Effects of imidacloprid metabolites on habituation in honeybees suggest the existence of two subtypes of nicotinic receptors differentially expressed during adult development

A potential trade-off between offense and defense in honeybee innate immunity: Reduced phagocytosis in honeybee hemocytes correlates with a protective response after exposure to imidacloprid and amitraz

Phagocytosis "offense" is a crucial process to protect the organism from diseases and the effects of foreign particles. Insects rely on the innate immune system and thus any hindrance to phagocytosis may greatly affect their resistance to diseases and response to pathogens. The European honeybee, a valuable species due to its economic and environmental contribution, is being challenged by colony collapse disorder leading to its decline. Exposure to multiple factors including pesticides like imidacloprid and amitraz may negatively alter their immune response and ultimately make them more susceptible to diseases. In this study, we compare the effect of different concentrations and mixtures of imidacloprid and amitraz with different concentrations of the immune stimulant, zymosan A. Results show that imidacloprid and amitraz have a synergistic negative effect on phagocytosis. The lowered phagocytosis induces significantly higher hemocyte viability suggesting a negatively correlated protective mechanism "defense" from pesticide-associated damage but may not be protective from pathogens.

Effects of pesticide-adjuvant combinations used in almond orchards on olfactory responses to social signals in honey bees (Apis mellifera)

Exposure to agrochemical sprays containing pesticides and tank-mix adjuvants has been implicated in post-bloom mortality, particularly of brood, in honey bee colonies brought into California almond orchards for pollination. Although adjuvants are generally considered to be biologically inert, some adjuvants have exhibited toxicity and sublethal effects, including decreasing survival rates of next-generation queens. Honey bees have a highly developed olfactory system to detect and discriminate among social signals. To investigate the impact of pesticide-adjuvant combinations on honey bee signal perception, we performed electroantennography assays to assess alterations in their olfactory responsiveness to the brood ester pheromone (BEP), the volatile larval pheromone β-ocimene, and the alarm pheromone 2-heptanone. These assays aimed to uncover potential mechanisms underlying changes in social behaviors and reduced brood survival after pesticide exposure. We found that combining the adjuvant Dyne-Amic with the fungicide Tilt (propiconazole) and the insecticide Altacor (chlorantraniliprole) synergistically enhanced olfactory responses to three concentrations of BEP and as well exerted dampening and compensatory effects on responses to 2-heptanone and β-ocimene, respectively. In contrast, exposure to adjuvant alone or the combination of fungicide and insecticide had no effect on olfactory responses to BEP at most concentrations but altered responses to β-ocimene and 2-heptanone. Exposure to Dyne-Amic, Altacor, and Tilt increased BEP signal amplitude, indicating potential changes in olfactory receptor sensitivity or sensilla permeability to odorants. Given that, in a previous study, next-generation queens raised by nurses exposed to the same treated pollen experienced reduced survival, these new findings highlight the potential disruption of social signaling in honey bees and its implications for colony reproductive success.

Lethal and Sublethal Dose of Thiamethoxam and Its Effects on the Behavior of a Non-target Social Wasp

In the last few years, with the increase in agricultural productivity, there has also been an increase in the use of insecticides to combat insects considered pests. However, these chemical compounds end up affecting nontarget insects that also interact with the crops. Studies have shown that social bees are among the insects that are suffering most from the effects of these compounds, resulting in negative ecological and economic impacts, considering that these insects provide pollination services in ecosystems. At the same time, social wasps also interact with plants, including cultivated ones, and perform ecological services similar to those of social bees, so it can be hypothesized that insecticides are also affecting social wasp colonies. Therefore, the purpose of this study was to evaluate contamination and sublethal effects of neonicotinoids on the mobility of the social wasp Protopolybia exigua (Saussure). In the first step, oral exposure experiments were performed to determine lethal and sublethal concentrations. In a second step, the wasps were exposed to sublethal concentrations, in order to evaluate the effects on their mobility. The results demonstrated that this species is more susceptible to exposure to neonicotinoids, compared to several bee species that have so far been studied, but lower than others. Exposure to sublethal concentrations can significantly reduce wasp mobility, which can have short-term consequences both for worker wasps and for the maintenance of their colonies.

Delayed effects of a single dose of a neurotoxic pesticide (sulfoxaflor) on honeybee foraging activity

Pesticide risk-assessment guidelines for honeybees (Apis mellifera) generally require determining the acute toxicity of a chemical over the short-term through fix-duration tests. However, potential long-lasting or delayed effects resulting from an acute exposure (e.g. a single dose) are often overlooked, although the modification of a developmental process may have life-long consequences. To investigate this question, we exposed young honeybee workers to a single sublethal field-realistic dose of a neurotoxic pesticide, sulfoxaflor, at one of two amounts (16 or 60 ng), at the moment when they initiated orientation flights (preceding foraging activity). We then tracked in the field their flight activity and lifespan with automated life-long monitoring devices. Both amounts of sulfoxaflor administered reduced the total number of flights but did not affect bee survival and flight duration. When looking at the time series of flight activity, effects were not immediate but delayed until foraging activity with a decrease in the daily number of foraging flights and consequently in their total number (24 and 33% less for the 16 and 60 ng doses, respectively). The results of our study therefore blur the general assumption in honeybee toxicology that acute exposure results in immediate and rapid effects and call for long-term recording and/or time-to-effect measurements, even upon exposure to a single dose of pesticide.

