Floral to green: mating switches moth olfactory coding and preference

Mating induces profound physiological changes in a wide range of insects, leading to behavioural adjustments to match the internal state of the animal. Here, we show for the first time, to our knowledge, that a noctuid moth switches its olfactory response from food to egg-laying cues following mating. Unmated females of the cotton leafworm (Spodoptera littoralis) are strongly attracted to lilac flowers (Syringa vulgaris). After mating, attraction to floral odour is abolished and the females fly instead to green-leaf odour of the larval host plant cotton, Gossypium hirsutum. This behavioural switch is owing to a marked change in the olfactory representation of floral and green odours in the primary olfactory centre, the antennal lobe (AL). Calcium imaging, using authentic and synthetic odours, shows that the ensemble of AL glomeruli dedicated to either lilac or cotton odour is selectively up- and downregulated in response to mating. A clear-cut behavioural modulation as a function of mating is a useful substrate for studies of the neural mechanisms underlying behavioural decisions. Modulation of odour-driven behaviour through concerted regulation of odour maps contributes to our understanding of state-dependent choice and host shifts in insect herbivores.     

Host odours enhance the responses of adult Rhyzopertha dominica to male-produced aggregation pheromone

The behavioural responses of adult male and female Rhyzopertha dominica (Coleoptera, Bostrichidae) to blends of host volatiles and male-produced aggregation pheromone were observed in a four-arm airflow olfactometer. The odour sources used were five pheromone-releasing males each on a single maize grain (lower maize-volatiles blend), five pheromone-releasing males on 500 g of maize (higher maize-volatiles blend) or the host volatiles emanating from 500g of maize (maize volatiles alone). Multiple-choice tests, in which volatiles from all three odour sources were presented in the exposure chamber at the same time, were used to study odour preferences of the males and females. Both sexes showed strongest attraction to the higher maize volatile blend but there were significant sex differences in response to the odour sources. Males spent significantly more time than females in the zone with only maize volatiles, and females spent significantly more time in the higher maize-volatiles zone. However, when odour sources were offered singly, females gave numerically greater responses than males to all sources although this difference was statistically significant for only the lower maize-volatiles blend. As males are more attracted than females to host odours alone it is suggested that they may be more highly adapted to seek out new hosts while females are more inclined to locate a food source by following the pheromone signals produced by males. However, both sexes responded most strongly to the odour source comprising aggregation pheromone with the higher proportion of maize volatiles.     

Differential interactions of sex pheromone and plant odour in the olfactory pathway of a male moth

Most animals rely on olfaction to find sexual partners, food or a habitat. The olfactory system faces the challenge of extracting meaningful information from a noisy odorous environment. In most moth species, males respond to sex pheromone emitted by females in an environment with abundant plant volatiles. Plant odours could either facilitate the localization of females (females calling on host plants), mask the female pheromone or they could be neutral without any effect on the pheromone. Here we studied how mixtures of a behaviourally-attractive floral odour, heptanal, and the sex pheromone are encoded at different levels of the olfactory pathway in males of the noctuid moth Agrotis ipsilon. In addition, we asked how interactions between the two odorants change as a function of the males' mating status. We investigated mixture detection in both the pheromone-specific and in the general odorant pathway. We used a) recordings from individual sensilla to study responses of olfactory receptor neurons, b) in vivo calcium imaging with a bath-applied dye to characterize the global input response in the primary olfactory centre, the antennal lobe and c) intracellular recordings of antennal lobe output neurons, projection neurons, in virgin and newly-mated males. Our results show that heptanal reduces pheromone sensitivity at the peripheral and central olfactory level independently of the mating status. Contrarily, heptanal-responding olfactory receptor neurons are not influenced by pheromone in a mixture, although some post-mating modulation occurs at the input of the sexually isomorphic ordinary glomeruli, where general odours are processed within the antennal lobe. The results are discussed in the context of mate localization.     

