Interactions with Combined Chemical Cues Inform Harvester Ant Foragers' Decisions to Leave the Nest in Search of Food

Social insect colonies operate without central control or any global assessment of what needs to be done by workers. Colony organization arises from the responses of individuals to local cues. Red harvester ants (Pogonomyrmex barbatus) regulate foraging using interactions between returning and outgoing foragers. The rate at which foragers return with seeds, a measure of food availability, sets the rate at which outgoing foragers leave the nest on foraging trips. We used mimics to test whether outgoing foragers inside the nest respond to the odor of food, oleic acid, the odor of the forager itself, cuticular hydrocarbons, or a combination of both with increased foraging activity. We compared foraging activity, the rate at which foragers passed a line on a trail, before and after the addition of mimics. The combination of both odors, those of food and of foragers, is required to stimulate foraging. The addition of blank mimics, mimics coated with food odor alone, or mimics coated with forager odor alone did not increase foraging activity. We compared the rates at which foragers inside the nest interacted with other ants, blank mimics, and mimics coated with a combination of food and forager odor. Foragers inside the nest interacted more with mimics coated with combined forager/seed odors than with blank mimics, and these interactions had the same effect as those with other foragers. Outgoing foragers inside the nest entrance are stimulated to leave the nest in search of food by interacting with foragers returning with seeds. By using the combined odors of forager cuticular hydrocarbons and of seeds, the colony captures precise information, on the timescale of seconds, about the current availability of food.     

Olfactory learning in the stingless bee Tetragonisca angustula (Hymenoptera, Apidae, Meliponini)

Tetragonisca angustula stingless bees are considered as solitary foragers that lack specific communication strategies. In their orientation towards a food source, these social bees use chemical cues left by co-specifics and the information obtained in previous foraging trips by the association of visual stimuli with the food reward. Here, we investigated their ability to learn the association between odors and reward (sugar solution) and the effect on learning of previous encounters with scented food either inside the hive or during foraging. During food choice experiments, when the odor associated with the food was encountered at the feeding site, the bees’ choice is biased to the same odor afterwards. The same was not the case when scented food was placed inside the nest. We also performed a differential olfactory conditioning of proboscis extension response with this species for the first time. Inexperienced bees did not show significant discrimination levels. However, when they had had already interacted with scented food inside the hive, they were able to learn the association with a specific odor. Possible olfactory information circulation inside the hive and its use in their foraging strategies is discussed.     

Bumble bee olfactory information flow and contact-based foraging activation

Nestmate foraging activation and interspecific variation in foraging activation is poorly understood in bumble bees, as compared to honey bees and stingless bees. We therefore investigated olfactory information flow and foraging activation in the New World bumble bee species, Bombus impatiens. We (1) tested the ability of foragers to associate forager-deposited odor marks with rewarding food, (2) determined whether potential foragers will seek out the food odor brought back by a successful forager, and (3) examined the role of intranidal tactile contacts in foraging activation. Bees learned to associate forager-deposited odor marks with rewarding food. They were significantly more attracted to an empty previously rewarding feeder presented at a random position within an array of eight previously non-rewarding feeders. However, foragers did not exhibit overall odor specificity for short-term, daily floral shifts. For two out of three tested scents, activated foragers did not significantly prefer the feeder providing the same scent as that brought back by a successful forager. Finally, bees contacted by the successful forager inside the nest were significantly more likely to leave the nest to forage (38.6% increase in attempts to feed from empty feeders) than were non-contacted bees. This is the first demonstration that tactile contact, a hypothesized evolutionary basal communication mechanism in the social corbiculate bees, is involved in bumble bee foraging activation. Received 4 September 2007; revised 30 May 2008; accepted 15 July 2008.     