A common neonicotinoid pesticide, thiamethoxam, alters honey bee activity, motor functions, and movement to light

Honey bees provide key ecosystem services. To pollinate and to sustain the colony, workers must walk, climb, and use phototaxis as they move inside and outside the nest. Phototaxis, orientation to light, is linked to sucrose responsiveness and the transition of work from inside to outside the nest, and is also a key component of division of labour. However, the sublethal effects of pesticides on locomotion and movement to light are relatively poorly understood. Thiamethoxam (TMX) is a common neonicotinoid pesticide that bees can consume in nectar and pollen. We used a vertical arena illuminated from the top to test the effects of acute and chronic sublethal exposures to TMX. Acute consumption (1.34 ng/bee) impaired locomotion, caused hyperactivity (velocity: +109%; time moving: +44%) shortly after exposure (30 min), and impaired motor functions (falls: +83%; time top: -43%; time bottom: +93%; abnormal behaviours: +138%; inability to ascend: +280%) over a longer period (60 min). A 2-day chronic exposure (field-relevant daily intakes of 1.42-3.48 ng/bee/day) impaired bee ability to ascend. TMX increased movement to light after acute and chronic exposure. Thus, TMX could reduce colony health by harming worker locomotion and, potentially, alter division of labour if bees move outside or remain outdoors.

Developmental and loco-like effects of a swainsonine-induced inhibition of α-mannosidase in the honey bee, Apis mellifera

Background

Deficiencies in lysosomal a-mannosidase (LAM) activity in animals, caused either by mutations or by consuming toxic alkaloids, lead to severe phenotypic and behavioural consequences. Yet, epialleles adversely affecting LAM expression exist in the honey bee population suggesting that they might be beneficial in certain contexts and cannot be eliminated by natural selection.

Methods

We have used a combination of enzymology, molecular biology and metabolomics to characterise the catalytic properties of honey bee LAM (AmLAM) and then used an indolizidine alkaloid swainsonine to inhibit its activity in vitro and in vivo.

Results

We show that AmLAM is inhibited in vitro by swainsonine albeit at slightly higher concentrations than in other animals. Dietary exposure of growing larvae to swainsonine leads to pronounced metabolic changes affecting not only saccharides, but also amino acids, polyols and polyamines. Interestingly, the abundance of two fatty acids implicated in epigenetic regulation is significantly reduced in treated individuals. Additionally, swainsonie causes loco-like symptoms, increased mortality and a subtle decrease in the rate of larval growth resulting in a subsequent developmental delay in pupal metamorphosis.

Discussion

We consider our findings in the context of cellular LAM function, larval development, environmental toxicity and colony-level impacts. The observed developmental heterochrony in swainsonine-treated larvae with lower LAM activity offer a plausible explanation for the existence of epialleles with impaired LAM expression. Individuals carrying such epialleles provide an additional level of epigenetic diversity that could be beneficial for the functioning of a colony whereby more flexibility in timing of adult emergence might be useful for task allocation.

Current knowledge of detoxification mechanisms of xenobiotic in honey bees

The western honey bee Apis mellifera is the most important managed pollinator species in the world. Multiple factors have been implicated as potential causes or factors contributing to colony collapse disorder, including honey bee pathogens and nutritional deficiencies as well as exposure to pesticides. Honey bees' genome is characterized by a paucity of genes associated with detoxification, which makes them vulnerable to specific pesticides, especially to combinations of pesticides in real field environments. Many studies have investigated the mechanisms involved in detoxification of xenobiotics/pesticides in honey bees, from primal enzyme assays or toxicity bioassays to characterization of transcript gene expression and protein expression in response to xenobiotics/insecticides by using a global transcriptomic or proteomic approach, and even to functional characterizations. The global transcriptomic and proteomic approach allowed us to learn that detoxification mechanisms in honey bees involve multiple genes and pathways along with changes in energy metabolism and cellular stress response. P450 genes, is highly implicated in the direct detoxification of xenobiotics/insecticides in honey bees and their expression can be regulated by honey/pollen constitutes, resulting in the tolerance of honey bees to other xenobiotics or insecticides. P450s is also a key detoxification enzyme that mediate synergism interaction between acaricides/insecticides and fungicides through inhibition P450 activity by fungicides or competition for detoxification enzymes between acaricides. With the wide use of insecticides in agriculture, understanding the detoxification mechanism of insecticides in honey bees and how honeybees fight with the xenobiotis or insecticides to survive in the changing environment will finally benefit honeybees' management.

Impacts of chronic sublethal exposure to clothianidin on winter honeybees

A wide application of systemic pesticides and detection of their residues in bee-collected pollen and nectar at sublethal concentrations led to the emergence of concerns about bees' chronic exposure and possible sublethal effects on insect pollinators. Therefore, special attention was given to reducing unintentional intoxications under field conditions. The sensitivity of winter bees throughout their long lifespan to residual exposure of pesticides is not well known, since most previous studies only looked at the effects on summer bees. Here, we performed various laboratory bioassays to assess the effects of clothianidin on the survival and behavior of winter bees. Oral lethal and sublethal doses were administered throughout 12-day. The obtained LD50 values at 48, 72, 96 h and 10 days were 26.9, 18.0, 15.1 and 9.5 ng/bee, respectively. Concentrations <20 µg/kg were found to be sublethal. Oral exposure to sublethal doses was carried out for 12-day and, the behavioral functions were tested on the respective 13th day. Although slight reductions in the responses at the concentrations 10 and 15 µg/kg were observed, all tested sublethal concentrations had showed non-significant effects on the sucrose responsiveness, habitation of the proboscis extension reflex and olfactory learning performance. Nevertheless, chronic exposure to 15 µg/kg affected the specificity of the early long-term memory (24 h). Since the tested concentrations were in the range of field-relevant concentrations, our results strongly suggest that related-effects on winter and summer bees' sensitivity should also be studied under realistic conditions.