Factors affecting the antennal and behavioural responses of the saw-toothed grain beetle Oryzaephilus surinamensis to food odour and aggregation pheromone

ABSTRACT. The effects of insect age, sex, mated state and culture density on the attraction of adult saw-toothed grain beetles, Oryzaephilus surinamensis (L.) (Coleoptera: Silvanidae), to sources of food odour and synthetic aggregation pheromone were investigated. In single-insect behavioural assays, insect age affected the attraction to the food odour, but not to the synthetic aggregation pheromone. Conversely, there were sexual differences in the attraction to the aggregation pheromone, but not to the food odour. The effects of the same factors on the antennal response to food odour and synthetic pheromone were also investigated, using the electroantennogram (EAG) technique. Insect age affected the EAG response to food odour, but not to the synthetic pheromone. There were no sexual differences in the EAG responses to either food odour or synthetic pheromone.
The correlation of factors affecting both behavioural and antennal responses, and the possible role of changes in peripheral olfactory receptor sensitivity in the modulation of behaviour are discussed.     

Host plant volatiles induce oriented flight behaviour in male European grapevine moths, Lobesia botrana

The European grapevine moth Lobesia botrana relies on a female produced sex pheromone for long-distance mate finding. Grapevine moth males compete heavily during limited time windows for females. The aim of this study was to investigate the perception of host plant volatiles by grapevine moth males and whether such compounds elicit upwind oriented flights. We compared five host plant headspace extracts by means of gas chromatography linked electroantennogram (EAG) recording. We identified 12 common host plant volatiles (aliphatic esters, aldehydes, and alcohols, aromatic compounds and terpenes) that elicit EAG responses from grapevine moth males and that occur in at least three of the host plant volatile headspace extracts tested. Subsequently the behavioural response of grapevine moth males to four these compounds presented singly and in mixtures (1-hexanol, 1-octen-3-ol, (Z)-3-hexenyl acetate and (E)-β-caryophyllene) was recorded in a wind tunnel. Grapevine moth males engaged in upwind flights to all of four compounds when released singly at 10,000 pg/min and to all, except 1-octen-3-ol, when released at 100 pg/min. A blend of the four host plant volatiles released at 10,000 pg/min and mixed at a ratio based on the analysis of Vitis vinifera cv. Solaris volatile emissions attracted significantly more males than any single compound. Grapevine moth males perceive and respond to host plant volatiles at biologically relevant levels indicating that host plant volatiles figure as olfactory cues and that L. botrana males can discern places where the likelihood of encountering females is higher.     

The roles of vision and olfaction in mate location by males of the tsetse fly Glossina morsitans morsitans

The roles of visual and/or olfactory stimuli in eliciting mating responses from male Glossina morsitans morsitans Westwood were examined, using a system for automatically recording the number and duration of mating strikes made towards decoys, under controlled conditions. The results confirm that there is no olfactory component of the female sex recognition pheromone sensed by the male antennae, and the attraction of males to females appears to be visual. The absence of male-male mating strikes was the result of the absence of female sex-pheromone, rather than the presence of a repellent mating deterrent in the male cuticle. Experiments with coloured, artificial, sex-pheromone-dosed, cotton decoys showed that colour had only weak effects on attractiveness and number of encounters with decoys, and that no colour caused significant enhancement of mating responses over those shown to decoy females.     

Peripheral modulation of olfaction by physiological state in the Egyptian leaf worm Spodoptera littoralis (Lepidoptera: Noctuidae)

Insects show behavioural plasticity based on their physiological state. Deprivation from a resource will normally make them more responsive to it or to perform behaviour increasing the probability of encountering such a resource. Modulation of the olfactory system has been shown mainly in the central nervous system, but also in the periphery. In this study, antennal sensitivity of females of the Egyptian cotton leaf worm Spodoptera littoralis to different plant and sex pheromone odours was measured using electroantennography (EAG). Different mating status, age, and feeding status were used to investigate peripheral changes in olfactory responses. Virgin females were found to be more sensitive to plant odours and sex pheromone compared to mated females. Age also had an impact on antennal sensitivity, 2 and 4 days old females being more sensitive than just-emerged ones. Such an increase in sensitivity could be explained by maturation of olfactory receptor neurons after emergence or by an increase in motivation as the available expected life time remaining decreases. Finally, feeding status did not modify the antennal sensitivity of females. A peripheral modulation could thus be demonstrated in the olfactory system of female Spodoptera littoralis at certain life stages.     