The scent of the waggle dance

The waggle dance of honey bee (Apis mellifera L.) foragers communicates to nest mates the location of a profitable food source. We used solid-phase microextraction and gas chromatography coupled with mass spectrometry to show that waggle-dancing bees produce and release two alkanes, tricosane and pentacosane, and two alkenes, Z-(9)-tricosene and Z-(9)-pentacosene, onto their abdomens and into the air. Nondancing foragers returning from the same food source produce these substances in only minute quantities. Injection of the scent significantly affects worker behavior by increasing the number of bees that exit the hive. The results of this study suggest that these compounds are semiochemicals involved in worker recruitment. By showing that honey bee waggle dancers produce and release behaviorally active chemicals, this study reveals a new dimension in the organization of honey bee foraging.     

Introduction of a scented carbohydrate resource into the nest increases departure rate in Polybia occidentalis

Social wasps do not possess a sophisticated, signal-based mechanism for recruiting foragers to food resources. Instead, in some species na?ve potential foragers use cues, specifically the scent of a resource obtained from successful foragers, to help locate the resource in the field. Prior studies have focused on the effectiveness of this mechanism on increasing the number of foragers at an artificial resource in the field; the increase is typically modest. Here, we focus on the activity at the nest in Polybia occidentalis, a tropical social wasp, and quantify the magnitude of the effect an influx of a known amount of scented carbohydrate solution added directly to the nest has on activating foragers to leave the nest in search of the resource. Under our experimental conditions, adding a scented 2.0 M sucrose solution to the nest doubled the average rate of departure. No increase occurred when the same amount of water was added as a control. This mass activation of foragers may give colonies of this species a competitive edge by enhancing their ability to rapidly exploit new resources.     

Differential foraging in presence of predator and conspecific odors in bank voles: a field enclosure study

Bank voles detect and discriminate other organisms, e.g., predators versus conspecifics, based on olfactory clues like urine, feces, or scent marks. The present field enclosure experiment was set up to determine whether scented food is sufficient for inducing such differentiated reactions in bank voles. Fourteen different odors were tested, divided into four categories: (1) terrestrial predators, (2) avian predators, (3) conspecifics, and (4) neutral origin (human, dog, solvent-treated, and unscented). The odors were extracted with methanol from pellets or feces, or were an artificial substitute. To test the influence of odors on the foraging behavior of voles, artificial food patches containing scented feeders were used. The feeders were cleaned and refilled daily with a calibrated amount of food sprayed with 1 ml of a different odor randomly chosen. The patches were also equipped with monochromatic cameras to monitor the occurrences of voles around the feeders. Both the visits at scented feeders and food consumption were analyzed for each odor. The results show that relative to their foraging activity on unscented food, bank voles significantly reduce it on food scented with odors of terrestrial predators, and increase it in the presence of conspecific odors. Their foraging activity was not affected by neutral scent, nor by scents from avian predators. These results prove that bank voles react to scented food in a way that varies according to the source of odor, even in the semi-natural conditions of an outdoor enclosure.     

Circadian Activity Rhythm of the Foragers of a Eusocial Bee (Scaptotrigona aff depilis,Hymenoptera, Apidae,Meliponinae) outside the Nest

In all bee colonies of the Meliponinae subfamily, activity inside the nest is temporally organized around the oviposition by the queen, assisted by nurse bees. This class is constituted by young bees that remain inside the nest. In a colony of Scaptotrigona aff depilis, the oviposition cycle occurs in a 3-hour period. The foragers are older bees that collect food for the colony in the field. Other tasks in the nest are performed by workers of ages intermediate between nurses and foragers. With the aim of studying activity rhythms, foragers were kept under constant light, with food constantly available and no flight restriction. The results showed that, although inside the nest the prevailing period is 3 hours, the activity of the foragers is a circadian rhythm, synchronized by the light/dark cycle and probably influenced by other environmental cycles as temperature and the availability of food sources.     

