The correlation between tick (Ixodes persulcatus Sch.) questing behaviour and synganglion neuronal responses to odours

We examined the behavioural and electrophysiological responses of taiga ticks (Ixodes persulcatus) to several olfactory stimuli: Osmopherone® (5-a-androst-16-en-3-ol), Osmopherine® (butanoic and 3-methylbutanoic acids), DEET® (N,N-diethyl-meta-toluamide), ethanol (96%), and water (control stimulus). To study individual tick behavioural reactions to these stimuli, we used a Y-shaped glass maze (n = 50). To study the electrophysiological reactions of the ticks’ synganglia to these olfactory stimuli, we recorded the shifts of total potential (TP) of pre-oesophageal neurons in response to odour stimulation of Haller’s organ (n = 25). We found that Osmopherine® attracted ticks and frequently evoked negative shifts of TP, whereas the response to Osmopherone® did not differ from the reaction to water. DEET® and ethanol acted as tick repellents and generally evoked positive shifts of TP. We also tested each tick for its questing height (QH) on a glass rod that was at an incline of 75°, and we tested for the presence of pathogens i.e., DNA of Borrelia burgdorferi sp. s.l. and RNA of tick-borne encephalitis virus (TBEV). The degree of response to Osmopherine® positively correlated with the QH. The ticks with the highest values of QH showed a greater prevalence of the tick-borne pathogen Borrelia sp. s.l. compared with the ticks that did not reach the maximum QH. The present results show a correlation between the electrophysiological reaction of the synganglia of ticks and their behavioural responses to different odours.     

Taste- and odor-reactivity in elderly demented patients

Previous studies demonstrated that hedonically different chemical(taste or smell) stimuli induceinnate, inherited, differentialand distinct fixed reflectory motion features in the oral andfacial area. In the present study 20 elderly demented patients,suffering from ‘probable’ or ‘possible’Alzheimer's disease, and 20 normally functioning elderly subjectswere tested. The facial expressive behavioral reactions triggeredby a set of common gustatory and olfactory stimuli were videotaped.Both psychophysical and stimulus-dependent behavioral responseswere obtained from the control group, while for the dementedpatients only behavioral reactions were recorded. Results revealedthat: (i) severely demented elderly subjects displayed differentialand distinct orofacial responses indicating ‘acceptance’and ‘aversion’. These were found to be analogousto but less intense than those displayed by control age mates;(ii) the duration of responses induced by aversive tastes islonger than that triggered by pleasant or indifferent ones,for both groups; (iii) all gustatory and olfactory stimuli triggera longer lasting behavioral response in demented than in normalsubjects; and (iv) psychophysical and behavioral responses ofthe control subjects gave similar results for taste- and odor-hedonicsas well as for their intensity. This finding clearly indicatesthe validity of the alternative use of psychophysical and behavioraltesting procedures.     

Multimodal cues drive host-plant assessment in Asian citrus psyllid (Diaphorina citri)

Asian citrus psyllid (Diaphorina citri) transmits the causal agent of Huanglongbing, a devastating disease of citrus trees. In this study we measured behavioral responses of D. citri to combinations of visual, olfactory, and gustatory stimuli in test arenas. Stimuli were presented to the psyllids in droplets or lines of an emulsified wax formulation in two different arena types in no-choice tests. First, when placed on a colored ring situated halfway between the center and perimeter of a petri dish, D. citri spent more time on yellow versus gray rings; however, this response disappeared when either gray or yellow wax droplets were applied. When the psyllids were presented with droplets scented with terpenes, the response to both scent and color was increased. The addition of a dilute (?0.1 M) sucrose solution to the wax droplets increased the magnitude of D. citri responses. Next, groups of D. citri were placed on plastic laboratory film covering a sucrose solution, to mimic a leaf surface. Test stimuli were presented via two 'midribs' made from lines of emulsified wax formulation. Probing levels were measured as a function of color saturation and scent composition, and concentration. The test scents were based on qualitatively major volatiles emitted by Murraya paniculata (L.) Jack, Citrus aurantifolia (Christm.) Swingle, and C. sinensis (L.) Osbeck. The highest probing response was observed on the middle concentration (20-μl scent/10 ml wax formulation) of the C. aurantifolia-scented wax lines. Results indicate that there are interactive effects between the different sensory modalities in directing host-plant assessment behavior.     