Flower-feeding affects mating performance in male oriental fruit flies Bactrocera dorsalis

1. Males of the oriental fruit fly Bactrocera dorsalis are attracted to and feed on flowers of the golden shower blossom Cassia fistula. Flowers of this plant contain methyl eugenol, the metabolites of which apparently function in the synthesis of male sex pheromone. 2. The goal of the study reported here was to determine whether feeding on C. fistula flowers enhanced male mating success. Mating frequencies of unfed (control) and fed (treated) males were compared in trials conducted 0 (same day), 2, 7, or 21 days after treated males were exposed to the flowers. Trials were performed using flowers from three trees of C. fistula to investigate whether the effects of floral feeding were similar among different plants. 3. For all three trees, treated males accounted for a disproportionately large number of matings in trials performed 0, 2, and 7 days after floral feeding by the treated males. For two of the trees, treated males also had a mating advantage 21 days after flower‐feeding. 4. Additional tests were conducted to compare female attraction to perch sites of control and treated males. When at a lek, males exhibit rigorous wing‐fanning behaviour, presumably to increase dispersal of the sex pheromone. Floral feeding had no significant effect on the level of wing‐fanning. Significantly more female sightings were recorded for perches of treated than control males, however, suggesting that the treated males produced a pheromone more attractive to females than did control males.     

Olfactory responses of the leafminer Liriomyza sativae (Dipt., Agromyzidae) to the odours of host and non-host plants

Abstract: Electroantennogram (EAG) responses of male and female leafminer Liriomyza sativae Blanchard to 15 species of host and non-host plant odours were recorded. The most distinct EAG responses of both sexes were obtained in the host plants bean and tomato. EAG responses to other host plants, such as cucumber, celery, chrysanthemum, towel gourd and cotton were reduced. Odours of non-host plants, such as Chinese rose, tobacco and morning glory elicited minimal EAG responses. In a behavioural study, both sexes of L. sativae were more attracted by the odours of host plants than of non-host plants. Thus, olfactory cues are important for host plant location by L. sativae . In addition, the combination of strong EAG and behavioural responses for both males and females to specific plant odours suggests that host plant volatiles can be important as aggregation cues for both sexes.     

Flower and fruit volatiles assist host‐plant location in the Tomato fruit fly Neoceratitis cyanescens

Mature females of the tomato fruit fly Neoceratitis cyanescens can detect host fruit at a short distance using only visual stimuli, but little is known about the role of airborne volatile cues in the host searching strategy. A series of experiments is conducted in a laboratory wind tunnel, in which the behavioural responses of individual flies to volatiles from Solanaceae host plants (including tomato Lycopersicum esculentum Mill., bug weed Solanum mauritianum Scop. and Turkey berry Solanum torvum Sw.) are observed, according to some environmental (air speed) and physiological (age and mating status of females, time of day) factors. Mature females respond primarily to specific olfactory cues from blends of flowers or host fruit, preferentially unripe fruit for bug weed, as opposed to ripe fruit for Turkey berry or tomato. Males are also highly attracted by the odour of unripe fruit of bug weed. Wind plays a key role, as shown by the proportion of flies that reach the upwind section of the tunnel in the presence of both fruit odour and air flow (66.7%) and in the absence of either fruit odour (13.3%) or wind (36.7%). In response to fruit volatiles carried by wind, flies embark in a ‘plume tracking’ or ‘aim and shoot' flight, consistent with odour‐conditioned anemotaxis. Females respond to host fruit odour regardless of their age, egg load or mating status, and also more consistently in the afternoon, which is their preferential time of day for egg‐laying. Searching behaviour and response to host volatiles in N. cyanescens are discussed in the light of host‐finding and an adaptive strategy.     