Effects of Sublethal Doses of Imidacloprid on Young Adult Honeybee Behaviour

Imidacloprid (IMI), a neonicotinoid used for its high selective toxicity to insects, is one of the most commonly used pesticides. However, its effect on beneficial insects such as the honeybee Apis mellifera L is still controversial. As young adult workers perform in-hive duties that are crucial for colony maintenance and survival, we aimed to assess the effect of sublethal IMI doses on honeybee behaviour during this period. Also, because this insecticide acts as a cholinergic-nicotinic agonist and these pathways take part in insect learning and memory processes; we used IMI to assess their role and the changes they suffer along early adulthood. We focused on appetitive behaviours based on the proboscis extension response. Laboratory reared adults of 2 to 10 days of age were exposed to sublethal IMI doses (0.25 or 0.50ng) administered orally or topically prior to behavioural assessment. Modification of gustatory responsiveness and impairment of learning and memory were found as a result of IMI exposure. These outcomes differed depending on age of evaluation, type of exposure and IMI dose, being the youngest bees more sensitive and the highest oral dose more toxic. Altogether, these results imply that IMI administered at levels found in agroecosystems can reduce sensitivity to reward and impair associative learning in young honeybees. Therefore, once a nectar inflow with IMI traces is distributed within the hive, it could impair in-door duties with negative consequences on colony performance.

Low doses of neonicotinoid pesticides in food rewards impair short-term olfactory memory in foraging-age honeybees

Neonicotinoids are often applied as systemic seed treatments to crops and have reported negative impact on pollinators when they appear in floral nectar and pollen. Recently, we found that bees in a two-choice assay prefer to consume solutions containing field-relevant doses of the neonicotinoid pesticides, imidacloprid (IMD) and thiamethoxam (TMX), to sucrose alone. This suggests that neonicotinoids enhance the rewarding properties of sucrose and that low, acute doses could improve learning and memory in bees. To test this, we trained foraging-age honeybees to learn to associate floral scent with a reward containing nectar-relevant concentrations of IMD and TMX and tested their short (STM) and long-term (LTM) olfactory memories. Contrary to our predictions, we found that none of the solutions enhanced the rate of olfactory learning and some of them impaired it. In particular, the effect of 10 nM IMD was observed by the second conditioning trial and persisted 24 h later. In most other groups, exposure to IMD and TMX affected STM but not LTM. Our data show that negative impacts of low doses of IMD and TMX do not require long-term exposure and suggest that impacts of neonicotinoids on olfaction are greater than their effects on rewarding memories.

Similar Comparative Low and High Doses of Deltamethrin and Acetamiprid Differently Impair the Retrieval of the Proboscis Extension Reflex in the Forager Honey Bee (Apis mellifera)

In the present study, the effects of low (10 ng/bee) and high (100 ng/bee) doses of acetamiprid and deltamethrin insecticides on multi-trial learning and retrieval were evaluated in the honey bee Apis mellifera. After oral application, acetamiprid and deltamethrin at the concentrations used were not able to impair learning sessions. When the retention tests were performed 1 h, 6 h, and 24 h after learning, we found a significant difference between bees after learning sessions when drugs were applied 24 h before learning. Deltamethrin-treated bees were found to be more sensitive at 10 ng/bee and 100 ng/bee doses compared to acetamiprid-treated bees, only with amounts of 100 ng/bee and at 6 h and 24 h delays. When insecticides were applied during learning sessions, none of the tested insecticides was able to impair learning performance at 10 ng/bee or 100 ng/bee but retention performance was altered 24 h after learning sessions. Acetamiprid was the only one to impair retrieval at 10 ng/bee, whereas at 100 ng/bee an impairment of retrieval was found with both insecticides. The present results therefore suggest that acetamiprid and deltamethrin are able to impair retrieval performance in the honey bee Apis mellifera.

Systemic insecticides (neonicotinoids and fipronil): trends, uses, mode of action and metabolites

Since their discovery in the late 1980s, neonicotinoid pesticides have become the most widely used class of insecticides worldwide, with large-scale applications ranging from plant protection (crops, vegetables, fruits), veterinary products, and biocides to invertebrate pest control in fish farming. In this review, we address the phenyl-pyrazole fipronil together with neonicotinoids because of similarities in their toxicity, physicochemical profiles, and presence in the environment. Neonicotinoids and fipronil currently account for approximately one third of the world insecticide market; the annual world production of the archetype neonicotinoid, imidacloprid, was estimated to be ca. 20,000 tonnes active substance in 2010. There were several reasons for the initial success of neonicotinoids and fipronil: (1) there was no known pesticide resistance in target pests, mainly because of their recent development, (2) their physicochemical properties included many advantages over previous generations of insecticides (i.e., organophosphates, carbamates, pyrethroids, etc.), and (3) they shared an assumed reduced operator and consumer risk. Due to their systemic nature, they are taken up by the roots or leaves and translocated to all parts of the plant, which, in turn, makes them effectively toxic to herbivorous insects. The toxicity persists for a variable period of time-depending on the plant, its growth stage, and the amount of pesticide applied. A wide variety of applications are available, including the most common prophylactic non-Good Agricultural Practices (GAP) application by seed coating. As a result of their extensive use and physicochemical properties, these substances can be found in all environmental compartments including soil, water, and air. Neonicotinoids and fipronil operate by disrupting neural transmission in the central nervous system of invertebrates. Neonicotinoids mimic the action of neurotransmitters, while fipronil inhibits neuronal receptors. In doing so, they continuously stimulate neurons leading ultimately to death of target invertebrates. Like virtually all insecticides, they can also have lethal and sublethal impacts on non-target organisms, including insect predators and vertebrates. Furthermore, a range of synergistic effects with other stressors have been documented. Here, we review extensively their metabolic pathways, showing how they form both compound-specific and common metabolites which can themselves be toxic. These may result in prolonged toxicity. Considering their wide commercial expansion, mode of action, the systemic properties in plants, persistence and environmental fate, coupled with limited information about the toxicity profiles of these compounds and their metabolites, neonicotinoids and fipronil may entail significant risks to the environment. A global evaluation of the potential collateral effects of their use is therefore timely. The present paper and subsequent chapters in this review of the global literature explore these risks and show a growing body of evidence that persistent, low concentrations of these insecticides pose serious risks of undesirable environmental impacts.