A novel test system for detection of tick repellents

A bioassay was developed to examine the response of ticks towards potential repellents that may protect vertebrates against tick bites. Such tick repellents must be effective despite the unavoidable presence of various attractive host-derived stimuli. Therefore, a moving-object-bioassay (MO-bioassay) was developed that mimicks body warmth and movement of vertebrates by a rotating and heated drum. Compounds which were tested for their effects on ticks were applied onto a small elevated area of the drum. Ticks were allowed to approach the drum by walking on a glass rod which ended 1 mm away from the local elevation. Ticks could cling to this elevation that intermittently passed by, whereas the remaining drum surface was too far away from the tip of the rod to be contacted by the ticks.Without the presence of any repellents, 85.5% of 600 hungry, field-collected Ixodes ricinus nymphs moved to the heated, rotating drum within 2 min. Further experiments with unfed I. ricinus nymphs were performed to test whether one established and two potential tick repellents elicit an avoidance reaction in the ticks despite the proven attractiveness of the drum. Freshly applied DEET (N,N-diethyl-m-toluamide) at a concentration of 0.11 mg cm^–2 proved active as a repellent in the MO-bioassay over a distance of a few mm as well as by direct contact. A similar repellent effect was observed with (–)-myrtenal at 1 mg cm^–2, but not at 0.1 mg cm^–2, indicating that this terpenaldehyde is a weaker repellent for I. ricinus nymphs than DEET. No repellent effect was observed with camphor (0.1 mg cm^–2).The MO-bioassay thus is a rapid, simple and low-cost test method allowing the investigation of tick host-contact behaviour as well as the screening of candidate repellents which are either perceived as volatiles or via contact chemoreception.     

Responses of Littoraria irrorata say (Mollusca: Gastropoda) to water-borne chemicals: a comparison of chemical sources and orientation mechanisms

The salt marsh periwinkle, Littoraria irrorata, exhibits orientation behaviors in response to a number of chemical stimuli. In this study, we quantified the response of L. irrorata to chemical stimuli derived from three sources: Spartina alterniflora (a food source), the blue crab Callinectes sapidus (a predator), and crushed conspecifics. Animals consistently moved toward sources of S. alterniflora extract and crushed conspecifics relative to controls, but did not approach or avoid extracts from C. sapidus relative to controls. Animals had lower response rates to stimuli from S. alterniflora than to controls, but not to putatively aversive chemical stimuli (C. sapidus or crushed conspecifics). However, periwinkles that moved in experimental groups and control groups did not differ in how far they crawled. In addition, we measured turning angles of pathways of periwinkles to determine the orientation mechanism used. Littoraria irrorata displayed a taxis in response to crushed conspecifics, but a kinesis in response to C. sapidus and S. alterniflora. Our results suggest that the ecological context of chemical cues influence the evolution of orientation mechanisms. Cues that require immediate response (crushed conspecifics) elicit directed movement using taxis, whereas those that represent less alarming stimuli (a blue crab in the vicinity) or that represent a ubiquitous resource (Spartina) elicit movement by kinesis.     

Influence of emotional stimuli on behavior and respiration of rabbits with various locomotor activity in the "open field"

Different behavioral reactivity of rabbit groups differentiated by locomotor activity in the "open field" was revealed during exposure to emotional stimuli (rustle, loud sound, pressuring on back of the neck, vibroacoustic tactile stimulation of an ear). In passive rabbits, the active locomotor reactions were induced harder and freezing was obtained easier than in active animals. During exposure to sound stimuli, passive rabbits increased their locomotion more rarely than active animals, pressing on back of the neck produced longer freezing, a threshold of defensive ear shaking in response to a vibroacoustic stimulus in passive animals was highest. Training to mild immobilization increased the threshold of defensive responses in active rabbits and animals of the intermediate type. Changes in respiratory parameters were correlated with behavioral reactions to emotional stimuli. The duration of exhalation and respiratory cycle increased during freezing and increased during enhanced locomotion. The duration of inhalation decreased in response to emotional stimuli irrespective of a behavioral reaction. The respiratory reactions to emotional stimuli differed in rabbits of different groups. The respiratory rate more frequently changed in passive rabbits than in animals of other groups. Passive animals reacted mainly by exhalation, active rabbits and animals from the intermediate group predominantly responded by inhalation.     

