Learning and memory in the mosquito Aedes aegypti shown by conditioning against oviposition deterrence

Biology and Behaviour of Aedes aegypti in the Human Environment: Opportunities for Vector Control of Arbovirus Transmission

Aedes aegypti is a ubiquitous vector of arboviruses mostly in urbanised areas throughout the tropics and subtropics and a growing threat beyond. Control of Ae. aegypti is difficult and costly, and no vaccines are available for most of the viruses it transmits. With practical control solutions our goal, ideally suitable for delivery by householders in affected communities, we reviewed the literature on adult Ae. aegypti biology and behaviour, within and close to the human home, the arena where such interventions must impact. We found that knowledge was vague or important details were missing for multiple events or activities in the mosquito life cycle, such as the duration or location of the many periods when females rest between blood feeding and oviposition. The existing body of literature, though substantial, is not wholly reliable, and evidence for commonly held "facts" range from untraceable to extensive. Source references of some basic information are poor or date back more than 60 years, while other information that today is accepted widely as "fact" is not supported by evidence in the literature. Many topics, e.g., sugar feeding, resting preferences (location and duration), and blood feeding, merit being revisited in new geographical regions and ecological contexts to identify vulnerabilities for exploitation in control.

Exploiting the chemical ecology of mosquito oviposition behavior in mosquito surveillance and control: a review

Vector control is an important component of the interventions aimed at mosquito-borne disease control. Current and future mosquito control strategies are likely to rely largely on the understanding of the behavior of the vector, by exploiting mosquito biology and behavior, while using cost-effective, carefully timed larvicidal and high-impact, low-volume adulticidal applications. Here we review the knowledge on the ecology of mosquito oviposition behavior with emphasis on the potential role of infochemicals in surveillance and control of mosquito-borne diseases. A search of PubMed, Embase, Web of Science, Global Health Archive, and Google Scholar databases was conducted using the keywords mosquito, infochemical, pheromone, kairomone, allomone, synomone, apneumone, attractant, host-seeking, and oviposition. Articles in English from 1974 to 2019 were reviewed to gain comprehensive understanding of current knowledge on infochemicals in mosquito resource-searching behavior. Oviposition of many mosquito species is mediated by infochemicals that comprise pheromones, kairomones, synomones, allomones, and apneumones. The novel putative infochemicals that mediate oviposition in the mosquito subfamilies Anophelinae and Culicinae were identified. The role of infochemicals in surveillance and control of these and other mosquito tribes is discussed with respect to origin of the chemical cues and how these affect gravid mosquitoes. Oviposition attractants and deterrents can potentially be used for manipulation of mosquito behavior by making protected resources unsuitable for mosquitoes (push) while luring them towards attractive sources (pull). In this review, strategies of targeting breeding sites with environmentally friendly larvicides with the aim to develop appropriate trap-and-kill techniques are discussed.

Attraction Versus Capture: Efficiency of BG-Sentinel Trap Under Semi-Field Conditions and Characterizing Response Behaviors for Female Aedes aegypti (Diptera: Culicidae)

Aedes aegypti (L.) is an important vector of viruses causing dengue, Zika, chikungunya, and yellow fever and as such is a threat to public health worldwide. Effective trapping methods are essential for surveillance of both the mosquito species and disease presence. The BG-Sentinel (BGS) is a widely used to trap Ae. aegypti but little is known of its efficiency, i.e., what proportion of the mosquitoes encountering the trap are captured. The first version of the BGS trap was predominantly white, and the current version is mostly navy blue. While this trap is often deployed without any olfactory lure, it can also be deployed with CO2 and/or a human skin odor mimic lure to increase capture rates. We tested the efficiency of capturing Ae. aegypti under semi-field conditions for the original white version without lures as well the blue version with and without various lure combinations. None of the configurations tested here captured 100% of the mosquitoes that encountered the trap. A navy-blue trap emitting CO2 and a skin odor mimic produced the highest capture (14% of the total insects in the semi-field cage), but its capture efficiency was just 5% (of mosquitoes encountering the trap). Mosquitoes often had multiple encounters with a trap that did not result in capture; they crossed over the trap entrance without being captured or landed on the sides of the trap. Understanding these behaviors and the factors that induce them has the potential to suggest improvement in trap design and therefore capture efficiency.

Sensory discrimination between aversive salty and bitter tastes in an haematophagous insect

Sensory aversion is essential for avoiding prospective dangers. We studied the chemical perception of aversive compounds of different gustatory modalities (salty, bitter) in the haematophagous bug, Rhodnius prolixus. Over a walking arena, insects avoided a substrate embedded with 1M NaCl or KCl if provided with water as an alternative. However, no preferences were expressed when both salts were opposed to each other. A pre-exposure to amiloride interfered with the repellency of NaCl and KCl equally, suggesting that amiloride-sensitive receptors are involved in the detection of both salts. Discriminative experiments were then performed to determine whether R. prolixus can distinguish between these salts. An aversive operant conditioning involving either NaCl or KCl modulated the repellency of the conditioned salt, but also of the novel salt. Repellency levels of both salts were rigid to a chemical pre-exposure to any of both salts. When gustatory modalities were crossed by presenting as a choice NaCl and a bitter molecule as caffeine (Caf), no innate preferences were expressed. Aversive operant conditionings with either NaCl or Caf rendered unspecific changes in the repellency of both compounds. A chemical pre-exposure to Caf modulated the response to Caf but not to NaCl, suggesting the existence of two independent neural pathways for the detection of salts and bitter compounds. Overall results suggest that R. prolixus cannot discriminate molecules of the same gustatory modality (i.e. salty), but can distinguish between salty and bitter tastes. The potential use of aversive gustatory stimuli as a complement of commercially available olfactory repellents is discussed.

