Learning in the Africanized honey bee: Apis mellifera L

Abramson C. Olfactory Learning in the Stingless Bee Melipona eburnea Friese (Apidae: Meliponini)

Olfactory learning and floral scents are co-adaptive traits in the plant–pollinator relationship. However, how scent relates to cognition and learning in the diverse group of Neotropical stingless bees is largely unknown. Here we evaluated the ability of Melipona eburnea to be conditioned to scent using the proboscis extension reflex (PER) protocol. Stingless bees did not show PER while harnessed but were able to be PER conditioned to scent when free-to-move in a mini-cage (fmPER). We evaluated the effect of: 1) unconditioned stimulus (US) reward, and 2) previous scent–reward associations on olfactory learning performance. When using unscented-US, PER-responses were low on day 1, but using scented-US reward the olfactory PER-response increased on day 1. On day 2 PER performance greatly increased in bees that previously had experienced the same odor and reward combination, while bees that experienced a different odor on day 2 showed poor olfactory learning. Bees showed higher olfactory PER conditioning to guava than to mango odor. The effect of the unconditioned stimulus reward was not a significant factor in the model on day 2. This indicates that olfactory learning performance can increase via either taste receptors or accumulated experience with the same odor. Our results have application in agriculture and pollination ecology.

Appetitive reversal learning differences of two honey bee subspecies with different foraging behaviors

We aimed to examine mechanistically the observed foraging differences across two honey bee, Apis mellifera, subspecies using the proboscis extension response assay. Specifically, we compared differences in appetitive reversal learning ability between honey bee subspecies: Apis mellifera caucasica (Pollman), and Apis mellifera syriaca (Skorikov) in a "common garden" apiary. It was hypothesized that specific learning differences could explain previously observed foraging behavior differences of these subspecies: A.m. caucasica switches between different flower color morphs in response to reward variability, and A.m. syriaca does not switch. We suggest that flower constancy allows reduced exposure by minimizing search and handling time, whereas plasticity is important when maximizing harvest in preparation for long winter is at a premium. In the initial or Acquisition phase of the test we examined specifically discrimination learning, where bees were trained to respond to a paired conditioned stimulus with an unconditioned stimulus and not to respond to a second conditioned stimulus that is not followed by an unconditioned stimulus. We found no significant differences among the subspecies in the Acquisition phase in appetitive learning. During the second, Reversal phase of the experiment, where flexibility in association was tested, the paired and unpaired conditioned stimuli were reversed. During the Reversal phase A.m. syriaca showed a reduced ability to learn the reverse association in the appetitive learning task. This observation is consistent with the hypothesis that A.m. syriaca foragers cannot change the foraging choice because of lack of flexibility in appetitive associations under changing contingencies. Interestingly, both subspecies continued responding to the previously rewarded conditioned stimulus in the reversal phase. We discuss potential ecological correlates and molecular underpinnings of these differences in learning across the two subspecies. In addition, in a supplemental experiment we demonstrated that these differences in appetitive reversal learning do not occur in other learning contexts.

Failure to Find Ethanol-Induced Conditioned Taste Aversion in Honey Bees (Apis mellifera L.)

BACKGROUND

Conditioned taste aversion (CTA) learning is a highly specialized form of conditioning found across taxa that leads to avoidance of an initially neutral stimulus, such as taste or odor, that is associated with, but is not the cause of, a detrimental health condition. This study examines if honey bees (Apis mellifera L.) develop ethanol (EtOH)-induced CTA.

METHODS

Restrained bees were first administered a sucrose solution that was cinnamon scented, lavender scented, or unscented, and contained either 0, 2.5, 5, 10, or 20% EtOH. Then, 30 minutes later, we used a proboscis extension response (PER) conditioning procedure where the bees were taught to associate either cinnamon odor, lavender odor, or an air-puff with repeated sucrose feedings. For some bees, the odor of the previously consumed EtOH solution was the same as the odor associated with sucrose in the conditioning procedure. If bees are able to learn EtOH-induced CTA, they should show an immediate low level of response to odors previously associated with EtOH.