Fetal Nicotine Exposure Increases Preference for Nicotine Odor in Early Postnatal and Adolescent,but Not Adult,Rats

Human studies demonstrate a four-fold increased possibility of smoking in the children of mothers who smoked during pregnancy. Nicotine is the active addictive component in tobacco-related products, crossing the placenta and contaminating the amniotic fluid. It is known that chemosensory experience in the womb can influence postnatal odor-guided preference behaviors for an exposure stimulus. By means of behavioral and neurophysiologic approaches, we examined whether fetal nicotine exposure, using mini-osmotic pumps, altered the response to nicotine odor in early postnatal (P17), adolescent (P35) and adult (P90) progeny. Compared with controls, fetal exposed rats displayed an altered innate response to nicotine odor that was evident at P17, declined in magnitude by P35 and was absent at P90 - these effects were specific to nicotine odor. The behavioral effect in P17 rats occurred in conjunction with a tuned olfactory mucosal response to nicotine odor along with an untoward consequence on the epithelial response to other stimuli – these P17 neural effects were absent in P35 and P90 animals. The absence of an altered neural effect at P35 suggests that central mechanisms, such as nicotine-induced modifications of the olfactory bulb, bring about the altered behavioral response to nicotine odor. Together, these findings provide insights into how fetal nicotine exposure influences the behavioral preference and responsiveness to the drug later in life. Moreover, they add to a growing literature demonstrating chemosensory mechanisms by which patterns of maternal drug use can be conveyed to offspring, thereby enhancing postnatal vulnerability for subsequent use and abuse.     

Role of visual and olfactory cues from agricultural hedgerows in the orientation behavior of multicolored Asian lady beetle (Coleoptera: Coccinellidae)

Harmonia axyridis Pallas is an introduced lady beetle common in eastern North American agroecosystems. Two-choice behavioral bioassays were performed to determine whether visual and olfactory stimuli from prey and host habitats could elicit taxis in wild-collected H. axyridis adults and whether beetles exhibit a preference among stimuli. Soybean aphid (Aphis glycines Matsumura) spends much of the year in agricultural hedgerows residing on buckthorn (Rhamnus cathartica L), and H. axyridis is frequently observed feeding on aphids in this habitat. Olfactory bioassays were performed in a Y-tube olfactometer and tested the response of beetles to the odor of buckthorn leaves, apple leaves (Malus domestica Borkh.), and buckthorn leaves both naturally and artificially infested with A. glycines. No differences were observed between the numbers of beetles moving toward the odor of buckthorn artificially infested with A. glycines and uninfested buckthorn, but more beetles preferred naturally infested buckthorn over uninfested buckthorn. Visual bioassays were performed in an acrylic tube arena,and tested beetle response to silhouettes and to apple and buckhorn leaves. Beetles were significantly more likely to choose silhouettes over blank space in visual trials. Significantly more beetles moved toward buckthorn leaves than blank space, but beetles did not discern between apple and buckthorn until olfactory cues were also included. This study lays the foundation for future work examining the response of H. axyridis to visual and olfactory cues in Ontario agroecosystems, which could help enhance effectiveness of H. axyridis as a biological control and mitigate its impacts as a pest species.     

The circadian rhythm of the sex-pheromone-mediated behavioral response in the turnip moth, Agrotis segetum, is not controlled at the peripheral level

The pheromone-mediated upwind flight of male turnip moths was observed in a flight tunnel at different times of day under conditions of a light-dark (LD) cycle, constant darkness (DD), and a shifted photoperiod. Under both LD and DD conditions, a significantly larger number of males flew to the pheromone during both the scotophase and the subjective scotophase than during the photophase and the subjective photophase for 2 consecutive days. When 1-day-old moths were transferred to a shifted LD cycle with lights turned off 4 h earlier, male behavioral responses to the pheromone advanced in time accordingly by 4 h. This showed that male behavioral responses to the pheromone are under the control of an endogenous oscillator. To further examine the level at which the circadian rhythm of the male behavioral response is regulated, the authors tested the olfactory responses of male antennal receptors to pheromone stimuli by means of electroantennograms (EAG) at different times of day. No significant variation in the sensitivity of the male antennal response to the pheromone was observed in terms of time of day. The results suggest that circadian regulation of the rhythmic behavioral response to pheromones in the male Agrotis occurs at the central nervous system level.     