Odor information transfer in the stingless bee<Emphasis Type="Italic"> Melipona quadrifasciata</Emphasis>: effect of in-hive experiences on classical conditioning of proboscis extension

A recent study showed that the stingless bee Melipona quadrifasciata could learn to discriminate odors in a classical conditioning of proboscis extension response (PER). Here we used this protocol to investigate the ability of these bees to use olfactory information obtained within the colony in an experimental context: the PER paradigm. We compared their success in solving a classical differential conditioning depending on the previous olfactory experiences received inside the nest. We found that M. quadrifasciata bees are capable of transferring the food-odor information acquired in the colony to a differential conditioning in the PER paradigm. Bees attained higher discrimination levels when they had previously encountered the rewarded odor associated to food inside the hive. The increase in the discrimination levels, however, was in some cases unspecific to the odor used indicating a certain degree of generalization. The influence of the food scent offered at a field feeder 24 h before the classical conditioning could also be seen in the discrimination attained by the foragers in the PER setup, detecting the presence of long-term memory. Moreover, the improved performance of recruited bees in the PER paradigm suggests the occurrence of social learning of nectar scents inside the stingless bees’ hives.     

Effectiveness of recruitment behavior in stingless bees (Apidae,Meliponini)

Summary We examined the ability of stingless bees to recruit nest mates to a food source (i) in group foraging species laying pheromone trails from the food to the nest (Trigona recursa Smith, T. hypogea Silvestri, Scaptotrigona depilis Moure), (ii) in solitary foraging species with possible but still doubtful communication of food location inside the nest (Melipona seminigra Friese, M. favosa orbignyi Guérin), and (iii) in species with a less precise (Nannotrigona testaceicornis Lep., Tetragona clavipes Fab.) or no communication (Frieseomelitta varia Lep.). The bees were allowed to collect food (sugar solution or liver in the necrophageous species) ad libitum and the forager number to accumulate, as it would do under normal unrestrained conditions. The median number of bees collecting differed considerably among the species (1.0–1436.5). It was highest in the species employing scent trails. The time course of recruitment was characteristic for most of the species and largely independent of the number of foragers involved. The two Melipona species recruited other bees significantly faster than T. recursa, S. depilis, and N. testaceicornis during the first 10 to 30 minutes of an experiment. In species laying a scent trail to guide nestmates to a food source the first recruits appeared with a delay of several minutes followed by a quick increase in forager number. The median time required to recruit all foragers available differed among the species between 95.0 and 240.0 min. These differences can at least partly be explained by differences in the recruitment mechanisms and do not simply follow from differences in colony biomass.     

How long will honey bees (<Emphasis Type="Italic">Apis mellifera</Emphasis> L.) be stimulated by scent to revisit past-profitable forage sites?

Honey bees utilise floral food sources that vary temporally in their relative and absolute quality. Via a sophisticated colony organisation, a honey bee colony allocates its foragers such that the colony focuses on the most profitable forage sites while keeping track of changes within its foraging environment. One important mechanism of the allocation of foragers is the ability of experienced foragers to revisit past-profitable forage sites after a period of temporary dearth caused by, for example, inclement weather. The scent of past-profitable forage within the colony brought back by other foragers is sufficient to reactivate these experienced foragers. Here I determine for how long bees react to the scent of a past-profitable forage site. I show that the ability of foragers to revisit the location of a past-profitable food source diminishes rapidly over a period of 10 days, until no forager reacts to the cue (scent). I discuss the implications of these findings with respect to the colony’s ability to react rapidly to changing foraging conditions.     

Recruitment in starved nests: the role of direct and indirect interactions between scouts and nestmates in the ant <Emphasis Type="Italic">Lasius niger</Emphasis>

In social insects, the foraging activity usually increases with the length of food deprivation. In Lasius niger, a mass-recruiting ant species, the foraging adjustment to the level of food deprivation is regulated by the scout that fed at the food source and by the response of the nestmates to signals performed by the scout inside the nest. In this study, we look at the role of these direct interactions (antennations or trophallaxes) and indirect interactions (pheromonal emission) and how they are influenced by the level of food deprivation. At the beginning of recruitment, the relative number of nestmates leaving the nest to forage increases with the level of deprivation. The nestmates’ propensity to exit the nest is not influenced by a previous trophallactic and/or antennal contact with a scout. Our results strongly suggest that the exit of nestmates is triggered by a chemical signal emitted by a scout. Deprivation lowers the response threshold of nestmates to this chemical signal resulting in a more important exit from the nest. Surprisingly, 27% of starved nestmates that receive food from the scout relay the information by depositing a chemical signal before having discovered and drunk the food source. Both phenomena boost the recruitment process. Though successful foragers returning to the nest have a significant role in the recruitment to the food source, we observed that the response of the nestmates inside the nest also greatly influence regulation of the foraging activity.     