Effects of neonicotinoids and fipronil on non-target invertebrates

We assessed the state of knowledge regarding the effects of large-scale pollution with neonicotinoid insecticides and fipronil on non-target invertebrate species of terrestrial, freshwater and marine environments. A large section of the assessment is dedicated to the state of knowledge on sublethal effects on honeybees (Apis mellifera) because this important pollinator is the most studied non-target invertebrate species. Lepidoptera (butterflies and moths), Lumbricidae (earthworms), Apoidae sensu lato (bumblebees, solitary bees) and the section "other invertebrates" review available studies on the other terrestrial species. The sections on freshwater and marine species are rather short as little is known so far about the impact of neonicotinoid insecticides and fipronil on the diverse invertebrate fauna of these widely exposed habitats. For terrestrial and aquatic invertebrate species, the known effects of neonicotinoid pesticides and fipronil are described ranging from organismal toxicology and behavioural effects to population-level effects. For earthworms, freshwater and marine species, the relation of findings to regulatory risk assessment is described. Neonicotinoid insecticides exhibit very high toxicity to a wide range of invertebrates, particularly insects, and field-realistic exposure is likely to result in both lethal and a broad range of important sublethal impacts. There is a major knowledge gap regarding impacts on the grand majority of invertebrates, many of which perform essential roles enabling healthy ecosystem functioning. The data on the few non-target species on which field tests have been performed are limited by major flaws in the outdated test protocols. Despite large knowledge gaps and uncertainties, enough knowledge exists to conclude that existing levels of pollution with neonicotinoids and fipronil resulting from presently authorized uses frequently exceed the lowest observed adverse effect concentrations and are thus likely to have large-scale and wide ranging negative biological and ecological impacts on a wide range of non-target invertebrates in terrestrial, aquatic, marine and benthic habitats.

Impaired olfactory associative behavior of honeybee workers due to contamination of imidacloprid in the larval stage

The residue of imidacloprid in the nectar and pollens of the plants is toxic not only to adult honeybees but also the larvae. Our understanding of the risk of imidacloprid to larvae of the honeybees is still in a very early stage. In this study, the capped-brood, pupation and eclosion rates of the honeybee larvae were recorded after treating them directly in the hive with different dosages of imidacloprid. The brood-capped rates of the larvae decreased significantly when the dosages increased from 24 to 8000 ng/larva. However, there were no significant effects of DMSO or 0.4 ng of imidacloprid per larva on the brood-capped, pupation and eclosion rates. Although the sublethal dosage of imidacloprid had no effect on the eclosion rate, we found that the olfactory associative behavior of the adult bees was impaired if they had been treated with 0.04 ng/larva imidacloprid in the larval stage. These results demonstrate that a sublethal dosage of imidacloprid given to the larvae affects the subsequent associative ability of the adult honeybee workers. Thus, a low dose of imidacloprid may affect the survival condition of the entire colony, even though the larvae survive to adulthood.

Improved Cholinergic Transmission is Detrimental to Behavioural Plasticity in Honeybees (Apis mellifera)

Unravelling the role of neuromessenger processes in learning and memory has long interested researchers. We investigated the effects of an acetylcholinesterase blocker, Methyl Parathion (MeP), on honeybee learning. We used visual and olfactory tasks to test whether MeP had a detrimental effect on the acquisition of new knowledge when this new knowledge contradicts previously acquired one. Our results indicate that treatment with MeP prior to conditioning was significantly detrimental to the acquisition of incongruous (but not irrelevant or congruous) new knowledge due to improved recall. The neurobiological and ecotoxicological consequences of these results are discussed.

The proboscis extension reflex to evaluate learning and memory in honeybees (Apis mellifera): some caveats

The proboscis extension reflex (PER) is widely used in a classical conditioning (Pavlovian) context to evaluate learning and memory of a variety of insect species. The literature is particularly prodigious for honeybees (Apis mellifera) with more than a thousand publications. Imagination appears to be the only limit to the types of challenges to which researchers subject honeybees, including all the sensory modalities and a broad diversity of environmental treatments. Accordingly, some remarkable insights have been achieved using PER. However, there are several challenges to evaluating the PER literature that warrant a careful and thorough review. We assess here variation in methods that makes interpretation of studies, even those researching the same question, tenuous. We suggest that the numerous variables that might influence experimental outcomes from PER be thoroughly detailed by researchers. Moreover, the influence of individual variables on results needs to carefully evaluated, as well as among two or more variables. Our intent is to encourage investigation of the influence of numerous variables on PER results.