Age and rearing environment interact in the retention of early olfactory memories in honeybees

Due to the changing behavioral contexts at which social insects are exposed during the adult lifespan, they are ideal models to analyze the effect of particular sensory stimuli during young adulthood on later behavior. Specifically, our goal is to understand early influences on later foraging behavior. For that, olfactory memories were established by worker honeybees to different pre-foraging ages using either (1) classical conditioning in the proboscis extension response (PER) paradigm or (2) the offering of scented-sugar solution under different rearing conditions. By testing long-term memories (LTM) through a single PER test in workers of foraging ages (17-25 days), we found that retention of the early olfactory memories in honey bees is age-dependent and not time-dependent. Independently of the environmental conditions in which they were reared (laboratory cages or hives), bees were able to retain food-odor association from 5 days after emergence, but rarely before. In most experiments we observed a bi-modal pattern of response: bees exposed to scented-food at 5-8 and 13-16 days showed better retention than those exposed at 9-12 days. These differences disappeared for bees reared in hives. Retrieval of LTMs depending on the timing and the continuous inputs of appropriate sensory stimuli are discussed.     

Effects of adrenergic blockers on corticotropin-releasing factor-induced behavioral changes in rats

The effects of adrenoreceptor blocking agents on corticotropin-releasing factor (CRF)-induced behavioral changes in rats were examined. The i.c.v. injection of 1 micrograms ovine CRF significantly increased the grooming frequency, number of occurrences of rearing and total distance moved. I.c.v. administered phentolamine at a dose of 10 nmol completely suppressed the increase in rearing and total distance moved induced by CRF without affecting the grooming frequency, whereas 100 nmol phentolamine significantly decreased the grooming frequency as well as the rearing and total distance moved. In contrast, propranolol reduced the increase in rearing induced by CRF only at a dose which induced ataxia in rats. The increases in rearing and total distance moved induced by CRF were reduced by 10 nmol of yohimbine and 100 nmol of prazosin. S.c. injection of caffeine (10 mg/kg) produced a significant increase in grooming frequency, rearing, and total movement. Administration of 10 nmol phentolamine and yohimbine did not affect these behavioral changes induced by caffeine, while 100 nmol prazosin suppressed them. Therefore, prazosin depressed the behavior of rats non-specifically. These results suggest that CRF-induced behavioral hyperactivity is mediated at least in part by alpha-noradrenergic, mainly alpha 2-noradrenergic, systems in the brain.     

Effects of olfactory stimuli on arm-reaching duration

The aim of the present study was to investigate the effects of olfactory stimuli on visually guided reaching. In Experiment 1, participants reached toward and grasped either a small (almond/strawberry) or a large (apple/orange) visual target. Any 1 of 4 odors corresponding to the visual stimuli or odorless air was administered before movement initiation. Within the same block of trials, participants smelled 1) an odor associated with an object of a different size than the target, 2) an odor associated with an object of a size equal to that of the target, or 3) odorless air. Results indicated that reaching duration was longer for trials in which the odor "size" and the visual target did not match than when they matched. In Experiment 2, the same procedures were applied but the "no-odor" trials were administered in a separate block to the "odor" trials. Similar results as for Experiment 1 were found. However, in contrast to Experiment 1, the presence of an odor increased the level of alertness resulting in a shortening of reaching duration. We contend that olfactory stimuli have the capacity to elicit motor plans interfering with those programmed for a movement toward a visual stimulus.     

A phenomenological model of the perceived intensity of single odorants

The response of a model olfactory system to a single odorant is quantified by interconnecting three separate stimulus-response relationships. Together, these relationships encompass the deposition of odorant molecules onto an olfactory organ, their movement to the dendrite of the olfactory receptor neuron, their subsequent induction of action potentials, and the processing of induced and spontaneous action potentials by the central nervous system, resulting in perception and a behavioral response. Phenomena discussed within the context of the model include the behavioral threshold, central summation of responses from a number of olfactory neurons, and the effect of organ shape on olfactory detection.The intent of the model is to provide a quantitative conceptual framework for designing and interpreting experiments relating sensory input to perception and behavior. Its utility is particularly evident for insect olfaction since it enables insect sex pheromone behavioral thresholds to be estimated from the literature when bioassays or electrophysiological studies are not possible. It also derives a physiologically meaningful method for comparing behavioral thresholds among different animals, and permits comparisons of different kinds of behavioral responses in the same species. Vertebrate olfaction is treated briefly in a discussion of the effect of sniffing on the threshold of detection.     