The Regulation of Ant Colony Foraging Activity without Spatial Information

Many dynamical networks, such as the ones that produce the collective behavior of social insects, operate without any central control, instead arising from local interactions among individuals. A well-studied example is the formation of recruitment trails in ant colonies, but many ant species do not use pheromone trails. We present a model of the regulation of foraging by harvester ant (Pogonomyrmex barbatus) colonies. This species forages for scattered seeds that one ant can retrieve on its own, so there is no need for spatial information such as pheromone trails that lead ants to specific locations. Previous work shows that colony foraging activity, the rate at which ants go out to search individually for seeds, is regulated in response to current food availability throughout the colony's foraging area. Ants use the rate of brief antennal contacts inside the nest between foragers returning with food and outgoing foragers available to leave the nest on the next foraging trip. Here we present a feedback-based algorithm that captures the main features of data from field experiments in which the rate of returning foragers was manipulated. The algorithm draws on our finding that the distribution of intervals between successive ants returning to the nest is a Poisson process. We fitted the parameter that estimates the effect of each returning forager on the rate at which outgoing foragers leave the nest. We found that correlations between observed rates of returning foragers and simulated rates of outgoing foragers, using our model, were similar to those in the data. Our simple stochastic model shows how the regulation of ant colony foraging can operate without spatial information, describing a process at the level of individual ants that predicts the overall foraging activity of the colony.     

Extracting the Behaviorally Relevant Stimulus: Unique Neural Representation of Farnesol,a Component of the Recruitment Pheromone of Bombus terrestris

To trigger innate behavior, sensory neural networks are pre-tuned to extract biologically relevant stimuli. Many male-female or insect-plant interactions depend on this phenomenon. Especially communication among individuals within social groups depends on innate behaviors. One example is the efficient recruitment of nest mates by successful bumblebee foragers. Returning foragers release a recruitment pheromone in the nest while they perform a ‘dance’ behavior to activate unemployed nest mates. A major component of this pheromone is the sesquiterpenoid farnesol. How farnesol is processed and perceived by the olfactory system, has not yet been identified. It is much likely that processing farnesol involves an innate mechanism for the extraction of relevant information to trigger a fast and reliable behavioral response. To test this hypothesis, we used population response analyses of 100 antennal lobe (AL) neurons recorded in alive bumblebee workers under repeated stimulation with four behaviorally different, but chemically related odorants (geraniol, citronellol, citronellal and farnesol). The analysis identified a unique neural representation of the recruitment pheromone component compared to the other odorants that are predominantly emitted by flowers. The farnesol induced population activity in the AL allowed a reliable separation of farnesol from all other chemically related odor stimuli we tested. We conclude that the farnesol induced population activity may reflect a predetermined representation within the AL-neural network allowing efficient and fast extraction of a behaviorally relevant stimulus. Furthermore, the results show that population response analyses of multiple single AL-units may provide a powerful tool to identify distinct representations of behaviorally relevant odors.     