Development of biomarkers of exposure to xenobiotics in the honey bee Apis mellifera: application to the systemic insecticide thiamethoxam

This study describes the development of acetylcholinesterase (AChE), carboxylesterases (CaE1, CaE2, CaE3), glutathion-S-transferase (GST), alkaline phosphatase (ALP) and catalase (CAT) as enzyme biomarkers of exposure to xenobiotics such as thiamethoxam in the honey bee Apis mellifera. Extraction efficiency, stability under freezing and biological variability were studied. The extraction procedure achieved good recovery rates in one extraction step and ranged from 65 percent (AChE) to 97.3 percent (GST). Most of the enzymes were stable at -20°C, except ALP that displayed a slight but progressive decrease in its activity. Modifications of enzyme activities were considered after exposure to thiamethoxam at the lethal dose 50 percent (LD(50), 51.16 ng bee(-1)) and two sublethal doses, LD(50)/10 (5.12 ng bee(-1)) and LD(50)/20 (2.56 ng bee(-1)). The biomarker responses revealed that, even at the lowest dose used, exposure to thiamethoxam elicited sublethal effects and modified the activity of CaEs, GST, CAT and ALP. Different patterns of biomarker responses were observed: no response for AChE, an increase for GST and CAT, and differential effects for CaEs isoforms with a decrease in CaE1 and CaE3 and an increase in CaE2. ALP and CaE3 displayed contrasting variations but only at 2.56 ng bee(-1). We consider that this profile of biomarker variation could represent a useful fingerprint to characterise exposure to thiamethoxam in the honey bee A. mellifera. This battery of honey bee biomarkers might be a promising option to biomonitor the health of aerial and terrestrial ecosystems and to generate valuable information on the modes of action of pesticides.

Neonicotinoids in bees: a review on concentrations, side-effects and risk assessment

Neonicotinoid insecticides are successfully applied to control pests in a variety of agricultural crops; however, they may not only affect pest insects but also non-target organisms such as pollinators. This review summarizes, for the first time, 15 years of research on the hazards of neonicotinoids to bees including honey bees, bumble bees and solitary bees. The focus of the paper is on three different key aspects determining the risks of neonicotinoid field concentrations for bee populations: (1) the environmental neonicotinoid residue levels in plants, bees and bee products in relation to pesticide application, (2) the reported side-effects with special attention for sublethal effects, and (3) the usefulness for the evaluation of neonicotinoids of an already existing risk assessment scheme for systemic compounds. Although environmental residue levels of neonicotinoids were found to be lower than acute/chronic toxicity levels, there is still a lack of reliable data as most analyses were conducted near the detection limit and for only few crops. Many laboratory studies described lethal and sublethal effects of neonicotinoids on the foraging behavior, and learning and memory abilities of bees, while no effects were observed in field studies at field-realistic dosages. The proposed risk assessment scheme for systemic compounds was shown to be applicable to assess the risk for side-effects of neonicotinoids as it considers the effect on different life stages and different levels of biological organization (organism versus colony). Future research studies should be conducted with field-realistic concentrations, relevant exposure and evaluation durations. Molecular markers may be used to improve risk assessment by a better understanding of the mode of action (interaction with receptors) of neonicotinoids in bees leading to the identification of environmentally safer compounds.

Pest insect olfaction in an insecticide-contaminated environment: info-disruption or hormesis effect

Most animals, including pest insects, live in an “odor world” and depend strongly on chemical stimuli to get information on their biotic and abiotic environment. Although integrated pest management strategies including the use of insect growth regulators (IGRs) are increasingly developed, most insect pest treatments rely on neurotoxic chemicals. These molecules are known to disrupt synaptic transmission, affecting therefore sensory systems. The wide-spread use of neurotoxic insecticides and the growing use of IGRs result in residual accumulation of low concentrations in the environment. These insecticide residues could act as an “info-disruptor” by modifying the chemical communication system, and therefore decrease chances of reproduction in target insects. However, residues can also induce a non-expected hormesis effect by enhancing reproduction abilities. Low insecticide doses might thus induce adaptive processes in the olfactory pathway of target insects, favoring the development of resistance. The effect of sublethal doses of insecticides has mainly been studied in beneficial insects such as honeybees. We review here what is known on the effects of sublethal doses of insecticides on the olfactory system of insect pests.

DNA methylation changes elicited by social stimuli in the brains of worker honey bees

Social environments are notoriously multifactorial, yet studies in rodents have suggested that single variables such as maternal care can in fact be disentangled and correlated with specific DNA methylation changes. This study assesses whether non-detrimental social environmental variation in a highly plastic social insect is correlated with epigenomic modifications at the DNA methylation level. Honey bee workers perform tasks such as nursing and foraging in response to the social environment in the hive, in an age-linked but not age-dependent manner. In this study, the methylation levels of 83 cytosine-phosphate-guanosine dinucleotides over eight genomic regions were compared between the brains of age-matched bees performing nursing or foraging tasks. The results reveal more changes correlated with task than with chronological age, and also hive-associated methylation at some sites. One methylation site from a gene encoding Protein Kinase C binding protein 1 was consistently more methylated in foragers than nurses, which is suggested to lead to production of task-specific protein isoforms via alternative splicing. This study illustrates the ability of the neural epigenome to dynamically respond to complex social stimuli.