Effect of Ninjin-yoei-to (Rensheng-Yangrong-Tang) on olfactory behavior after olfactory nerve transection

Ninjin-yoei-to (NYT), a Japanese traditional medicine, is used to treat athrepsia due to surgery, anorexia, cold constitution, and anemia. There are reports of the effects of NYT on the nervous system; however, there have been no behavioral studies of the effect of NYT on olfactory function. The olfactory system undergoes continuous replacement of sensory neurons. Morphologic and behavioral studies have shown that the olfactory system recovers after bilateral olfactory nerve transection (BNX). However, in the humans, olfactory function does not always recover. In this study, we examined the effect of oral NYT on behavioral recovery after BNX. Fourteen mice were subjected to BNX. The regular diet was mixed with 2% NYT (NYT diet). Mice were separated into two groups; seven mice were fed the regular diet (control group), and seven mice were fed the NYT diet (NYT group). NYT was administered beginning 7 days prior to BNX and continuing for 35 days after BNX. Mice in both groups had free access to food and water. Olfactory function was evaluated by testing each mouse's ability to avoid cotton balls treated with acetic acid. After BNX, mice lost their ability to avoid cotton balls treated with acetic acid. In the control group, the time for behavioral recovery after BNX was 28 days. In the NYT group, the time for behavioral recovery after BNX was 21 days. NYT hastened behavioral recovery after BNX. NYT may have therapeutic benefits for patients with olfactory disorders.     

Responses of the tick, Amblyomma hebraeum (Acari: Ixodidae), to carbon dioxide

We investigated the effect of physiological state of adult ticks (Amblyomma hebraeum Koch; Acari: Ixodidae) on their responses to a CO2-enriched atmosphere. Ticks were exposed to CO2 or air in an experimental chamber; we measured the effects on questing, movement and latency (the time after exposure to CO2 or air when ticks began questing or moving). This study confirmed the hypothesis that responsiveness to CO2 is enhanced during host-seeking periods of the life cycle and reduced at other times. Thus, whereas newly moulted ticks quested infrequently when exposed to CO2, questing increased over the 6 week period following moulting. Likewise, in 6 week old ticks, movement in CO2 was significantly greater than in air. When a partially fed female below approximately ten times its unfed weight is removed from a host, it reattaches if subsequently presented to another host. Such females quested vigorously in CO2 but not in air. When a partially fed female above approximately ten times its unfed weight is removed from a host, it usually does not subsequently reattach. Such females quested infrequently, there being no significant difference between air and CO2. Males removed from a host at any stage of feeding reattach readily when given the opportunity. They quested frequently, moved readily and the latency was very low in air or CO2. Exp Appl Acarol 22: 667–681 © 1998 Kluwer Academic Publishers     

Women with a History of Childhood Maltreatment Exhibit more Activation in Association Areas Following Non-Traumatic Olfactory Stimuli: A fMRI Study

Background

The aim of this study was investigating how women with a history of childhood maltreatment (CM) process non-threatening and non-trauma related olfactory stimuli. The focus on olfactory perception is based on the overlap of brain areas often proposed to be affected in CM patients and the projection areas of the olfactory system, including the amygdala, orbitofrontal cortex, insula and hippocampus.

Methods

Twelve women with CM and 10 controls participated in the study. All participants were, or have been, patients in a psychosomatic clinic. Participants underwent a fMRI investigation during olfactory stimulation with a neutral (coffee) and a pleasant (peach) odor. Furthermore, odor threshold and odor identification (Sniffin'' Sticks) were tested.

Principal Findings

Both groups showed normal activation in the olfactory projection areas. However, in the CM-group we found additionally enhanced activation in multiple, mainly neocortical, areas that are part of those involved in associative networks. These include the precentral frontal lobe, inferior and middle frontal structures, posterior parietal lobe, occipital lobe, and the posterior cingulate cortex.

Conclusions

The results indicate that in this group of patients, CM was associated with an altered processing of olfactory stimuli, but not development of a functional olfactory deficit. This complements other studies on CM insofar as we found the observed pattern of enhanced activation in associative and emotional regions even following non-traumatic olfactory cues.     

Altered behavior of the snail Biomphalaria glabrata as a result of infection with Schistosoma mansoni

In this comparative behavioral study, the effect of infection with Schistosoma mansoni on its snail intermediate host Biomphalaria glabrata was investigated. Three groups of snails were compared for their activity: (1) uninfected, (2) infected with male parasites, and (3) infected with female parasites. In solitary movement trials, uninfected snails traveled greater distances at faster rates, explored more surface area, and had shorter rest periods than snails infected with either male or female schistosomes. In Y-maze experiments designed to determine attraction, the uninfected snails more often and more quickly moved toward other snails than the infected individuals. Snails from all 3 groups were more attracted to infected individuals than to uninfected ones. There was no difference in attraction toward snails infected with male or female parasites. These experiments provide evidence that behavioral alterations as a result of infection may lead to aggregation of infected snails in the field. We propose that such an effect may result in enhanced parasite transmission.     