RESULTS

We found that bees did not develop CTA despite the substantial inhibitory and aversive effects EtOH has on behavior. Instead, bees receiving a conditioning odor that was previously associated with EtOH showed an immediate high level of response. While this demonstrates bees are capable of one-trial learning common to CTA experiments, this high level of response is the opposite of what would occur if the bees developed a CTA. Responding on subsequent trials also showed a general inhibitory effect of EtOH. Finally, we found that consumption of cinnamon extract reduced the effects of EtOH.

CONCLUSIONS

The honey bees' lack of learned avoidance to EtOH mirrors that seen in human alcoholism. These findings demonstrate the usefulness of honey bees as an insect model for EtOH consumption.

Advances and limitations of visual conditioning protocols in harnessed bees

Bees are excellent invertebrate models for studying visual learning and memory mechanisms, because of their sophisticated visual system and impressive cognitive capacities associated with a relatively simple brain. Visual learning in free-flying bees has been traditionally studied using an operant conditioning paradigm. This well-established protocol, however, can hardly be combined with invasive procedures for studying the neurobiological basis of visual learning. Different efforts have been made to develop protocols in which harnessed honey bees could associate visual cues with reinforcement, though learning performances remain poorer than those obtained with free-flying animals. Especially in the last decade, the intention of improving visual learning performances of harnessed bees led many authors to adopt distinct visual conditioning protocols, altering parameters like harnessing method, nature and duration of visual stimulation, number of trials, inter-trial intervals, among others. As a result, the literature provides data hardly comparable and sometimes contradictory. In the present review, we provide an extensive analysis of the literature available on visual conditioning of harnessed bees, with special emphasis on the comparison of diverse conditioning parameters adopted by different authors. Together with this comparative overview, we discuss how these diverse conditioning parameters could modulate visual learning performances of harnessed bees.

Classical Conditioning of Proboscis Extension in Harnessed Africanized Honey Bee Queens (Apis Mellifera L.)

Experiments are reported on learning in virgin Africanized honey bee queens ( Apis mellifera L.). Queens restrained in a “Pavlovian harness” received a pairing of hexanal odor with a 1.8-M feeding of sucrose solution. Compared to explicitly unpaired controls, acquisition was rapid in reaching about 90%. Acquisition was also rapid in queens receiving an unconditioned stimulus of “bee candy” or an unconditioned stimulus administered by worker bees. During extinction the conditioned response declines. The steepest decline was observed in queens receiving an unconditioned stimulus of bee candy. These findings extend previous work on learning of Africanized honey bee workers to a population of queen bees.

The effect of ethanol on reversal learning in honey bees (Apis mellifera anatolica): Response inhibition in a social insect model

We investigated the effects of ethanol on reversal learning in honey bees (Apis mellifera anatolica). The rationale behind the present experiment was to determine the species generality of the effect of ethanol on response inhibition. Subjects were originally trained to associate either a cinnamon or lavender odor with a sucrose feeding before a reversal of the conditioned stimuli. We administered 15 μL of ethanol at varying doses (0%, 2.5%, 5%, 10%, or 20%) according to group assignment. Ethanol was either administered 5 min before original discrimination training or 5 min before the stimuli reversal. We analyzed the effects of these three manipulations via a recently developed individual analysis that eschews aggregate assessments in favor of a model that conceptualizes learning as occurring in individual organisms. We measured responding in the presence of conditioned stimuli associated with a sucrose feeding, responding in the presence of conditioned stimuli associated with distilled water, and responding in the presence of the unconditioned stimulus (sucrose). Our analyses revealed the ethanol dose manipulation lowered responding for all three measures at increasingly higher doses, which suggests ethanol served as a general behavioral suppressor. Consistent with previous ethanol reversal literature, we found administering ethanol before the original discrimination phase or before the reversal produced inconsistent patterns of responding at varying ethanol doses.

