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.     

Evaluating the Induced-Odour Emission of a Bt Maize and its Attractiveness to Parasitic Wasps

The current discussion on the safety of transgenic crops includes their effects on beneficial insects, such as parasitoids and predators of pest insects. One important plant trait to consider in this context is the emission of volatiles in response to herbivory. Natural enemies use the odours that result from these emissions as cues to locate their herbivorous prey and any significant change in these plant-provided signals may disrupt their search efficiency. There is a need for practical and reliable methods to evaluate transgenic crops for this and other important plant traits. Moreover, it is imperative that such evaluations are done in the context of variability for these traits among conventional genotypes of a crop. For maize and the induction of volatile emissions by caterpillar feeding this variability is known and realistic comparisons can therefore be made. Here we used a six-arm olfactometer that permits the simultaneous collection of volatiles emitted by multiple plants and testing of their attractiveness to insects. With this apparatus we measured the induced odour emissions of Bt maize (Bt11, N4640Bt) and its near-isogenic line (N4640) and the attractiveness of these odours to Cotesia marginiventris and Microplitis rufiventris, two important larval parasitoids of common lepidopteran pests. Both parasitoid species were strongly attracted to induced maize odour and neither wasp distinguished between the odours of the transgenic and the isogenic line. Also wasps that had previously experienced one of the odours during a successful oviposition divided their choices equally between the two odours. However, chemical analyses of collected odours revealed significant quantitative differences. The same 11 compounds dominated the blends of both genotypes, but the isogenic line released a larger amount of most of these. These differences may be due to altered resource allocation in the transgenic line, but it had no measurable effect on the wasps’ behaviour. All compounds identified here had been previously reported for maize and the differential quantities in which they were released fall well within the range of variability observed for other maize genotypes.     

Reactions of adult female parasitoids, particularly Aphidius rhopalosiphi, to volatile chemical cues from the host plants of their aphid prey

Abstract. In Y-tube olfactometer tests, Aphidius ervi Hal., Trioxys sp., Praon sp., Aphelinus flavus (Nees), Lysiphlebus fabarum (Marsh.) and Aphidius rophalosiphi De Stef. responded positively to the odour of the plant on which aphid mummies containing them had been collected. The response to host plant odour was greater than the response to the odour of host aphids, their honeydew or a combination of the two. The strongest response was to a combination of plant and host aphids. A. rhopalosiphi showed a strong positive response to three wheat volatiles (cis-3-hexenyl acetate, cis-3-hexen-1-ol and trans-2-hexenal) as well as to indole-3-acetaldehyde (a breakdown product of tryptophan in aphid honeydew). In both olfactometer tests with odours and choice trials with whole plants, newly emerged A. rhopalosiphi distinguished and preferred the variety of wheat on which their development had occurred to other wheat varieties.     

Larval and adult nutrition effects on blood/nectar choice of Culex nigripalpus mosquitoes

Abstract. The impact of nutritional variables on the development of host-seeking and biting behaviours after emergence by female Culex nigripalpus mosquitoes were studied using air-flow olfactometer and close-range biting assays, respectively. Unfed females failed to develop resting stage ovarian follicles. When offered a bird host in the absence of competing stimuli, sugar-fed mosquitoes were significantly more responsive in both host-seeking and biting than unfed controls. In a choice olfactometer assay using nectar odours (honey scented with artificial apple-blossom oil) versus host odours (a bird), unfed females preferred honey over bird odours except when honey odour was weak. After sucrose feeding, females switched from honey to bird preference. This change in behaviour was accompanied by significant accumulation of lipid and by follicular growth to the resting stage. Elevation of host responsiveness after sugar feeding was reversible; starvation ultimately resulted in females preferring honey over bird odours. When the larval diet was restricted by crowding, the wing-length and total lipid of resultant adult females were reduced. Although differences were subtle, unfed bird-responding females tended to have longer wings and more lipid than their honey-responding counterparts.     