The hidden cost of information in collective foraging

Many animals nest or roost colonially. At the start of a potential foraging period, they may set out independently or await information from returning foragers. When should such individuals act independently and when should they wait for information? In a social insect colony, for example, information transfer may greatly increase a recruit's probability of finding food, and it is commonly assumed that this will always increase the colony's net energy gain. We test this assumption with a mathematical model. Energy gain by a colony is a function both of the probability of finding food sources and of the duration of their availability. A key factor is the ratio of pro-active foragers to re-active foragers. When leaving the nest, pro-active foragers search for food independently, whereas re-active foragers rely on information from successful foragers to find food. Under certain conditions, the optimum strategy is totally independent (pro-active) foraging because potentially valuable information that re-active foragers may gain from successful foragers is not worth waiting for. This counter-intuitive outcome is remarkably robust over a wide range of parameters. It occurs because food sources are only available for a limited period. Our study emphasizes the importance of time constraints and the analysis of dynamics, not just steady states, to understand social insect foraging.     

Bumble bees (Bombus terrestris) store both food and information in honeypots

Social insect foragers often transmit information about foodsources to nest mates. In bumble bees (Bombus terrestris), forexample, successful foragers use excited motor displays anda pheromone as communication signals. In addition, bees couldmake use of an indirect pathway of information flow, via thehoney stores. We show here that, indeed, bees in the nest continuouslymonitor honeypots and sample their contents, thus obtaininginformation on supply and demand of nectar. When there is aninflux of nectar into the nest, the colony deploys more workersfor foraging. The number of new foragers depends on sugar concentration.Foragers returning with high-quality sugar solution displaymore "excited runs" on the nest structure. The recruits' response,however, does not depend on modulated behavior by foragers:more workers start to forage with high quality of incoming nectar,even when this nectar is brought by a pipette. Moreover, weshow that the readiness of bees to respond to recruitment signalsor incoming nectar also depends on colony demand. When colonynectar stores are full, the response of bees to equal amountsof nectar influx is smaller than when stores are empty. Whencolony nectar stores are depleted, foragers spend more timerunning excitedly and less time probing pots in the nest andrun with higher average speed, possibly to disperse the alertingpheromone more efficiently. However, more bees respond to nectarinflux to empty stores, whether or not this is accompanied byforager signals. Thus, honeypots serve to store informationas well as food.     

Stingless bees (<Emphasis Type="Italic">Scaptotrigona pectoralis</Emphasis>) learn foreign trail pheromones and use them to find food

Foragers of several species of stingless bees (Hymenoptera, Apidae and Meliponini) deposit pheromone marks in the vegetation to guide nestmates to new food sources. These pheromones are produced in the labial glands and are nest and species specific. Thus, an important question is how recruited foragers recognize their nestmates’ pheromone in the field. We tested whether naïve workers learn a specific trail pheromone composition while being recruited by nestmates inside the hive in the species Scaptotrigona pectoralis. We installed artificial scent trails branching off from trails deposited by recruiting foragers and registered whether newly recruited bees follow these trails. The artificial trails were baited with trail pheromones of workers collected from foreign S. pectoralis colonies. When the same foreign trail pheromone was presented inside the experimental hives while recruitment took place a significant higher number of bees followed the artificial trails than in experiments without intranidal presentation. Our results demonstrate that recruits of S. pectoralis can learn the composition of specific trail pheromone bouquets inside the nest and subsequently follow this pheromone in the field. We, therefore, suggest that trail pheromone recognition in S. pectoralis is based on a flexible learning process rather than being a genetically fixed behaviour.     

Body Size Shapes Caste Expression,and Cleptoparasitism Reduces Body Size in the Facultatively Eusocial Bees <Emphasis Type="Italic">Megalopta</Emphasis> (Hymenoptera: Halictidae)

