of 18 nests), most probably a consequence of colony usurpation during the early phase of the colony cycle before worker emergence. Third, nonqueen's workers may, but the queen's own workers do not, lay fertilized eggs in the presence of the queen that successfully develop into gynes, in agreement with so-called 'concession' models of reproductive skew.

Genetic variation within and among populations of an arctic/alpine sweat bee (Hymenoptera: Halictidae)

AbstractWe present the results of electrophoretic analyses of allozymes for eight population samples of the arctic/alpine sweat bee,Lasioglossum(Evylaeus)borealeSvensson, Sakagami and Ebmer. This holarctic species is found at high latitudes and at increasingly high elevations in the mountains of western North America as far south as southern Arizona. Our samples encompass a large proportion of the species' range; three samples are from Arizona, one from Utah, two from northern Canada, one from Mount Washington (the highest point in the eastern United States), and one from Sweden. Most samples had high levels of genetic variation compared with other bees, but the one from Sweden had low heterozygosity, suggesting that this location may have been comparatively recently colonized. The three northern North American samples were genetically similar despite the large geographic distances separating the localities (average &gt; 3000 km). In contrast, the southern United States samples were (with the exception of one pairwise estimate) genetically divergent despite the small geographic distances separating them (average &lt; 500 km). These results are consistent with earlier divergence among the southern populations, which are currently separated by regions of low elevation and inhospitable desert, than among the more northern ones. Although the data are not conclusive, they are suggestive of northward dispersal from refugia south of the ice sheets since the last glaciation.

Fossil bee nests, coleopteran pupal chambers and tuffaceous paleosols from the Late Cretaceous Laguna Palacios Formation, Central Patagonia (Argentina)

The Late Cretaceous Laguna Palacios Formation in Central Patagonia (San Jorge Basin), southern South America, is composed of tuffaceous deposits supplied by periodical volcanic ash falls partly reworked by rivers, on broad plains. Variations in ash-fall rates allowed the formation of stacked, mature paleosols, which are one of the most characteristic features of this formation. The mature paleosols show well-developed horizons, ped structure and bear an intricate network of trace fossils mostly produced by small roots and invertebrates. Two different insect trace fossils could be recognized in this formation: sweat bee nests and coleopteran pupal chambers. Fossil bee nests are composed of inclined tunnels with cells attached to them by means of short necks, a typical construction of bees of the subfamily Halictinae. Similar halictine constructions were reported from the Cretaceous of the USA. Coleopteran pupal chambers are discrete, ovoid structures, having an internal cavity with a smooth surface, and an outer wall of lumpy appearance composed of different layers of soil material. They are commonly constructed by the larvae of different families of Coleoptera. Similar trace fossils were previously reported from the Asencio Formation (Late Cretaceous–Early Tertiary) of Uruguay and from the Djadokhta Formation of Mongolia (Late Cretaceous). These trace fossils constitute some of the only paleontological data from the Laguna Palacios Formation, allowing inferences about its paleoecology, paleoclimatology and paleogeography. Ecological preferences of Halictinae, as well as some features of the nests, suggest a temperate, seasonal climate and an environment dominated by low vegetation for the Laguna Palacios Formation, which is also compatible with sedimentologic and pedogenic evidence. The morphology of the nests, typical of North American halictinae, adds more evidence to the hypothesis of the existence of faunal interchange between North and South America by the Late Cretaceous. The fossil nests constitute some of the oldest evidence of bees in the fossil record, the third known record of bees of Cretaceous age and the first for the Southern Hemisphere. The two traces described are, together with those of Dakota and the Gobi Desert, the only trace fossils from paleosols of Cretaceous age that can be certainly attributed to insects.