Tip-recording from ixodid tick olfactory sensilla: Responses to tick related odours

Summary The tip-recording technique was used to obtain electrophysiological responses from the tarsal olfactory sensilla, md3 and apl, in ixodid ticks,Rhipicephalus appendiculatus andAmblyomma variegatum. Stimuli tested were phenolic compounds, 2,6-dichlorophenol,P-cresol, phenol and salicylaldehyde, reported to act as sex pheromone of various ixodid ticks. Certain cells responded to vapours from cartridges charged with as little as 0.04ng of 2,6-dichlorophenol while at least 200 g of any of the other compounds was required to exert the minimum effect. Response patterns of cells in sexually immature and mature males were similar but response levels were higher among sexually immature males. Vapours fromn-hexane washings of adult and larval ticks or from live virgin females contained potent stimuli which caused response patterns similar to those evoked by 2,6-dichlorophenol. The chemical identity and behavioural significance of those stimuli are still under investigation.     

Can audio–visual or visual stimuli from a prospective mate stimulate a reproductive neuroendocrine response in sheep?

Stimuli from a prospective mate increase the secretion of luteinising hormone (LH) in sheep. This 'male effect' in ewes and 'female effect' effect in rams is predominantly mediated by olfactory signals, though it is thought that non-olfactory signals play synergistic or substitutive roles. In this study, we tested whether exposure to visual or audio-visual stimuli from a prospective mate would stimulate an increase in LH secretion in ewes (Experiment 1) and rams (Experiment 2). In Experiment 1, groups of eight Merino ewes were exposed to one of three stimuli midway through a frequent blood-sampling regimen: full ram contact, still images of rams, a video of ewes and rams mating. Control ewes (n = 8) were completely isolated from rams. Exposure to still images of rams appeared to stimulate an increase in mean LH concentrations (P < 0.05) and tended to increase LH pulse frequency (P < 0.1), but the response was significantly smaller than that observed in ewes exposed to rams (P < 0.01). Audio-visual stimuli had no effect on any parameters of LH secretion (P > 0.1). In Experiment 2, Merino rams were allocated to either an Exposure (n = 7) or a Control (n = 7) group. Exposure rams underwent two exposure periods midway through a frequent blood-sampling regimen; exposure to still images of ewes and audio recorded during mating of ewes and rams (audio-visual exposure); exposure to oestrous ewes (ewe exposure). Control rams were sampled at the same frequency but remained isolated from ewe stimuli. Exposure of rams to the audio-visual stimuli did not affect any parameters of LH secretion (P > 0.1). In contrast, exposure to oestrous ewes increased LH pulse frequency (P < 0.05) and advanced the onset of the next LH pulse (P < 0.05). In conclusion, visual signals appear to be involved in eliciting the neuroendocrine response of ewes to rams and are of greater importance to this phenomenon in ewes (male effect) than rams (female effect). However, overall the visual and audio-visual signals used in this study were far less effective than stimulus animals, suggesting that these stimuli are less important than olfactory signals, or a combination of olfactory and audio-visual signals.     

Peripheral regulation by ecdysteroids of olfactory responsiveness in male Egyptian cotton leaf worms, Spodoptera littoralis

Physiological and behavioral plasticity allows animals to adapt to changes in external (environmental) and internal (physiological) factors. In insects, the physiological state modulates adult behavior in response to different odorant stimuli. Hormones have the potential to play a major role in the plasticity of the olfactory responses. To explore if peripheral olfactory processing could be regulated by steroid hormones, we characterized the molecular, electrophysiological, and behavioral response to changes in endogenous hormone levels in adult male Spodoptera littoralis. The expression of the receptor complex (EcR/USP) was localized by in situ hybridization in the olfactory sensilla of antennae. Injections of 20-hydroxyecdysone (20E) induced an ecdysteroid signaling pathway in antennae and increased expression of the nuclear receptors EcR, USP and E75. Diacylglycerol kinase (DGK) and CaM expression were also up-regulated by 20E. Taken together, these molecular, electrophysiological, and behavioral results suggest a hormonal regulation of the peripheral olfactory processing in S. littoralis.