蚜虫雄蚜与雌性母对性信息素及植物挥发物的田间反应

田间观察了桃蚜Myzus persicae (Sulzer)、绣线菊蚜Aphis spiraecola Patch 、山楂圆疣蚜Ovatus crataegarius (Walker)等3种蚜虫对性信息素[(4aS,7S,7aR)-荆芥内酯和 (1R,4aS,7S,7aR)-荆芥醇]的反应,并且调查了性信息素与植物挥发物对桃蚜的田间引诱活性的相互作用.在有冬寄主或夏寄主植物的田中,性信息素诱捕器诱捕到桃蚜雄蚜与雌性母的数量显著多于对照诱捕器的诱捕数;但在非寄主植物的田中,却引诱不到桃蚜.苯甲醛(冬寄主植物桃树Prunus persica的主要挥发物组分之一)能够增强桃蚜雄蚜的引诱作用.绣线菊蚜雄蚜和雌性母对植物中提取的荆芥内酯有反应,而山楂圆疣蚜雄蚜和雌性母对植物中提取的和人工合成的荆芥内酯都没有反应,但对荆芥醇有反应.并且在荆芥醇中添加荆芥内酯之后对山楂圆疣蚜雄蚜引诱活性显著提高.还讨论了雌性蚜产生化合物被雄蚜作为性信息素、被雌性母作为聚集信息素以及性信息素与寄主植物挥发物的相互作用.     

一种测定植物挥发物对棉铃虫引诱作用的无选择性风洞嗅觉仪(英文)

发展了一套测定棉铃虫Helicoverpaarmigera (H櫣ｂｎｅｒ)对植物挥发物的行为反应的生测系统。这套系统由嗅觉仪、风洞和风洞室组成。用这套系统测定了棉铃虫对萎蔫后的枫杨Pterocaryastenoptera叶、加拿大杨Populuscanadensis叶、香椿Toonasisnensis叶、新鲜的芹菜与胡萝卜花挥发物以及苯乙醛的行为反应。结果显示 ,萎蔫的枫杨叶、加拿大杨叶、香椿叶的挥发物均对棉铃虫处女雌蛾表现出显著的引诱作用 ,而对已交配过的雌蛾和雄蛾无明显的引诱作用 ;新鲜的芹菜与胡萝卜花挥发物对棉铃虫两性性成虫均不表现出显著的引诱作用 ;苯乙醛对棉铃虫处女雌、雄蛾有较强的引诱作用。讨论了杨树枝把引诱棉铃虫的原因。     

Plasticity of olfactory-guided behaviour and its neurobiological basis: lessons from moths and locusts

The sense of smell plays an important role in guiding the behaviour of many animals including insects. The attractiveness of a volatile is not only dependent on the nature of the chemical, but might change with the physiological status (e.g., age/hormone or mating status) or environmental conditions (e.g., photoperiod or temperature) of the individual. Here we summarize our studies focused on the plasticity of olfactory-guided behaviour and its neurobiological basis linked with the physiological status in Lepidoptera and migratory locusts. In moths and locusts, age and juvenile hormone changed the behavioural responses to pheromones. In moths, mating had an effect on pheromone responses in males and plant odour responses in females. In all cases of behavioural plasticity studied, we found changes in the sensitivity of olfactory interneurons in the antennal lobe, whereas the peripheral system does not seem to show any plasticity in that context. Changes in the central nervous system were slow under the influence of juvenile hormone (days) or fast after mating (minutes). The olfactory system seems thus to adapt to the physiological or environmental situation of an animal to avoid a waste of energy. We discuss possible mechanisms underlying the observed plasticity.     

Synergism of pheromone and host‐plant volatile blends in the attraction of Grapholita molesta males

Control of Grapholita molesta (Busck) (Lepidoptera: Tortricidae), a major pest of stone and pome fruits, is successfully achieved by mating disruption. Under these conditions, tools other than conventional pheromone dispensers are needed for flight monitoring. The objective of the present work was to determine whether plant volatiles synergize male G. molesta attraction to a suboptimal dose of synthetic sex pheromone. The plant blend (referred to as 5VB), a mixture of three green leaf volatiles [(Z)‐3‐hexenyl acetate, (Z)‐3‐hexenol, and (E)‐2‐hexenal] and two aromatics [benzaldehyde (BZA) and benzonitrile (BZN)], was added to the suboptimal pheromone dose (2 ng on filter paper) in log steps (up to 10 000× the pheromone dose) to test synergism of pheromone and plant blends. In addition, the effect of individual plant volatiles on male responses was investigated by adding to the suboptimal pheromone dose each of the four‐compound plant‐volatile blends, resulting from eliminating one volatile from the 5VB at a time, or each plant volatile alone. Flight behaviour and the time to reach the source were recorded. The 5VB alone was not attractive to G. molesta males, but at a ratio of 1:1 000 (Ph:5VB) or higher, the attractiveness of the suboptimal pheromone dose increased, to a level similar to that of the optimal pheromone dose (10 ng). All tested plant volatiles, except BZA, synergized the response to the pheromone when added individually, but only (Z)‐3‐hexenol and BZN did so to a level not significantly different from the Ph:5VB blend. Aromatics had a stronger effect than green leaf volatiles (GLVs), because their removal, but not the removal of GLVs, decreased landing responses. The addition of the 5VB decreased significantly the time males needed to reach the odour source. The observed enhanced male attraction to mixtures of pheromone and plant volatiles will facilitate the development of lures for G. molesta adult flight monitoring.     