A genomic approach to inferring kinship reveals limited intergenerational dispersal in the yellow fever mosquito

Understanding past dispersal and breeding events can provide insight into ecology and evolution and can help inform strategies for conservation and the control of pest species. However, parent-offspring dispersal can be difficult to investigate in rare species and in small pest species such as mosquitoes. Here, we develop a methodology for estimating parent-offspring dispersal from the spatial distribution of close kin, using pairwise kinship estimates derived from genome-wide single nucleotide polymorphisms (SNPs). SNPs were scored in 162 Aedes aegypti (yellow fever mosquito) collected from eight close-set, high-rise apartment buildings in an area of Malaysia with high dengue incidence. We used the SNPs to reconstruct kinship groups across three orders of kinship. We transformed the geographical distances between all kin pairs within each kinship category into axial standard deviations of these distances, then decomposed these into components representing past dispersal events. From these components, we isolated the axial standard deviation of parent-offspring dispersal and estimated neighbourhood area (129 m), median parent-offspring dispersal distance (75 m) and oviposition dispersal radius within a gonotrophic cycle (36 m). We also analysed genetic structure using distance-based redundancy analysis and linear regression, finding isolation by distance both within and between buildings and estimating neighbourhood size at 268 individuals. These findings indicate the scale required to suppress local outbreaks of arboviral disease and to target releases of modified mosquitoes for mosquito and disease control. Our methodology is readily implementable for studies of other species, including pests and species of conservation significance.

Effects of methylmercury on mosquito oviposition behavior: Maladaptive response to non-toxic exposure

Animals can modulate their own exposure to environmental contaminants through behavioral plasticity such as diet and habitat choice. However, it remains unclear if behavior also has cascading effects on contaminant exposure across multiple generations. In insects, oviposition site selection is an important behavior females can use to modify offspring contaminant exposure risk. In this study, we use the yellow fever mosquito, Aedes aegypti, to test how methylmercury (MeHg) affects oviposition site selection. We found that mosquito larval development rate and survival were negatively affected at MeHg concentrations ≥100 ppb. Adult females not exposed to MeHg as larvae avoided oviposition sites with high MeHg concentrations (>50 ppb), but MeHg exposure at the larval stage significantly affected this oviposition site selection. Specifically, females raised from larvae exposed to non-toxic MeHg levels (i.e., five-50 ppb) showed a significant increase in preference for oviposition sites contaminated with toxic MeHg concentrations (≥500 ppb), compared to unexposed controls. This maladaptive behavioral response could be because, when conditioned with non-toxic MeHg concentrations, MeHg-associated olfactory cues act as a "supernormal" stimulus during oviposition site selection. Importantly, however, this maladaptive behavioral response is eliminated in female mosquitoes raised from larvae exposed to toxic MeHg concentrations (i.e. 100 ppb), and these mosquitoes showed a significant increase in preference for MeHg uncontaminated oviposition sites, compared to unexposed controls. Thus, in mosquitoes, the magnitude of MeHg exposure in one generation can impact MeHg exposure in subsequent generations by altering oviposition site selection behavior. Our results have broad implications for our understanding of how contaminant-mediated behavioral modifications can feedback on contaminant exposure risk across multiple generations, and consequently how behavior can affect the evolutionary trajectory of organisms inhabiting a heterogeneously contaminated environment.

Dengue virus infection changes Aedes aegypti oviposition olfactory preferences

Aedes aegypti mosquitoes, main vectors for numerous flaviviruses, have olfactory preferences and are capable of olfactory learning especially when seeking their required environmental conditions to lay their eggs. In this study, we showed that semiochemical conditions during Aedes aegypti larval rearing affected future female choice for oviposition: water-reared mosquitoes preferred to lay eggs in water or p-cresol containers, while skatole reared mosquitoes preferred skatole sites. Using two independent behavioural assays, we showed that this skatole preference was lost in mosquitoes infected with dengue virus. Viral RNA was extracted from infected female mosquito heads, and an increase of virus load was detected from 3 to 10 days post infection, indicating replication in the insect head and possibly in the central nervous system. Expression of selected genes, potentially implied in olfactory learning processes, were also altered during dengue infection. Based on these results, we hypothesise that dengue virus infection alters gene expression in the mosquito’s head and is associated with a loss of olfactory preferences, possibly modifying oviposition site choice of female mosquitoes.

Fine-scale landscape genomics helps explain the slow spatial spread of Wolbachia through the Aedes aegypti population in Cairns, Australia

The endosymbiotic bacterium Wolbachia suppresses the capacity for arbovirus transmission in the mosquito Aedes aegypti, and can spread spatially through wild mosquito populations following local introductions. Recent introductions in Cairns, Australia have demonstrated slower than expected spatial spread. Potential reasons for this include: (i) barriers to Ae. aegypti dispersal; (ii) higher incidence of long-range dispersal; and (iii) intergenerational loss of Wolbachia. We investigated these three potential factors using genome-wide single-nucleotide polymorphisms (SNPs) and an assay for the Wolbachia infection wMel in 161 Ae. aegypti collected from Cairns in 2015. We detected a small but significant barrier effect of Cairns highways on Ae. aegypti dispersal using distance-based redundancy analysis and patch-based simulation analysis. We detected a pair of putative full-siblings in ovitraps 1312 m apart, indicating long-distance female movement likely mediated by human transport. We also found a pair of full-siblings of different infection status, indicating intergenerational loss of Wolbachia in the field. These three factors are all expected to contribute to the slow spread of Wolbachia through Ae. aegypti populations, though from our results it is unclear whether Wolbachia loss and long-distance movement are sufficiently common to reduce the speed of spatial spread appreciably. Our findings inform the strategic deployment of Wolbachia-infected mosquitoes during releases, and show how parameter estimates from laboratory studies may differ from those estimated using field data. Our landscape genomics approach can be extended to other host/symbiont systems that are being considered for biocontrol.

Presence of a predator image in potential breeding sites and oviposition responses of a dengue vector

In dengue vector control, attempts to minimize or replace the use of pesticides have mostly involved use of predators, but success has been severely impeded by difficulties associated with financial and environmental costs, predator mass production, and persistence in target habitats. Visual deterrents have been used successfully to control animal pests, in some cases in an effort to replace pesticide use. Despite evidence that visual signals are crucial in site choice for egg deposition by dengue vectors, and that female mosquitoes respond to artificial predation, the role of predator intimidation as it affects the oviposition behavior of dengue vectors remains largely unexplored. Here, we examined the oviposition responses of Aedes aegypti exposed to various mosquito predator pictures. Gravid females were presented with equal opportunities to oviposit in two cups with predator images [Toxorhynchites splendens-TXI, Goldfish (Carassius auratus)-small (SFI) and large (LFI) and Tx. splendens+Goldfish-TXFI] and two others without pictures. Differences in egg deposition were examined between sites with and without these images. When given a chance to oviposit in cups with and without TXI, Ae. aegypti females were similarly attracted to both sites. When provided an opportunity to oviposit in cups displaying pictures of fish (SFI or LFI) and blank cups, egg deposition rates were much lower in the fish picture sites. Females showed a preference for blank cups over TXFI for egg deposition. They also equally avoided cups with pictures of fish, regardless of the size of the picture. Our results indicate that the presence of images of goldfish and their association with Tx. larvae significantly reduced egg deposition by Ae. aegypti, and this was not the case with the predatory larvae alone. The observations that the images of natural predators can repel gravid females of a dengue vector provide novel possibilities to develop effective and inexpensive alternative tools to harmful insecticides.