APRENDIZAGEM DA EXTENSÃO DA PROBÓSCIDE EM ZANGÕES AFRICANIZADOS (Apis mellifera L.) CONFINADOS

Estudos sobre a aprendizagem olfativa em abelhas (Apis mellifera L.) são predominantes nas operárias. Neste estudo, utilizou-se o condicionamento clássico da extensão da probóscide (PER) para avaliar o efeito de 5 odores como estímulos condicionantes (EC). Foram utilizados dez grupos de 20 zangões (A. mellifera L.) cada. Os estímulos condicionantes foram Citral, Hexanal, Geraniol, cera de abelha em favo e cera de abelha alveolada. Além da aquisição de aprendizagem, mediu-se a persistência do condicionamento quando o estímulo incondicional (EI) não foi mais oferecido (i.e. extinção). O intervalo entre testes, o tempo de apresentação de EC e EI foram10 minutos, 2 segundos e 3 segundos, respectivamente. Os zangões foram capazes de demonstrar condicionamento e armazenamento de informação. Citral, Hexanal e cera de abelha foram os estímulos mais eficientes no condicionamento clássico (CC) com zangões.

Honey Bee Location- and Time-Linked Memory Use in Novel Foraging Situations: Floral Color Dependency

Learning facilitates behavioral plasticity, leading to higher success rates when foraging. However, memory is of decreasing value with changes brought about by moving to novel resource locations or activity at different times of the day. These premises suggest a foraging model with location- and time-linked memory. Thus, each problem is novel, and selection should favor a maximum likelihood approach to achieve energy maximization results. Alternatively, information is potentially always applicable. This premise suggests a different foraging model, one where initial decisions should be based on previous learning regardless of the foraging site or time. Under this second model, no problem is considered novel, and selection should favor a Bayesian or pseudo-Bayesian approach to achieve energy maximization results. We tested these two models by offering honey bees a learning situation at one location in the morning, where nectar rewards differed between flower colors, and examined their behavior at a second location in the afternoon where rewards did not differ between flower colors. Both blue-yellow and blue-white dimorphic flower patches were used. Information learned in the morning was clearly used in the afternoon at a new foraging site. Memory was not location-time restricted in terms of use when visiting either flower color dimorphism.

Color dependent learning in restrained Africanized honey bees

Associative color learning has been demonstrated to be very poor using restrained European honey bees unless the antennae are amputated. Consequently, our understanding of proximate mechanisms in visual information processing is handicapped. Here we test learning performance of Africanized honey bees under restrained conditions with visual and olfactory stimulation using the proboscis extension response (PER) protocol. Restrained individuals were trained to learn an association between a color stimulus and a sugar-water reward. We evaluated performance for "absolute" learning (learned association between a stimulus and a reward) and "discriminant" learning (discrimination between two stimuli). Restrained Africanized honey bees (AHBs) readily learned the association of color stimulus for both blue and green LED stimuli in absolute and discriminatory learning tasks within 7 presentations, but not with violet as the rewarded color. Additionally, 24-hour memory improved considerably during the discrimination task, compared to absolute association (15%-55%). We found that antennal amputation was unnecessary and reduced performance in AHBs. Thus color learning can now be studied using the PER protocol with intact AHBs. This finding opens the way toward investigating visual and multimodal learning with application of neural techniques commonly used in restrained honey bees.

Learning impairment in honey bees caused by agricultural spray adjuvants

Background

Spray adjuvants are often applied to crops in conjunction with agricultural pesticides in order to boost the efficacy of the active ingredient(s). The adjuvants themselves are largely assumed to be biologically inert and are therefore subject to minimal scrutiny and toxicological testing by regulatory agencies. Honey bees are exposed to a wide array of pesticides as they conduct normal foraging operations, meaning that they are likely exposed to spray adjuvants as well. It was previously unknown whether these agrochemicals have any deleterious effects on honey bee behavior.