Circadian mating activity and effect of pheromone pre-exposure on pheromone response rhythms in the moth Spodoptera littoralis

Mating in moths is generally mediated by female-produced sex pheromones. Mating activity, female pheromone production/release and male pheromone responsiveness all show diurnal variations in many species. We found that the response of the male Egyptian cotton leafworm, Spodoptera littoralis, to sex pheromone gland extracts showed a diel rhythm in olfactometer tests, and the variation was persistent for at least 1 day in constant darkness. High male response to sex pheromone was correlated in time with high mating and locomotor activity. Male S. littoralis, maintained in constant darkness and exposed to pheromone gland extracts on a daily basis, showed an induced temporal variation in response after several days, in contrast to unexposed males. This suggests that in the absence of other external zeitgebers, exposure to sex pheromone may function to synchronise circadian behavioural rhythms in male moths. The daily rhythm in mating activity in S. littoralis is also shown to be persistent for at least 2 days in constant darkness. Pairs mated significantly less when either the male or female had been raised in a light:dark cycle 10 h out of phase, indicating that the proposed circadian rhythm in mating activity is composed of rhythmic mating preference/ability in both sexes.     

Electroantennogram responses of Apanteles obliquae (Hym., Braconidae) to various infochemicals

Abstract:   The electroantennogram recording technique (EAG) was used to study the olfactory sensitivity of Apanteles obliquae (Hym., Braconidae), a gregarious larval endoparasitoid of Spilosoma obliqua (Walker) (Lep., Arctiidae), to 25 general plant volatiles belonging to alcohol, aldehyde and terpenoid groups and also to volatiles from the host and plant–host complex. The EAG data indicated different olfactory sensitivity between the sexes, not only to individual plant volatiles but also to the volatiles from host and plant–host complex. Females were found to be more responsive than males. However the synthetic sex pheromone blend of the host insect elicited similar EAG responses in both sexes. The EAG data of the present study is correlated with the reported behaviour observed in other parasitoids.     

A NO‐CHOICE WIND TUNNEL OLFACTOMETER SYSTEM FOR EVALUATING THE ATTRACTIVENESS OF PLANT VOLATILES TO ADULT HELICOVERPA ARMIGERA (HÜBNER) (LEPIDOPTERA:NOCTUIDAE) *

Abstract A bioassay system to evaluate adult cotton bollworm (CBW), Helicoverpa armigera, behavioral responses to plant volatiles was described. The system was comprised of a no‐choice olfactometer, a wind tunnel, and a wind tunnel room. The olfactometer was designed to provide for the testing and evaluation of volatiles from a variety of natural and synthetic sources. The behavioral responses in this system demonstrated that the volatiles of wilted leaves of Pterocarya stenoptera, Toona sinensis and Populus canadensis, the main materials of which poplar bundles were made, only could attract virgin females but not mated females and males. The volatiles emitted from fresh carrot and celery flowers did not exhibit significant attractiveness to both sexes of CBW. 0.1% phenylacetaldehyde showed significant attraction for virgin females and males. The biological significance of attraction of volatiles from these plants for CBW was discussed.     