Experience-dependent modulation of the attraction to faeces in the kissing bug Triatoma infestans

Triatoma infestans is the main vector of the Chagas disease in Latin America. These nocturnal bugs spend most of the daylight hours aggregated with conspecifics inside crevices in roofs and walls. Around the entrances of the shelters T. infestans deposits faeces that contain chemical cues that attract conspecifics. In this work we investigated whether attraction to faeces can be modulated by experience in this insect species. First, we analyzed if the attraction of nymphs to faeces is innate or acquired through previous sensory experiences. Results show that after hatching, 1st instar nymphs are attracted to faeces even if they had never been in contact with them before, thus indicating that this attraction is innate. Second, we studied if attraction to faeces can be influenced by the presence of con-specifics. No differences were found in the attraction to faeces of nymphs released alone or in groups, suggesting that attraction to faeces is independent of the presence of other individuals. Third, we examined if the innate response to faeces of nymphs can be modulated by experience. After pre-exposing nymphs to faeces during 24h, insects were no longer attracted to faeces. Finally, by pairing the presence of faeces with an aversive mechanical disturbance, nymphs switched from attraction to avoidance of faeces. These results show that although faeces attraction has a strong innate component, it can be modulated by experience. The learning and memory capacities of triatomines have been studied only recently, and our work is the first report on the effects of experience in the aggregation context.

Olfactory learning and chemical ecology of olfaction in disease vector mosquitoes: a life history perspective

Mosquitoes transmit many debilitating diseases including malaria, dengue and Zika. Odors mediate behaviors that directly impact disease transmission (blood-feeding) as well as life history events that contribute to mosquito survival and fitness (mating and oviposition, nectar foraging, larval foraging and predator avoidance). In addition to innate olfaction-mediated behaviors, mosquitoes rely on olfactory experience throughout their life to inform advantageous choices in many of these important behaviors. Previous reviews have addressed either the chemical ecology of mosquitoes, or olfactory-driven behaviors including host-feeding or oviposition. Adding to this literature, we use a holistic life history perspective to integrate and compare innate and learned olfactory behavior at various stages of mosquito development.

Learning in mosquito larvae (Aedes aegypti): Habituation to a visual danger signal

In spite of the mosquito Aedes aegypti being a vector of several infectious diseases, a limited number of studies has been undertaken on learning in this species. Moreover, larval stages have been neglected as model organisms, although they are active, aquatic and perform stereotyped behavioural responses, e.g. the escape response when disturbed. To study the learning abilities of mosquito larvae, we focused on habituation, a form of non-associative learning widely studied in vertebrates and invertebrates. Habituation was defined as the progressive and reversible decrease in response to a reiterative stimulus. We first aimed at confirming habituation of the escape response in mosquito larvae (4th instar). Then, we determined whether a mnesic trace was established. Larvae were individually stimulated with a visual danger stimulus inducing the escape response. We set up a protocol for testing larvae individually, allowing the control of different parameters that are crucial for the study of cognitive abilities. After 15 trials, the escape response of mosquitoes was significantly lower. A disturbance stimulus presented after the 15th trial, induced the escape response and reversed habituation. Retention was confirmed up to 1h after the last habituation trial. This original bioassay can be adapted for studying the physiology of learning and memory in mosquito larvae, for analysing the effects of chemicals in the water, the characterisation of the cognitive abilities related to the life history of different mosquito species across preimaginal stages.

Male Psyllids Differentially Learn in the Context of Copulation

In the Asian citrus psyllid, Diaphorina citri Kuwayama, stimulatory cuticular hydrocarbons act as sex pheromone attractants. Male psyllids locate aggregations of females using those olfactory cues, as well as vibrational communication on the plant surface. Although previous research has indicated that learning plays a role in modulating female reproductive behaviors in psyllids, it is unknown whether males similarly use learning to increase the likelihood of copulatory success. We used an olfactometer-based bio-assay to study the effects of experience on male response to female odor. First, we compared male attraction to female odor in virgin and previously mated males. Second, we tested the effect of several modes of experience with a novel odor, vanillin, to determine whether mating, feeding, or general environmental exposure elicited a learned response. We found that male attraction to female odor significantly increased after mating experience. In addition, we found that males learn about odor specifically in the context of mating, rather than feeding or general exposure. Electrophysiological measurements of antennal response to odorants confirmed that mating status did not affect the sensitivity of the peripheral nervous system to volatile stimuli implicating learning at the level of the central nervous system. These results suggest that male response to female odor is not an entirely innate behavior. Males may require mating experience with female conspecifics to develop attraction to those olfactory cues produced by the female and in association with the female’s habitat. This adaptive plasticity may allow males to detect females in an ever-changing environment and promote diversification and further specialization on different host genotypes.

Anopheles gambiae females readily learn to associate complex visual cues with the quality of sugar sources

The ability to learn plays a key role in tuning and adapting the behaviours of animals for their unpredictable biotopes. This also applies to insect vectors of disease. Anautogenous mosquitoes need to find both sugar and blood for survival and reproduction. Learning processes are expected to contribute to a mosquito's ability to undertake repeated feeding behaviours more efficiently with time, serving to decrease energy demands and avoid hazards. Here we report how visual learning by the Afrotropical malaria mosquito Anopheles gambiae allows it to readily associate visual cues with the quality of a sugar source. Circular black and white patterns were used as visual cues. An. gambiae females were conditioned in cages with a chequered pattern paired with sucrose and a concentric pattern paired with non-palatable sucrose-NaCl and with reverse combinations. Hours later, significantly higher numbers of feeding attempts were counted on sucrose paired with the chequered pattern following conditioning with the same combination. This was also the case on the concentric pattern paired with sucrose following conditioning with this combination. However, the effect was weaker than with sucrose paired with the chequered pattern. These findings indicate a differential capacity of visual stimuli to induce learning, explained in our experiments by a significantly higher mosquito appetence on sucrose paired with a chequered pattern that mimics floral shape. Training that induced a higher propensity for feeding attempts was found to allow the females to display a fast learning curve (<4min) on the less suitable concentric pattern paired with sucrose, several hours after conditioning on the chequered pattern paired with sucrose. This has important implications for mosquito behavioural ecology, suggesting that An. gambiae shows plasticity in its learning capacities that would allow it to readily turn to alternative sources for a sugar meal once initiated in the process by an appropriate stimulus combination.