Olfactory responses of seed wasps Megastigmus pinus Parfitt and Megastigmus rafni Hoffmeyer (Hym., Torymidae) to host-tree odours and some monoterpenes

Megastigmus pinus and Megastigmus rafni are able to colonize the seed cones in eight different species of Abies in Denmark. Olfactory responses of M. pinus females from Abies alba and Abies procera seeds and of M. rafni females from A. alba and Abies grandis seeds to odours of different host-trees cones, needles and pure alpha-pinene and limonene were studied in a Y tube Plexiglas olfactometer. The odours of young cones significantly influenced the behaviour of females. The results obtained in experiments with cones from different trees indicate the occurrence of biotypes depending on the host tree within the M. pinus and M. rafni species. Megastigmus pinus females from A. procera cones preferred the cone odours of A. procera and were significantly repelled by the cone odours of other possible host trees. Megastigmus pinus females from A. alba cones preferred A. alba cone odour but were not indifferent to A. procera cones. This suggests that the M. pinus population in A. alba apparently originates from the A. procera population. Megastigmus rafni from A. grandis cones preferred only A. grandis cone odours. Megastigmus rafni females from A. alba cones preferred A. alba cone odour but were also significantly attracted by A. grandis cone odours. This probably indicates that the M. rafni population living in A. alba originates from the A. grandis population. Neither M. pinus nor M. rafni responded to the odours of the host-tree needles or alpha-pinene. The limonene odour significantly acted as a repellent to both seed chalcid species, independent of the origin-tree.     

Host feeding experience affects host plant odour preference of the polyphagous leafminer Liriomyza bryoniae

Herbivorous insects use highly specific volatiles or blends of volatiles characteristic to particular plant species to locate their host plants. Thus, data on olfactory preferences can be valuable in developing integrated pest management tools that deal with manipulation of pest insect behaviour. We examined host plant odour preferences of the tomato leafminer, Liriomyza bryoniae (Kaltenbach) (Diptera: Agromyzidae), which is an economically important agricultural pest widespread throughout Europe. The odour preferences of leafminers were tested in dependence of feeding experiences. We ranked host plant odours by their appeal to L. bryoniae based on two‐choice tests using a Y‐tube olfactometer with five host plants: tomato, Solanum lycopersicum Mill.; bittersweet, Solanum dulcamara L.; downy ground‐cherry, Physalis pubescens L. (all Solanaceae); white goosefoot, Chenopodium album L. (Chenopodiaceae); and dead nettle, Lamium album L. (Lamiaceae). The results imply that ranking of host plant odours by their attractiveness to L. bryoniae is complicated due to the influence of larval and adult feeding experiences. Without any feeding experience as an adult, L. bryoniae males showed a preference for the airflow with host plant odour vs. pure air, whereas females did not display a preference. Further tests revealed that adult feeding experience can alter the odour choice of L. bryoniae females. After feeding experience, females showed a preference for host plant odour vs. pure air. Feeding experience in the larval stage influenced the choice by adults of both sexes: for males as well as females reared on bittersweet the odour of that plant was the most attractive. Thus, host feeding experience both in larval and/or adult stage of polyphagous tomato leafminer L. bryoniae influences host plant odour preference by adults.     

Olfactory response towards its prey Frankliniella occidentalis of wild and laboratory‐reared Orius insidiosus and Orius laevigatus

Orius species are important biological control agents of thrips in protected crops. Rearing conditions in mass production facilities may affect their performance in the crop when searching for the target prey. The aim of this study was to evaluate and compare the search behaviour and orientation towards prey of two Orius species, O. laevigatus (Fieber) and O. insidiosus (Say) that have been reared in the laboratory under different conditions, with wild (field‐collected) individuals. Adult predator females were placed in a Y‐tube olfactometer and offered a choice between the odours released by plants of different species (cotton, common bean, sweet pepper and cucumber), which were either non‐infested or infested with Frankliniella occidentalis (Pergande) adults.O. laevigatus and O. insidiosus responded to odours from thrips‐infested plants and these responses were influenced by the origin of the colonies. A larger percentage of laboratory‐reared O. laevigatus females (42%) did not made a choice between thrips‐infested or clean plants, compared with wild individuals (17%). Of those females that did respond to plant odours, a smaller percentage of laboratory‐reared O. laevigatus females (34%) responded to the odours from thrips‐infested plants compared with wild insects (76%). No significant differences were found inO. insidiosus females that did not make a choice between thrips‐infested or clean plants (14% for wild vs. 17% for lab individuals). Also, no significant differences were found between O. insidiosus females that selected thrips‐infested plants at the corresponding proportion of wild (75%) and laboratory‐reared (70%) individuals. We propose that the olfactometer test could be a complementary evaluation aspect to the already developed quality criteria for performance of mass‐reared Orius predators.     