Epigenomic communication systems in humans and honey bees: from molecules to behavior

A 2010 Nature editorial entitled "Time for the Epigenome" trumpets the appearance of the International Human Epigenome Consortium and likens it to Biology's equivalent of the Large Hadron Collider. It strongly endorses the viewpoint that selective modifications of "marks" on DNA and histones constitute the crucial codes of life, a proposition which is hotly contested (Ptashne et al., in 2010). This proposition reflects the current mindset that DNA and histone modifications are the prime movers in gene regulation during evolution. This claim is perplexing, since the well characterized organisms, Drosophila melanogaster and Caenorhabditis elegans, lack methylated DNA "marks" and the DNA methytransferase enzymology. Despite their complete absence, D. melanogaster nevertheless has extensive gene regulatory networks which drive sophisticated development, gastrulation, migration of germ cells and yield a nervous system with significant neural attributes. In stark contrast, the honey bee Apis mellifera deploys its human-type DNA methyltransferase enzymology to "mark" its DNA and it too has sophisticated development. What roles therefore is DNA methylation playing in different animals? The honey bee brings a fresh perspective to this question. Its combinatorial chemistry of pheromones, tergal and cuticular exudates provide an exquisite communication system between thousands of individuals. The development of queen and worker is strictly controlled by differential feeding of royal jelly and their adult behaviors are accompanied by epigenomic changes. Their interfaces with different "environments" are extensive, allowing an evaluation of the roles of epigenomes in behavior in a natural environment, in the space of a few weeks, and at requisite levels of experimental rigor.

Enhanced cholinergic transmission promotes recall in honeybees

The involvement of the cholinergic system in learning and memory in honeybees has been well established using olfactory conditioning. We examined the effect of Methyl Parathion (MeP), an acetylcholinesterase inhibitor of the organo-phosphate family, on the learning and recall of visual and olfactory discrimination tasks in honeybees. One of our expectations was to observe the effects induced by both the nicotinic and muscarinic systems, as the blocking of acetylcholinesterase should induce an increase in the activity of both systems. We were also interested in knowing whether the type of tasks could influence the results. The visual tasks involved learning to discriminate the orientation of gratings in a Y-maze; the olfactory task involved learning to discriminate odours in a proboscis extension reflex (PER) paradigm. The results indicate that MeP treatment enhances recall of learned tasks in the visual and olfactory domains, but it does not affect the acquisition phase in either domain. Surprisingly, MeP treatment led to muscarinic-like effects but failed to mimic the nicotinic-like effects already described in relation to learning phases in honeybees. Implications for the role of cholinergic pathways in learning and memory and the nature of their involvement are discussed, and a hypothesis relating to the organisation of the cholinergic system and the relationship between the nicotinic and muscarinic systems in honeybees is proposed. The results are also discussed in terms of their ecotoxicological consequences.

Risk assessment for side-effects of neonicotinoids against bumblebees with and without impairing foraging behavior

Bombus terrestris bumblebees are important pollinators of wild flowers, and in modern agriculture they are used to guarantee pollination of vegetables and fruits. In the field it is likely that worker bees are exposed to pesticides during foraging. To date, several tests exist to assess lethal and sublethal side-effects of pesticides on bee survival, growth/development and reproduction. Within the context of ecotoxicology and insect physiology, we report the development of a new bioassay to assess the impact of sublethal concentrations on the bumblebee foraging behavior under laboratory conditions. In brief, the experimental setup of this behavior test consists of two artificial nests connected with a tube of about 20 cm and use of queenless micro-colonies of 5 workers. In one nest the worker bees constructed brood, and in the other food (sugar and pollen) was provided. Before exposure, the worker bees were allowed a training to forage for untreated food; afterwards this was replaced by treated food. Using this setup we investigated the effects of sublethal concentrations of the neonicotinoid insecticide imidacloprid, known to negatively affect the foraging behavior of bees. For comparison within the family of neonicotinoid insecticides, we also tested different concentrations of two other neonicotinoids: thiamethoxam and thiacloprid, in the laboratory with the new bioassay. Finally to evaluate the new bioassay, we also tested sublethal concentrations of imidacloprid in the greenhouse with use of queenright colonies of B. terrestris, and here worker bees needed to forage/fly for food that was placed at a distance of 3 m from their hives. In general, the experiments showed that concentrations that may be considered safe for bumblebees can have a negative influence on their foraging behavior. Therefore it is recommended that behavior tests should be included in risk assessment tests for highly toxic pesticides because impairment of the foraging behavior can result in a decreased pollination, lower reproduction and finally in colony mortality due to a lack of food.

Diverse actions and target-site selectivity of neonicotinoids: structural insights

The nicotinic acetylcholine receptors (nAChRs) are targets for human and veterinary medicines as well as insecticides. Subtype-selectivity among the diverse nAChR family members is important for medicines targeting particular disorders, and pest-insect selectivity is essential for the development of safer, environmentally acceptable insecticides. Neonicotinoid insecticides selectively targeting insect nAChRs have important applications in crop protection and animal health. Members of this class exhibit strikingly diverse actions on their nAChR targets. Here we review the chemistry and diverse actions of neonicotinoids on insect and mammalian nAChRs. Electrophysiological studies on native nAChRs and on wild-type and mutagenized recombinant nAChRs have shown that basic residues particular to loop D of insect nAChRs are likely to interact electrostatically with the nitro group of neonicotinoids. In 2008, the crystal structures were published showing neonicotinoids docking into the acetylcholine binding site of molluscan acetylcholine binding proteins with homology to the ligand binding domain (LBD) of nAChRs. The crystal structures showed that 1) glutamine in loop D, corresponding to the basic residues of insect nAChRs, hydrogen bonds with the NO(2) group of imidacloprid and 2) neonicotinoid-unique stacking and CH-pi bonds at the LBD. A neonicotinoid-resistant strain obtained by laboratory-screening has been found to result from target site mutations, and possible reasons for this are also suggested by the crystal structures. The prospects of designing neonicotinoids that are safe not only for mammals but also for beneficial insects such as honey bees (Apis mellifera) are discussed in terms of interactions with non-alpha nAChR subunits.