Psychology of Learning: A New Approach to Study Behavior of Rhodnius Prolixus Stal under Laboratory Conditions

Conditioning methodologies associated with the psychology of learning are suggested as a new strategy to investigate behavior of the assassin bug Rhodnius prolixus, which is the main vector of Chagas disease in Venezuela. Chagas disease is the fourth leading cause of death in Latin America, as it causes severe chronic illness and approximately 43,000 deaths per year. To illustrate this strategy, two preliminary experiments are reported. In the first, Pavlovian conditioning was examined by pairing an olfactory conditioned stimulus with a temperature unconditioned stimulus. A temperature of 42°C elicits a complex behavioral sequence in R. prolixus consisting of proboscis extension and crawling. Over the course of 12 training trials, this behavioral sequence was not elicited by an olfactory conditioned stimulus. In the second experiment, a latent inhibition paradigm was used to pre-expose R. prolixus to an olfactory conditioned stimulus before pairing the odor with temperature. Over the course of training, an effect of pre-exposure was found. Suggestions for research are discussed and potential conditioned and unconditioned stimuli identified.

Learning and Memory in Disease Vector Insects

Learning and memory plays an important role in host preference and parasite transmission by disease vector insects. Historically there has been a dearth of standardized protocols that permit testing their learning abilities, thus limiting discussion on the potential epidemiological consequences of learning and memory to a largely speculative extent. However, with increasing evidence that individual experience and associative learning can affect processes such as oviposition site selection and host preference, it is timely to review the recently acquired knowledge, identify research gaps and discuss the implication of learning in disease vector insects in perspective with control strategies.

Neuroethology of Olfactory-Guided Behavior and Its Potential Application in the Control of Harmful Insects

Harmful insects include pests of crops and storage goods, and vectors of human and animal diseases. Throughout their history, humans have been fighting them using diverse methods. The fairly recent development of synthetic chemical insecticides promised efficient crop and health protection at a relatively low cost. However, the negative effects of those insecticides on human health and the environment, as well as the development of insect resistance, have been fueling the search for alternative control tools. New and promising alternative methods to fight harmful insects include the manipulation of their behavior using synthetic versions of "semiochemicals", which are natural volatile and non-volatile substances involved in the intra- and/or inter-specific communication between organisms. Synthetic semiochemicals can be used as trap baits to monitor the presence of insects, so that insecticide spraying can be planned rationally (i.e., only when and where insects are actually present). Other methods that use semiochemicals include insect annihilation by mass trapping, attract-and- kill techniques, behavioral disruption, and the use of repellents. In the last decades many investigations focused on the neural bases of insect's responses to semiochemicals. Those studies help understand how the olfactory system detects and processes information about odors, which could lead to the design of efficient control tools, including odor baits, repellents or ways to confound insects. Here we review our current knowledge about the neural mechanisms controlling olfactory responses to semiochemicals in harmful insects. We also discuss how this neuroethology approach can be used to design or improve pest/vector management strategies.

Experience- and age-mediated oviposition behaviour in the yellow fever mosquito Stegomyia aegypti (=Aedes aegypti)

In repeated behaviours such as those of feeding and reproduction, past experiences can inform future behaviour. By altering their behaviour in response to environmental stimuli, insects in highly variable landscapes can tailor their behaviour to their particular environment. In particular, female mosquitoes may benefit from plasticity in their choice of egg-laying site as these sites are often temporally variable and clustered. The opportunity to adapt egg-laying behaviour to past experience also exists for mosquito populations as females typically lay eggs multiple times throughout their lives. Whether experience and age affect egg-laying (or oviposition) behaviour in the mosquito Stegomyia aegypti (=Aedes aegypti) (Diptera: Culicidae) was assessed using a wind tunnel. Initially, gravid mosquitoes were provided with a cup containing either repellent or well water. After ovipositing in these cups, the mosquitoes were blood-fed and introduced into a wind tunnel. In this wind tunnel, an oviposition cup containing repellent was placed in the immediate vicinity of the gravid mosquitoes. A cup containing well water was placed at the opposite end of the tunnel so that if the females flew across the chamber, they encountered the well water cup, in which they readily laid eggs. Mosquitoes previously exposed to repellent cups became significantly more likely to later lay eggs in repellent cups, suggesting that previous experience with suboptimal oviposition sites informs mosquitoes of the characteristics of nearby oviposition sites. These results provide further evidence that mosquitoes modify behaviour in response to environmental information and are demonstrated in a vector species in which behavioural plasticity may be ecologically and epidemiologically meaningful.

Circadian modulation of learning abilities in a disease vector insect, Rhodnius prolixus

Despite the drastic consequences it may have on the transmission of parasites, the ability of disease vectors to learn and retain information have just begun to be characterized. The kissing-bug Rhodnius prolixus, vectors of the Chagas disease, is an excellent model, particularly because conditioning the proboscis extension response (PER) constitutes a valuable paradigm to study their cognitive abilities under carefully controlled conditions. Another characteristic of these bugs is the temporal organisation of their different activities in a bimodal endogenous daily rhythm. This offers the opportunity to address the implication of the circadian system in learning and memory. Using aversive conditioning of the PER, we have tested whether the ability of kissing-bugs to learn and remember information varies during the day. We found that bugs perform well during the night, but not during the day: their ability to acquire information -but not that to retrieve it- is modulated in time. When keeping bugs under constant conditions to analyse the origin of this rhythm, the rhythm continues to freerun, showing its endogenous and truly circadian nature. These results are the first ones to evince the implication of the circadian system in the learning abilities of disease vectors and one of the few in insects in general.