Foraging and mate-finding in the silver Y moth, Autographa gamma (Lepidoptera: Noctuidae) under the risk of predation

Animal foraging and reproductive behaviour is influenced by other simultaneous demands such as predator avoidance. The trade-offs between these demands may depend on sex or mating experience. This study demonstrates that the olfactory-mediated foraging and mate-seeking behaviours in the silver Y moths, Autographa gamma , are affected by auditory cues mimicking their bat predators. Both males and females changed their foraging behaviour under simulated predation risk. Fewer moths reached the odour source following sound stimulation and the time to find the odour source increased by up to 250%. However, there were no significant differences between male and female ability to reach the plant odour source or the duration of the flight towards the source when stimulated with ultrasound. Hence females are not more cautious than males when observed in the same behavioural context. Risk-taking in males was independent of whether they were flying toward a flower odour or sex pheromones having equal attractive value. This indicates that the trade-off between olfactory and acoustic cues is independent the type of odour. Mated females were not as strongly affected by sound as non-mated, indicating that flower odours have a higher adaptive value for mated females, suggesting that some processes following mating experience influence the trade-off between flower odours and simulated bat sounds.     

Electroantennogram and behavioural responses of different forms of the bird cherry-oat aphid,Rhopalosiphum padi,to sex pheromone and a plant volatile

Winged (alate) virginoparae were induced in the laboratory by crowding the bird cherry-oat aphid, Rhopalosiphum padi (L.) under long-day conditions. Males and gynoparae (the winged female form that produces sexual females) were induced by short days. Electroantennogram (EAG) and behavioural responses were investigated in each of the three forms to two aphid sex pheromone components, (-)-(4aS,7S,7aR)-nepetalactol and (+)-(4aS,7S,7aR)-nepetalactone, and benzaldehyde, a volatile which is released by the winter host plant. All three compounds elicited EAG responses with the males showing the highest sensitivity to each compound. Both the nepetalactol and the nepetalactone elicited larger EAG responses in gynoparae than in the winged virginoparae but antennae from virginoparae were more responsive to benzaldehyde. Although the nepetalactone is not a sex pheromone component in R. padi the EAG responses were similar to those evoked by the nepetalactol, the sex pheromone, in all three aphid forms. In a linear-track olfactometer, significantly more male R. padi moved into air containing nepetalactol, nepetalactone or benzaldehyde than into a simultaneous choice of clean air (i.e. attraction) but nepetalactol was more attractive than nepetalactone. Males, however, showed no response to a mixture of nepetalactol and nepetalactone. Gynoparae were attracted only to the nepetalactol but were less sensitive than the males and showed no response to the nepetalactone or benzaldehyde. In contrast, alate virginoparae showed no behavioural responses to any of the compounds. The present study supports the idea that the male R. padi utilise both sex pheromone and benzaldehyde for mate/host-plant location in autumn. It also demonstrates, for the first time, polyphenic differences in the olfactory responses at the peripheral level between the two female forms. Such differences impact on the life-cycle strategy where winged virginoparae move between graminaceous summer host plants while gynoparae move from the summer hosts to the bird cherry, winter host. The latter move appears to be assisted by the sex pheromone released by sexual females, already present on that host, acting as an aggregation pheromone.     

Specificity of contact sex pheromones in tsetse flies, Glossina spp.

ABSTRACT. Interspecific mating tests among seven tsetse species covering the 'morsitans' and 'palpalis' groups showed a degree of reaction that extends beyond the traditional taxonomic groupings. Tests with either live or dead females as targets produced essentially similar results which shows that differing responses of males are largely the consequence of differences in composition of the female surface cuticular paraffins and not of female behaviour. Gas chromatographic (GLC) analysis of cuticular paraffins tends to confirm this view. Responses of males of different species to dead conspecific males baited with the most active synthetic G.morsitans pheromone were variable. Comparison of behavioural responses and of GLC profiles of cuticular paraffins suggest that the species tested do not share a pheromone structure in common with G.morsitans. Mating tests with G.morsitans males using dead conspecific males baited with a variety of synthetic compounds provided evidence of the molecular configuration required to elicit a mating response. Synthetic 15,19-dimethyltritriacontane induced a suboptimal response in male G.morsitans but is a likely candidate for a sex pheromone in G.austeni. Although not conclusive, a comparison of behavioural evidence and GLC data has permitted speculation on the nature of sex pheromones in tsetse of the 'palpalis' group.