We used the facultatively social sweat bee Megalopta genalis (Halictidae) to test whether body size is associated with social caste. Behavioral observations showed that non-reproductive foragers were significantly smaller than reproductive nest mate queens, and foragers were also smaller than presumed pre-dispersal reproductives. Moreover, among females from field-collected nests without behavioral observations, relative body size correlated with relative ovary size. Reproductive status is not a direct result of body size, as body size was not significantly associated with either ovary size or fecundity among both solitary and social reproductives. Reproductive status is apparently an outcome of social competition for reproductive dominance, and status is influenced by size relative to nest mates. Our study is the first to demonstrate an association of body size with caste expression in a facultatively social species with relatively weak seasonal constraints on independent nesting. Larvae of a parasitic fly (Fiebrigella sp., Chloropidae) consume pollen provisions stored in nest cells of M. genalis and M. ecuadoria. We tested whether fly parasitism of M. genalis reduces body size. Parasitized females are significantly smaller as adults than their unparasitized nestmates. This reduction is of a similar magnitude to the size differences between castes, and has the potential to shape host reproductive options by influencing competition with nest mates. We present data on the prevalence of parasitism from four collections of M. genalis and two collections of M. ecuadoria from Barro Colorado Island, Panama, and La Selva, Costa Rica.     

Crop scents affect the occurrence of trophallaxis among forager honeybees

Previous evidence indicates that the recognition of the nectar delivered by forager honeybees within the colony may have been a primitive method of communication on food resources. Thus, the association between scent and reward that nectar foragers establish while they collect on a given flower species should be retrieved during trophallaxis, i.e., the transfer of liquid food by mouth, and, accordingly, foraging experience could affect the occurrence of these interactions inside the nest. We used experimental arenas to analyze how crop scents carried by donor bees affect trophallaxis among foragers, i.e., donors and receivers, which differ in their foraging experience. Results showed that whenever the foragers had collected unscented sugar solution from a feeder the presence of scents in the solution carried by donors did not affect the occurrence of trophallaxis nor its dynamics. In contrast, whenever the foragers had previous olfactory information, new scents present in the crop of the donors negatively affected the occurrence, but not the dynamics of trophallaxis. Thus, the association learned at the food source seems to be retrieved during trophallaxis, and it is possible that known scents present in the mouthparts of nest-mates may operate as a triggering stimulus to elicit trophallactic behavior within the hive.     

Interactions Increase Forager Availability and Activity in Harvester Ants

Social insect colonies use interactions among workers to regulate collective behavior. Harvester ant foragers interact in a chamber just inside the nest entrance, here called the ''entrance chamber''. Previous studies of the activation of foragers in red harvester ants show that an outgoing forager inside the nest experiences an increase in brief antennal contacts before it leaves the nest to forage. Here we compare the interaction rate experienced by foragers that left the nest and ants that did not. We found that ants in the entrance chamber that leave the nest to forage experienced more interactions than ants that descend to the deeper nest without foraging. Additionally, we found that the availability of foragers in the entrance chamber is associated with the rate of forager return. An increase in the rate of forager return leads to an increase in the rate at which ants descend to the deeper nest, which then stimulates more ants to ascend into the entrance chamber. Thus a higher rate of forager return leads to more available foragers in the entrance chamber. The highest density of interactions occurs near the nest entrance and the entrances of the tunnels from the entrance chamber to the deeper nest. Local interactions with returning foragers regulate both the activation of waiting foragers and the number of foragers available to be activated.     

Increase in dance imprecision with decreasing foraging distance in the honey bee <Emphasis Type="Italic">Apis mellifera</Emphasis> L. is partly explained by physical constraints

Honey bee foragers communicate the direction and distance of both food sources and new nest sites to nest mates by means of a symbolic dance language. Interestingly, the precision by which dancers transfer directional information is negatively correlated with the distance to the advertised food source. The ‘tuned-error’ hypothesis suggests that colonies benefit from this imprecision as it spreads recruits out over a patch of constant size irrespective of the distance to the advertised site. An alternative to the tuned-error hypothesis is that dancers are physically incapable of dancing with great precision for nearby sources. Here we revisit the tuned-error hypothesis by studying the change in dance precision with increasing foraging distance over relatively short distances while controlling for environmental influences. We show that bees indeed increase their dance precision with the increase in foraging distance. However, we also show that dances performed by swarm-scouts for a nearby (30 m) nest site, where there could be no benefit to imprecision, are either without or with only limited directional information. This result suggests that imprecision in dance communication is caused primarily by physical constraints in the ability of dancers to turn around quickly enough when the advertised site is nearby.