Olfactory learning and memory in the disease vector mosquito Aedes aegypti

Olfactory learning in blood-feeding insects, such as mosquitoes, could play an important role in host preference and disease transmission. However, standardised protocols allowing testing of their learning abilities are currently lacking, and how different olfactory stimuli are learned by these insects remains unknown. Using a Pavlovian conditioning paradigm, we trained individuals and groups of Aedes aegypti mosquitoes to associate an odorant conditioned stimulus (CS) with a blood-reinforced thermal stimulus (unconditioned stimulus; US). Results showed, first, that mosquitoes could learn the association between L-lactic acid and the US, and retained the association for at least 24 h. Second, the success of olfactory conditioning was dependent upon the CS--some odorants that elicited indifferent responses in naïve mosquitoes, such as L-lactic acid and 1-octen-3-ol, were readily learned, whereas others went from aversive to attractive after training (Z-3-hexen-1-ol) or were untrainable (β-myrcene and benzyl alcohol). Third, we examined whether mosquitoes' ability to learn could interfere with the action of the insect repellent DEET. Results demonstrated that pre-exposure and the presence of DEET in the CS reduced the aversive effects of DEET. Last, the nature of the formed memories was explored. Experiments using cold-shock treatments within the first 6 h post-training (for testing anaesthesia-resistant memory) and a protein synthesis inhibitor (cycloheximide; to disrupt the formation of long-term memory) both affected mosquitoes' performances. Together, these results show that learning is a crucial component in odour responses in A. aegypti, and provide the first evidence for the functional role of different memory traces in these responses.

Habitat discrimination by gravid Anopheles gambiae sensu lato--a push-pull system

BACKGROUND

The non-random distribution of anopheline larvae in natural habitats suggests that gravid females discriminate between habitats of different quality. Whilst physical and chemical cues used by Culex and Aedes vector mosquitoes for selecting an oviposition site have been extensively studied, those for Anopheles remain poorly explored. Here the habitat selection by Anopheles gambiae sensu lato (s.l.), the principal African malaria vector, was investigated when presented with a choice of two infusions made from rabbit food pellets, or soil.

METHODS

Natural colonization and larval survival was evaluated in artificial ponds filled randomly with either infusion. Dual-choice, egg-count bioassays evaluated the responses of caged gravid females to (1) two- to six-day old infusions versus lake water; (2) autoclaved versus non-autoclaved soil infusions; and assessed (3) the olfactory memory of gravid females conditioned in pellet infusion as larvae.

RESULTS

Wild Anopheles exclusively colonized ponds with soil infusion and avoided those with pellet infusion. When the individual infusions were tested in comparison with lake water, caged An. gambiae sensu stricto (s.s.) showed a dose response: females increasingly avoided the pellet infusion with increasing infusion age (six-day versus lake water: odds ratio (OR) 0.22; 95% confidence interval (CI) 0.1-0.5) and showed increasing preference to lay eggs as soil infusion age increased (six-day versus lake water: OR 2.1; 95% CI 1.4-3.3). Larvae survived in soil infusions equally well as in lake water but died in pellet infusions. Anopheles gambiae s.s. preferred to lay eggs in the non-autoclaved soil (OR 2.6; 95% CI 1.8-3.7) compared with autoclaved soil. There was no change in the avoidance of pellet infusion by individuals reared in the infusion compared with those reared in lake water.

CONCLUSION

Wild and caged An. gambiae s.l. females discriminate between potential aquatic habitats for oviposition. These choices benefit the survival of the offspring. Although the study was not designed to distinguish between stimuli that acted over a distance or on contact, it could be demonstrated that the choice of habitat is mediated by chemical cues based on both preference and avoidance. These cues, if identified, might be developed for 'push-pull' strategies to improve malaria vector monitoring and control.

The main component of an alarm pheromone of kissing bugs plays multiple roles in the cognitive modulation of the escape response

Innate responses in animals can be modulated by experience. Disturbed adults of the triatomine bug Triatoma infestans release an alarm pheromone (AP) that elicits an escape response in conspecific larvae. The main component of this AP, the isobutyric acid (IsoAc), alone has already shown to generate an escape response in this species. However, not much is known about the modulation of this behavior by non-associative and associative cognitive processes. We present here evidences of the cognitive capacities of T. infestans larvae in an escape context under different conditioning paradigms, including IsoAc in different roles. We show that: (1) the duration of a pre-exposure to IsoAc plays a main role in determining the type of non-associative learning expressed: short time pre-exposures elicit a sensitization while a longer pre-exposure time triggers a switch from repellence to attractiveness; (2) a simple pre-exposure event is enough to modulate the escape response of larvae to the AP and to its main component: IsoAc; (3) IsoAc and the AP are perceived as different chemical entities; (4) an association between IsoAc and an aversive stimulus can be created under a classical conditioning paradigm; (5) an association between IsoAc and a self-action can be generated under an operant conditioning. These results evince that IsoAc can attain multiple and different cognitive roles in the modulation of the escape response of triatomines and show how cognitive processes can modulate a key behavior for surviving, as it is the escaping response in presence of a potential danger in insects.

Aedes aegypti mosquitoes exhibit decreased repellency by DEET following previous exposure

DEET (N,N-Diethyl-m-toluamide) is one of the most widely used mosquito repellents. Although DEET has been shown to be extremely effective, recent studies have revealed that certain individual insects are unaffected by its presence. A genetic basis for this has been shown in Aedes aegypti mosquitoes and the fruit fly Drosophila melanogaster, but, for the triatomine bug, Rhodnius prolixus, a decrease in response to DEET occurred shortly after previous exposure, indicating that non-genetic factors may also be involved in DEET “insensitivity”. In this study, we examined host-seeking behaviour and electrophysiological responses of A. aegypti after pre-exposure to DEET. We found that three hours after pre-exposure the mosquitoes showed behavioural insensitivity, and electroantennography revealed this correlated with the olfactory receptor neurons responding less to DEET. The change in behaviour as a result of pre-exposure to DEET has implications for the use of repellents and the ability of mosquitoes to overcome them.