Methodology/Principal Findings

An improved, automated version of the proboscis extension reflex (PER) assay with a high degree of trial-to-trial reproducibility was used to measure the olfactory learning ability of honey bees treated orally with sublethal doses of the most widely used spray adjuvants on almonds in the Central Valley of California. Three different adjuvant classes (nonionic surfactants, crop oil concentrates, and organosilicone surfactants) were investigated in this study. Learning was impaired after ingestion of 20 µg organosilicone surfactant, indicating harmful effects on honey bees caused by agrochemicals previously believed to be innocuous. Organosilicones were more active than the nonionic adjuvants, while the crop oil concentrates were inactive. Ingestion was required for the tested adjuvant to have an effect on learning, as exposure via antennal contact only induced no level of impairment.

Conclusions/Significance

A decrease in percent conditioned response after ingestion of organosilicone surfactants has been demonstrated here for the first time. Olfactory learning is important for foraging honey bees because it allows them to exploit the most productive floral resources in an area at any given time. Impairment of this learning ability may have serious implications for foraging efficiency at the colony level, as well as potentially many social interactions. Organosilicone spray adjuvants may therefore contribute to the ongoing global decline in honey bee health.

Africanized honeybees are slower learners than their European counterparts

Does cognitive ability always correlate with a positive fitness consequence? Previous research in both vertebrates and invertebrates provides mixed results. Here, we compare the learning and memory abilities of Africanized honeybees (Apis mellifera scutellata hybrid) and European honeybees (Apis mellifera ligustica). The range of the Africanized honeybee continues to expand, superseding the European honeybee, which led us to hypothesize that they might possess greater cognitive capabilities as revealed by a classical conditioning assay. Surprisingly, we found that fewer Africanized honeybees learn to associate an odor with a reward. Additionally, fewer Africanized honeybees remembered the association a day later. While Africanized honeybees are replacing European honeybees, our results show that they do so despite displaying a relatively poorer performance on an associative learning paradigm.

Odor discrimination in classical conditioning of proboscis extension in two stingless bee species in comparison to Africanized honeybees

Learning in insects has been extensively studied using different experimental approaches. One of them, the proboscis extension response (PER) paradigm, is particularly well suited for quantitative studies of cognitive abilities of honeybees under controlled conditions. The goal of this study was to analyze the capability of three eusocial bee species to be olfactory conditioned in the PER paradigm. We worked with two Brazilian stingless bees species, Melipona quadrifasciata and Scaptotrigona aff. depilis, and with the invasive Africanized honeybee, Apis mellifera. These three species present very different recruitment strategies, which could be related with different odor-learning abilities. We evaluated their gustatory responsiveness and learning capability to discriminate floral odors. Gustatory responsiveness was similar for the three species, although S. aff. depilis workers showed fluctuations along the experimental period. Results for the learning assays revealed that M. quadrifasciata workers can be conditioned to discriminate floral odors in a classical differential conditioning protocol and that this discrimination is maintained 15 min after training. During conditioning, Africanized honeybees presented the highest discrimination, for M. quadrifasciata it was intermediate, and S. aff. depilis bees presented no discrimination. The differences found are discussed considering the putative different learning abilities and procedure effect for each species.

Ethanol levels in honeybee hemolymph resulting from alcohol ingestion

Our previous work on a social insect model of ethanol-induced behavior focused on behavioral studies of honeybees (Apis mellifera L.). We now investigate the dependence of honeybee blood ethanol concentration on both the amount of ethanol consumed and time elapsed since ingestion. Blood ethanol level was determined using gas chromatograph using hemolymph taken from harnessed bees. Significantly increased levels of ethanol in honeybee hemolymph were detected within 15 min of feeding bees alcohol. Within 30 min, ethanol concentration increased 2.7 times. The concentration of ethanol ingested also had a significant effect on blood ethanol level. However, postfeeding times greater than 30 min did not significantly increase ethanol concentration in bee hemolymph. This study integrates with our behavioral data on the effect of ethanol on honeybees. Our laboratory and field experiments show a correlation between the time frame for behavioral changes and significant increases of blood ethanol levels shown in this study.