Involvement of NO-synthase and nicotinic receptors in learning in the honey bee

Restrained worker honey bees (Apis mellifera) are one of the main models for the comparative study of learning and memory processes. Bees easily learn to associate a sucrose reward to antennal tactile scanning of a small metal plate (associative learning). Their proboscis extension response can also be habituated through repeated sucrose stimulations (non-associative learning). We studied the role of nitric oxide synthase and nicotinic acetylcholine receptors in these two forms of learning. The nicotinic antagonist MLA or the nitric oxide synthase inhibitor l-NAME impaired the formation of tactile associative long-term memory that specifically occurs during multiple-trial training; however these drugs had no effect on single-trial training. None of the drugs affected retrieval processes. These pharmacological results are consistent with data previously obtained with olfactory conditioning and indicate that MLA-sensitive nicotinic receptors and NO-synthase are specifically involved in long-term memory. MLA and l-NAME both reduced the number of trials required for habituation to occur. This result suggests that a reduction of cholinergic nicotinic neurotransmission promotes PER habituation in the honey bee.

Effects of sublethal doses of acetamiprid and thiamethoxam on the behavior of the honeybee (Apis mellifera)

Acetamiprid and thiamethoxam are insecticides introduced for pest control, but they can also affect non-target insects such as honeybees. In insects, these neonicotinoid insecticides are known to act on acetylcholine nicotinic receptors but the behavioral effects of low doses are not yet fully understood. The effects of acetamiprid and thiamethoxam were studied after acute sublethal treatment on the behavior of the honeybee (Apis mellifera) under controlled laboratory conditions. The drugs were either administered orally or applied topically on the thorax. After oral consumption acetamiprid increased sensitivity to antennal stimulation by sucrose solutions at doses of 1 microg/bee and impaired long-term retention of olfactory learning at the dose of 0.1 microg/bee. Acetamiprid thoracic application induced no effect in these behavioral assays but increased locomotor activity (0.1 and 0.5 microg/bee) and water-induced proboscis extension reflex (0.1, 0.5, and 1 microg/bee). Unlike acetamiprid, thiamethoxam had no effect on bees' behavior under the conditions used. Our results suggest a particular vulnerability of honeybee behavior to sublethal doses of acetamiprid.

Exploring the pharmacological properties of insect nicotinic acetylcholine receptors

Insect nicotinic acetylcholine (nACh) receptors are molecular targets of insecticides such as neonicotinoids that are used to control disease-carrying insects and agricultural pests. To date, several insect nACh receptor subunits have been identified, indicating different nACh receptor subtypes and pharmacological profiles. Because of the difficulty in expressing functional insect nACh receptors in heterologous systems, new research tools are needed. Studies on insects resistant to the insecticide imidacloprid and on laboratory-generated hybrid and chimaeric nACh receptors in vitro have provided information about the molecular basis of receptor diversity, neonicotinoid resistance and selectivity. Additionally, recent results indicate that the sensitivity of insect nACh receptors to imidacloprid can be modulated by intracellular phosphorylation mechanisms, which offers a new approach to studying insect nACh receptor pharmacology.

Genomic dissection of behavioral maturation in the honey bee

Honey bees undergo an age-related, socially regulated transition from working in the hive to foraging that has been previously associated with changes in the expression of thousands of genes in the brain. To understand the meaning of these changes, we conducted microarray analyses to examine the following: (i) the ontogeny of gene expression preceding the onset of foraging, (ii) the effects of physiological and genetic factors that influence this behavioral transition, and (iii) the effects of foraging experience. Although >85% of approximately 5,500 genes showed brain differences, principal component analysis revealed discrete influences of age, behavior, genotype, environment, and experience. Young bees not yet competent to forage showed extensive, age-related expression changes, essentially complete by 8 days of age, coinciding with previously described structural brain changes. Subsequent changes were not age-related but were largely related to effects of juvenile hormone (JH), suggesting that the increase in JH that influences the hive bee-forager transition may cause many of these changes. Other treatments that also influence the onset age of foraging induced many changes but with little overlap, suggesting that multiple pathways affect behavioral maturation. Subspecies differences in onset age of foraging were correlated with differences in JH and JH-target gene expression, suggesting that this endocrine system mediates the genetic differences. We also used this multifactorial approach to identify candidate genes for behavioral maturation. This successful dissection of gene expression indicates that, for social behavior, gene expression in the brain can provide a robust indicator of the interaction between hereditary and environmental information.