Plant species- and status-specific odorant blends guide oviposition choice in the moth Manduca sexta

The reproductive success of herbivorous insects largely depends on the mother's oviposition preference. In nocturnal insects, olfaction is arguably the most important sensory modality mediating mate finding, foraging, and host location. In most habitats, gravid females select among a number of plants of varying suitability, yet assessment of the neuroethological mechanisms underlying odor-guided choice between host plants is rare. Using a series of behavioral, electrophysiological, and chromatographic analyses in the Hawk moth, Manduca sexta, we show that gravid females perform a hierarchical choice among host plants of different species and qualities using olfactory cues. Both relevant plant species and qualities can be distinguished by volatile profiles collected from the headspace of these plants, and olfactory sensilla on female antennae detect more than half of the about 120 analytically detected volatiles in host plant headspace samples. Although olfactory sensory neurons present in antennal sensilla are mainly broadly tuned to multiple host compounds, some sensilla exhibit species and condition-specific responses. In fact, species and quality can be distinguished by the physiologically active components alone. Our findings thus suggest that distinguishing characteristics of both host species and quality are already represented at the sensory periphery.

Associative learning in the dengue vector mosquito, Aedes aegypti: avoidance of a previously attractive odor or surface color that is paired with an aversive stimulus

Associative learning has been shown in a variety of insects, including the mosquitoes Culex quinquefasciatus and Anopheles gambiae. This study demonstrates associative learning for the first time in Aedes aegypti, an important vector of dengue, yellow fever and chikungunya viruses. This species prefers to rest on dark surfaces and is attracted to the odor of 1-octen-3-ol. After training in which a dark surface alone or a dark surface with odor was paired with electric shock, mosquitoes avoided the previously attractive area. The association was stronger when odor was included in training, was retained for at least 60 min but not for 24 h, and was equal for males and females. These results demonstrate the utility of a bulk-training paradigm for mosquitoes similar to that used with Drosophila melanogaster.

Learned host preference in a Chagas disease vector, Rhodnius prolixus

It has been largely assumed that the individual experience of disease vector insects may affect host choice and, as a consequence, have an important influence on parasite transmission. In particular, it is speculated that vector insects should be able to learn and remember the most and/or less defensive hosts, shifting their preference accordingly. Nevertheless, despite the invested efforts in testing the capacity to learn and remember information of blood-sucking insects, only little conclusive information has been obtained hitherto. Recently, the ability of Rhodnius prolixus to associate a behaviourally neutral odour to the perspective of either obtaining a blood-meal or being punished has been demonstrated, the same odour becoming attractant or repellent for the bugs, respectively, according to the individual previous experience. The present work represents a step forward in the study of the cognitive abilities of Chagas disease vectors and their influence on host choice. We tested whether or not bugs bias their choice for a host based on the association of its odour with a negative experience. Our results show that whereas naïve bugs presented no preference when confronted to the odour of two different hosts, bugs previously exposed to the contingency of the odour of one host and a mechanical perturbation mimicking defensive behaviour, biased their preference towards the other host. This constitutes the first evidence of olfactory conditioning to host odours in triatomine bugs, vectors of Chagas disease and one of the few available up to date on haematophagous insects. The epidemiological significance of this finding is discussed.

Visual and olfactory associative learning in the malaria vector Anopheles gambiae sensu stricto

BACKGROUND

Memory and learning are critical aspects of the ecology of insect vectors of human pathogens because of their potential effects on contacts between vectors and their hosts. Despite this epidemiological importance, there have been only a limited number of studies investigating associative learning in insect vector species and none on Anopheline mosquitoes.

METHODS

A simple behavioural assays was developed to study visual and olfactory associative learning in Anopheles gambiae, the main vector of malaria in Africa. Two contrasted membrane qualities or levels of blood palatability were used as reinforcing stimuli for bi-directional conditioning during blood feeding.

RESULTS

Under such experimental conditions An. gambiae females learned very rapidly to associate visual (chequered and white patterns) and olfactory cues (presence and absence of cheese or Citronella smell) with the reinforcing stimuli (bloodmeal quality) and remembered the association for up to three days. Associative learning significantly increased with the strength of the conditioning stimuli used. Importantly, learning sometimes occurred faster when a positive reinforcing stimulus (palatable blood) was associated with an innately preferred cue (such as a darker visual pattern). However, the use of too attractive a cue (e.g. Shropshire cheese smell) was counter-productive and decreased learning success.

CONCLUSIONS

The results address an important knowledge gap in mosquito ecology and emphasize the role of associative memory for An. gambiae's host finding and blood-feeding behaviour with important potential implications for vector control.

Effects of natal habitat odour, reinforced by adult experience, on choice of oviposition site in the mosquito Aedes aegypti

The effects of natal experience on the oviposition behaviour of adult female mosquitoes were investigated in the laboratory using Aedes aegypti (L.) (Diptera: Culicidae). 'Treatment' mosquitoes were exposed to a dilute repellent (inducing stimulus) in their breeding water (aquatic stages) and/or in the air (adults) during various combinations of life stages [larval only (L regime); larval and pupal (LP regime); larval, pupal and emergent adult (LPE regime); larval, pupal, emergent adult and adult (LPEA regime); pupal, emergent adult and adult (PEA regime); adult only (A regime)]. 'Control' mosquitoes were raised in an identical manner, but were not exposed to the inducing stimulus. The oviposition behaviour of treatment and control females was assessed in an oviposition assay that presented a choice of water with or without the inducing stimulus. Of the 435 mosquitoes tested in the experiment, 176 were non-distributors (i.e. laid all of their eggs in only one of the choices). Treatment females (distributors plus non-distributors) reared in the presence of the inducing stimulus throughout their lives (LPEA regime) showed a significant preference for the oviposition option containing the inducing stimulus (24/36 females) compared with corresponding controls (5/39 females). Distributors reared under the LPEA and PEA regimes also showed this preference (6/6 treatment vs. 2/29 control females, and 13/18 treatment vs. 7/23 control females, respectively). Females that had been exposed to the inducing stimulus as either immatures or adults only showed no preference for, and some showed an aversion to, the treatment oviposition option. This is interpreted as evidence for a natal habitat preference induction (NHPI) in this species, albeit one that requires extensive reinforcement in the adult stage. This adult experience-reinforced NHPI (AER-NHPI) is discussed in terms of its adaptive significance for container breeders, the possible timing mechanism and sensory basis of induction and potential practical consequences.