Development of an ethanol model using social insects: III. Preferences for ethanol solutions

Experiments are designed to assess whether free-flying honey bees have an aversion to an ethanol solution when given a choice between targets containing an ethanol solution in sucrose or sucrose only. Animals given a choice between a 1% ethanol solution and sucrose only show no aversion to the ethanol solution either in acquisition or extinction. Honey bees given a choice between a 5% ethanol solution and sucrose only show no differences in the initial choice of targets but some ees do switch over to the sucrose-only target. Performance during extinction indicates that bees landed on the previously reinforced sucrose-only target more than the target previously containing the 5% ethanol solution. An experiment in which bees were given a single 5%, ethanol target showed that of 20 bees, 11 returned for the entire 12 trials of the experiment. All bees returned at least 6 times to the 5% ethanol target. Additional experiments were run on harnessed foragers in a palatability study of alcoholic beverages consumed by humans. The results of the palatability experiment indicate that in general, bees prefer more sweet drinks with less alcohol.

Development of an ethanol model using social insects: II. Effect of Antabuse on consumatory responses and learned behavior of the honey bee (Apis mellifera L.)

The ability of Antabuse (disulfiram) to influence ethanol consumption and learning in harnessed honey bees was investigated. In the first series of experiments a factorial design was used with 5 levels of ethanol concentration (0%, 1%, 5%, 10%, 20%), 4 doses of Antabuse (0, 37 microg/g, 3.7 microg/g, .37 microg/g), and 2 testing intervals (1 min., 10 min.). Animals were fed a single 1 microl dose of Antabuse and contact time with an ethanol solution measured. A second series of experiments investigated the influence of Antabuse on the formation of Pavlovian conditioning of the proboscis extension reflex. A factorial design was used with two levels of training (paired, unpaired), three levels of ethanol (0%, 1%, 5%), and 2 levels of pretreatment (distilled water, 3.7 microg/g). Analysis of the consumption experiments indicate that pretreatment with Antabuse reduces ethanol intake, although there was substantial variability. The findings of the Pavlovian experiments suggest that pretreatment with Antabuse significantly reduced responding to a CS signaling the availability of ethanol.

Exploratory studies of classical conditioning of the preoral cavity in harnessed carpenter ants (Camponotus pennsylvanicus)

An attempt was made to classically condition the mouthparts of harnessed worker ants (Camponotus pennsylvanicus) in anticipation of feeding. Experiments were designed to investigate classical conditioning with one CS, discrimination between two CSs, and pseudoconditioning. Analysis indicated a small acquisition effect that could be accounted for by pseudoconditioning. The preparation can be used to study nonassociative learning and some instrumental conditioning situations.

Some preliminary studies on the ability of Africanized honey bees (Apis mellifera L.) to tolerate cold temperatures when placed inside a refrigerator

Cold is often suggested as an ecological mechanism to prevent the migration of Africanized honey bees. The ability of Africanized honey bees to tolerate cold temperatures was investigated. In one study an observation hive was placed inside a refrigerator at 25 degrees C. The study was conceptualized as a choice experiment in which the colony could remain in a cold environment or leave for a warm environment. Analysis indicated that the bees remained at 9 +/- 1 degrees C for 14 days before leaving. In a second series of studies, testing the tolerance to 0 degree C, 280 bees were placed individually in small metal tubes. The data gathered included survival rate, time to regain consciousness, and ability to feed. Analysis indicated that Africanized bees can survive for up to 3 hr. at 0 degree C with few ill effects. At 4 hr., however, the survival rate is low. Limitations of the study, the use of cold as a possible deterrent to honey bee mites, and suggestions for additional research are discussed.