Involvement of α-bungarotoxin-sensitive nicotinic receptors in long-term memory formation in the honeybee (Apis mellifera)

In the honeybee Apis mellifera, multiple-trial olfactory conditioning of the proboscis extension response specifically leads to long-term memory (LTM) which can be retrieved more than 24 h after learning. We studied the involvement of nicotinic acetylcholine receptors in the establishment of LTM by injecting the nicotinic antagonists mecamylamine (1 mM), alpha-bungarotoxin (alpha-BGT, 0.1 mM) or methyllycaconitine (MLA, 0.1 mM) into the brain through the median ocellus 20 min before or 20 min after multiple-trial learning. The retention tests were performed 1, 3, and 24 h after learning. Pre-training injections of mecamylamine induced a lower performance during conditioning but had no effect on LTM formation. Post-training injections of mecamylamine did not affect honeybees' performances. Pre-training injections of MLA or post-training injection of alpha-BGT specifically induced LTM impairment whereas acquisition as well as memory retrieval tested 1 or 3 h after learning was normal. This indicates that brain injections of alpha-BGT and MLA did not interfere with learning or medium-term memory. Rather, these blockers affect the LTM. To explain these results, we advance the hypothesis that honeybee alpha-BGT-sensitive acetylcholine receptors are also sensitive to MLA. These receptors could be essential for triggering intracellular mechanisms involved in LTM. By contrast, medium-term memory is not dependent upon these receptors but is affected by mecamylamine.

Acetylcholine, GABA and glutamate induce ionic currents in cultured antennal lobe neurons of the honeybee, Apis mellifera

The honeybee, Apis mellifera, is a valuable model system for the study of olfactory coding and its learning and memory capabilities. In order to understand the synaptic organisation of olfactory information processing, the transmitter receptors of the antennal lobe need to be characterized. Using whole-cell patch-clamp recordings, we analysed the ligand-gated ionic currents of antennal lobe neurons in primary cell culture. Pressure applications of acetylcholine (ACh), gamma-amino butyric acid (GABA) or glutamate induced rapidly activating ionic currents. The ACh-induced current flows through a cation-selective ionotropic receptor with a nicotinic profile. The ACh-induced current is partially blocked by alpha-bungarotoxin. Epibatidine and imidacloprid are partial agonists. Our data indicate the existence of an ionotropic GABA receptor which is permeable to chloride ions and sensitive to picrotoxin (PTX) and the insecticide fipronil. We also identified the existence of a chloride current activated by pressure applications of glutamate. The glutamate-induced current is sensitive to PTX. Thus, within the honeybee antennal lobe, an excitatory cholinergic transmitter system and two inhibitory networks that use GABA or glutamate as their neurotransmitter were identified.

Quantification of imidacloprid uptake in maize crops

The systemic imidacloprid is one of the most used insecticides in the world for field and horticultural crops. This neurotoxicant is often used as seed-dressing, especially for maize, sunflower, and rape. Using a LC/MS/MS technique (LOQ = 1 microg/kg and LOD = 0.1 microg/kg), the presence of imidacloprid has been measured in maize from field samples at the time of pollen shed, from less than 0.1 microg/kg up to 33.6 microg/kg. Numerous random samples were collected throughout France from 2000 to 2003. The average levels of imidacloprid measured are 4.1 microg/kg in stems and leaves, 6.6 microg/kg in male flowers (panicles), and 2.1 microg/kg in pollen. These values are similar to those found previously in sunflower and rape. These results permit evaluation of the risk to honeybees by using the PEC/PNEC ratios (probable exposition concentrations/predicted no effect concentration). PEC/PNEC risk ratios were determined and ranged between 500 and 600 for honeybees foraging on maize treated with imidacloprid by seed dressing. Such a high risk factor can be related to one of the main causes of honeybee colony losses.

Apisα2, Apisα7-1 and Apisα7-2: three new neuronal nicotinic acetylcholine receptor α-subunits in the honeybee brain

Acetylcholine is the principal excitatory neurotransmitter in the central nervous system of insects. Nicotinic acetylcholine receptors, which belong to the ligand-gated ion channel family, constitute important targets for insecticides. In the honeybee Apis mellifera, pharmacological evidence supports the existence of several nicotinic acetylcholine receptors. In this paper, we report the identification of three new genes that encode nicotinic acetylcholine receptor alpha-subunits in the honeybee. Phylogenetic comparisons with other ligand-gated ion channel subunit sequences support their classification as Apisalpha2, Apisalpha7-1 and Apisalpha7-2 subunits. Based on in situ hybridization experiments, we determined their expression patterns in the different brain regions of pupae and adult honeybees. Our results show that these nicotinic acetylcholine receptor subunits are differently expressed among the brain regions and that they appear at different stages of honeybee development.

Effects of NMDA receptor antagonists on olfactory learning and memory in the honeybee (Apis mellifera)

In contrast to vertebrates the involvement of glutamate and N-methyl-D-aspartate (NMDA) receptors in brain functions in insects is both poorly understood and somewhat controversial. Here, we have examined the behavioural effects of two noncompetitive NMDA receptor antagonists, memantine (low affinity) and MK-801 (high affinity), on learning and memory in honeybees (Apis mellifera) using the olfactory conditioning of the proboscis extension reflex (PER). We induced memory deficit by injecting harnessed individuals with a glutamate transporter inhibitor, L-trans-2,4-PDC (L-trans-2,4-pyrrolidine dicarboxylate), that impairs long-term (24 h), but not short-term (1 h), memory in honeybees. We show that L-trans-2,4-PDC-induced amnesia is 'rescued' by memantine injected either before training, or before testing, suggesting that memantine restores memory recall rather than memory formation or storage. When injected alone memantine has a mild facilitating effect on memory. The effects of MK-801 are similar to those of L-trans-2,4-PDC. Both pretraining and pretesting injections lead to an impairment of long-term (24 h) memory, but have no effect on short-term (1 h) memory of an olfactory task. The implications of our results for memory processes in the honeybee are discussed.