Learning the way to blood: first evidence of dual olfactory conditioning in a blood-sucking insect, Rhodnius prolixus. I. Appetitive learning

It has been largely assumed that the individual experience of insects that are disease vectors might not only contribute to animal fitness, but also have an important influence on parasite transmission. Nevertheless, despite the invested efforts in testing the capacity to learn and remember information in blood-sucking insects, only little conclusive information has been obtained to date. Adapting a classical conditioning approach to our haematophagous model, we trained larvae of Rhodnius prolixus to associate l-lactic-acid, an odour perceived by these bugs but behaviourally neutral when presented alone, with food (i.e. positive reinforcement). Naive bugs – those exposed either to a conditioned stimulus (CS, l-lactic acid), unconditioned stimulus (US, heat) and reward (blood) alone or CS, US and reward in the absence of contingency – remained indifferent to the presence of an air stream loaded with l-lactic acid when tested in an olfactometer (random orientation), whereas the groups previously exposed to the contingency CS–US–reward (blood) were significantly attracted by l-lactic-acid. In a companion paper, the opposite, i.e. repellence, was induced in bugs exposed to the contingency of the same odour with a negative reinforcement. This constitutes the first evidence of olfactory conditioning in triatomine bugs, vectors of Chagas disease, and one of the few substantiations available to date of olfactory conditioning in haematophagous insects.

Learning in a sedentary insect predator: antlions (Neuroptera: Myrmeleontidae) anticipate a long wait

Pit-building antlions, the larvae of a winged adult insect, capture food by digging funnel-shaped pits in sand and then lying in wait, buried at the vertex, for prey to fall inside. The sedentary nature of this sit-and-wait predatory behaviour and, especially, antlions' innate ability to detect prey arrival, do not fit the typical profile of insects that possess learning capabilities. However, we show, for the first time, that learning can play an important role in this unique form of predation. In three separate experiments, individual antlions received, once per training day, either a vibrational cue presented immediately before the arrival of food or that same cue presented independently of food arrival. Signalling of food not only produced a learned anticipatory behavioural response (Experiment 1), but also conferred a fitness advantage: Associative learning enabled antlions to dig better pits (Experiments 2 and 3), extract food more efficiently (Experiments 2 and 3), and, in turn, moult sooner (Experiment 3) than antlions not receiving the associative learning treatment.

Melanotaenia duboulayi influence oviposition site selection by Culex annulirostris (Diptera: Culicidae) and Aedes notoscriptus (Diptera: Culicidae) but not Culex quinquefasciatus (Diptera: Culicidae)

Some species of mosquito can detect the presence of larvivorous fish and select against ovipositing in pools supporting them. The effect of kairomones released by the crimson-spotted rainbowfish Melanotaenia duboulayi (Castelnau) on the oviposition behavior of the freshwater mosquitoes Culex annulirostris Skuse, Culex quinquefasciatus Say, and Aedes notoscriptus Skuse was evaluated in both laboratory and outdoor artificial pool experiments. In the laboratory, colony-reared Cx. annulirostris selected against ovipositing in water that had contained M. duboulayi at densities of one fish per 5, 30, 180, and 1,080 liters. In contrast, gravid Ae. notoscriptus showed a preference for water that had contained the highest density of M. duboulayi (1 in 5 liters). Gravid Cx. quinquefasciatus were neither repelled nor attracted to water that had previously contained M. duboulayi. In outdoor artificial pool experiments, wild Cx. annulirostris females selected against ovipositing in pools containing caged M. duboulayi stocked at rates of 1, 4, and 10 g per 1,000 liters. When fish were removed from the pools, the repellent effect persisted for at least 24 h, whereas at the lower stocking rate, an avoidance response was not noted until 48 h after fish were placed in pools. This suggests a volatile substance, however, the exact nature of the kairomone/s has not been identified.

[Spatial and/or olfactory memory in sandflies in an endemic area for American cutaneous leishmaniasis, southern Brazil]

The results from an investigation on the possibility that sandflies in an endemic area for American cutaneous leishmaniasis, in the state of Paraná, may have memory are reported. Sandflies were caught in Recanto Marista, Doutor Camargo, State of Paraná, Brazil, using Falcão traps in two chicken sheds (G1 and G2), between November 15 and 26, 2007. A total of 2,080 sandflies were caught (1,000 in G1 and 1,080 in G2) and these were marked and released. Nyssomyia neivai was the most (90.5%) frequent species. Out of the total released, 168 sandflies (8%) were recaptured and the recapture rate in G2 was significant. The results show that it is possible that spatial or olfactory memory and/or host loyalty exists, and that this will guide the sandflies in recognizing the places where sources of blood are available.

Influence of container size, location, and time of day on oviposition patterns of the dengue vector, Aedes aegypti, in Thailand

We conducted a study to determine the effect of container size and location on oviposition site selection by Ae. aegypti in large outdoor field enclosures (10 x 10 x 4 m high). There was a strong positive relationship between increasing container diameter, container volume, and water surface area with egg numbers over both high (rainy, July) and low (cool-dry, January) dengue transmission seasons. Location of containers (indoors versus immediately outdoors and underneath houses) did not influence the number of eggs deposited for containers 5-32 cm in diameter in either season. No trends based on container color (black, brown, or grey) were observed. A slight trend with a greater numbers of eggs laid outdoors in the largest containers (42 cm diameter) during the dry season was observed. Three separate models were run using the mixed model procedure in SAS for each container attribute. Controlling for season, time, and date, the most important container attribute predicting total egg numbers was container volume (total capacity) explaining 88% of the variation, followed by water surface area (85%), and container diameter opening (83%). Oviposition peaked in the afternoon at 1600 hrs and 2000 hrs in the dry and rainy seasons, respectively. Few eggs were laid overnight (2000 hrs-0600 hrs). Our results indicate that physical attributes of oviposition sites, such as size, light-dark contrasts, and specular reflectance from water surfaces, play a significant role in oviposition site selection.

Naive and conditioned responses of Culex pipiens pipiens biotype molestus (Diptera: Culicidae) to flower odors

Flower odors are important signals for chemical communication between plants and flower visitors. Here, we studied the naive responses of Culex pipiens pipiens biotype molestus Forskal 1775 (Diptera: Culicidae) to typical flower odors and assessed the learning capacity of mosquitoes to floral volatiles. The odor compounds used in the bioassay, phenyl acetaldehyde, veratrole, and 2-methoxyphenol, are typically found in the floral odor of Silene otites (L.) Wibel, a plant that is pollinated by nectar-drinking mosquitoes and moths, and/or in other closely related Silene species. Wind tunnel bioassays with a mixture of these compounds revealed that attraction of mosquitoes to odors was positively correlated with time passed since the last feeding. In single component bioassays, mosquitoes showed strong innate responses to phenyl acetaldehyde and only moderate or weak responses to veratrole and 2-methoxyphenol. Furthermore, in comparison with naive mosquitoes, conditioned mosquitoes were significantly more attracted to the mixture and single volatiles. These results indicate that naive mosquitoes are effectively attracted by appropriate floral scent compounds and that learning can increase the attractiveness of these compounds.

Naive and conditioned responses of Culex pipiens pipiens biotype molestus (Diptera: Culicidae) to flower odors

Flower odors are important signals for chemical communication between plants and flower visitors. Here, we studied the naive responses of Culex pipiens pipiens biotype molestus Forskal 1775 (Diptera: Culicidae) to typical flower odors and assessed the learning capacity of mosquitoes to floral volatiles. The odor compounds used in the bioassay, phenyl acetaldehyde, veratrole, and 2-methoxyphenol, are typically found in the floral odor of Silene otites (L.) Wibel, a plant that is pollinated by nectar-drinking mosquitoes and moths, and/or in other closely related Silene species. Wind tunnel bioassays with a mixture of these compounds revealed that attraction of mosquitoes to odors was positively correlated with time passed since the last feeding. In single component bioassays, mosquitoes showed strong innate responses to phenyl acetaldehyde and only moderate or weak responses to veratrole and 2-methoxyphenol. Furthermore, in comparison with naive mosquitoes, conditioned mosquitoes were significantly more attracted to the mixture and single volatiles. These results indicate that naive mosquitoes are effectively attracted by appropriate floral scent compounds and that learning can increase the attractiveness of these compounds.

The 'ghosts' that pester studies on learning in mosquitoes: guidelines to chase them off

The identification of memory and learning in medically important mosquito species has been of epidemiological interest mainly because of the implications of learning on the pattern of contact between vectors and hosts. Empirical results either showing or suggesting the existence of cognitive abilities in mosquitoes have been reported in a number of experimental studies, mainly based on the observation of individual fidelity towards subsets of specific resources, such as hosts, resting sites or breeding sites. A closer inspection of the design of these experiments shows that, with the exception of recent studies providing stronger evidence of learning in the genus Culex (Diptera: Culicidae), methodological shortcomings still hinder the possibility of eliminating alternative interpretations for these findings, in some cases because the experiments were not specifically designed to identify the phenomenon, but mostly because of a lack of appropriate controls or replication. By highlighting these limitations, while acknowledging the practical difficulties that are inherent to the field, we aim to help expel from future research the 'ghosts' that still preclude the achievement of more definite conclusions about the prevalence of memory and learning in this group of insects.

Experience-altered oviposition responses to a neem-based product, Neemix, by the diamondback moth, Plutella xylostella

The oviposition responses of the diamondback moth, Plutella xylostella L., to Neemix 4.5, a neem-based oviposition repellent/deterrent, with or without previous experience were studied in the laboratory on plain aluminum foil sheets, on aluminum foil sheets coated with cabbage juice, and on cabbage plants. In the plain aluminum foil sheet experiment, the females without prior experience of Neemix (inexperienced females) deposited more eggs (66.2%) on sections with untreated sheets than on sections with Neemix-treated sheets (33.8%), indicating that the inexperienced females were significantly repelled and/or deterred by Neemix. In contrast, the Neemix-experienced females deposited more eggs (69.2-69.7%) on sections with Neemix-treated sheets than on sections with untreated sheets (30.3-30.8%), implying that the females were not repelled and/or deterred by Neemix following an experience. In the tests using egg-laying sheets coated with cabbage juice or with cabbage juice plus Neemix, the inexperienced females deposited more eggs (63.8%) on the sections with sheets treated with cabbage juice only than on the sections with sheets coated with cabbage juice and Neemix (36.2%), indicating that inexperienced females were repelled and/or deterred by Neemix. Similarly, Neemix-experienced females deposited more eggs (62.2-65.6%) on the sections with sheets treated with cabbage juice plus Neemix than on the sections with sheets treated with juice only (34.4-37.8%), suggesting that the females were not repelled and/or deterred by Neemix following an experience. On cabbage plants, inexperienced females oviposited similar numbers of eggs on both Neemix-treated (50.8%) and untreated plants (49.2%). However, the Neemix-experienced females oviposited more eggs on the plants treated with Neemix (56.3%) than on untreated plants (43.7%), indicating that the females were attracted by Neemix-treated plants following an experience. The significance of this study for applications of insect repellents/deterrents in pest management is discussed.

Aspects of bioecology of two Rift Valley Fever Virus vectors in Senegal (West Africa): Aedes vexans and Culex poicilipes (Diptera: Culicidae)

The dispersal, population dynamics, and age structure of two Rift Valley Fever Virus (Phlebovirus: Bunyaviridae) (RVFV) vectors, Aedes vexans Meigen and Culex poicilipes Theobald, were investigated in northern Senegal. The main objective was to investigate possible factors that mediate RVFV emergence and propagation at a site where humans and livestock live in proximity to temporary surface pools. In mark-release-recapture studies, recapture rates of 0.18% (156/85,500) and 3.46% (201/5,800) were obtained for Ae. vexans and Cx. poicilipes, respectively. The number of mosquitoes recaptured decreased with increasing distance from the release point and over time. The estimated daily survival rate for released females ranged from 91 to 96% for Ae. vexans and 70-79% for Cx. poicilipes. The maximum time after release when marked mosquitoes were collected was 23 and 12 d for Ae. vexans and Cx. poicilipes, respectively. The maximum distances from the release point that marked females were recaptured was 620 and 550 m for Ae vexans and Cx. poicilipes, respectively. Rainfall periodicity was a key factor controlling Ae. vexans population abundance. In contrast, rainfall had no discernible effect on the fluctuation of Cx. poicilipes numbers. The involvement of these two species in the transmission of RVFV is discussed with respect to their longevity and daily survival rate.