Host location and behavioural response patterns of the parasitoid,Tachinaephagus zealandicus Ashmead (Hymenoptera: Encyrtidae), to host and host‐habitat odours

Abstract 1. To overcome the challenge of host location in patchy and complex environments, many parasitoids exploit host‐habitat derived odour cues. This study investigated the role of odour cues used during host location by the generalist parasitoid, Tachinaephagus zealandicus, a common parasitoid of Dipteran larvae found in association with decomposing carrion. 2. A Y‐tube olfactometer was used to test aspects of host searching behaviour of T. zealandicus females under different choice scenarios: combinations of host–liver complex, eaten liver, unwashed host larvae, washed host larvae, liver in six stages of decay, and control (no odour). 3. T. zealandicus females demonstrated an innate response to, and preference for, odours arising from liver substrates following interaction with the host. However, this was dependent on the stage of meat decay. Females were not attracted to liver that had never been in contact with a host, regardless of the stage of decay. There were discernible differences in parasitoid movement and behaviour under different choice scenarios. The absence of odour stimulus elicited limited movement about the olfactometer. In the presence of any odour stimuli, females were more active within the olfactometer, and when presented with two odour options T. zealandicus females demonstrate a heightened investigation response, moving across all regions of the olfactometer. 4. Knowledge of the cues that mediate host location for T. zealandicus can contribute to fields such as forensic entomology, where the presence of parasitoids on decomposing remains can be used to estimate time since death.     

Aphid parasitoids detect that an alien plant was present nearby during their development

Abstract Glasshouse and laboratory experiments were conducted with Aphidius rhopalosiphi parasitizing Metopolophium dirhodum to test whether the presence in the same environment of another plant species (tomato or marigold) induced aerial allelopathy in wheat plants.
Emerging parasitoids were offered the choice of two odours of wheat plants in an olfactometer – wheat grown alone and wheat grown in the presence of the second plant species.
Female parasitoids responded more strongly than males, but both sexes preferred the odour of wheat grown in the environment in which they had developed (i.e. with or without the second plant species present).
Moving the tomato plants from the cages where they had been placed with wheat to the cages that had previously had wheat alone just before the aphid mummy formed showed that the odour environment the emerging parasitoid responded to was that of the mummy rather than that of the developing parasitoid larva. From this it was concluded that the parasitoids in the olfactometer were responding not only to wheat volatiles, but also to tomato volatiles absorbed on the surface of the mummy and encountered by the emerging adult parasitoid.
By excising the parasitoid pupa from the mummy formed on one wheat cultivar, and offering the emerging parasitoids the surface of aphid mummies formed on a different cultivar, it was confirmed that olfactometer responses represented a learning of the chemistry of the surface of the mummy encountered at or after emergence.     

Use of olfaction for sexual recognition in the subterranean rodent<Emphasis Type="Italic">Ctenomys talarum</Emphasis>

The ability of the tuco-tucoCtenomys talarum Thomas, 1898 to recognize sex by olfactory cues contained in urine, faeces and soiled shavings was tested by using preference tests. Nonbreeding tuco-tucos selected odours from opposite-sex rather than same-sex conspecifics. This pattern differed between sexes: females spent more time sniffing male than female odours for all scent sources whereas males did not show any difference in the time they spent investigating odours of each sex for each tested odour sources. Dissimilarities in odour selections between sexes may be attributed to a different combination of factors involved in olfactory interest for each sex. The function of gender cues recognition is discussed.     

Preliminary experiments on the olfactory responses of Pterostichus melanarius Illiger (Coleoptera: Carabidae) to intact plants

1 In two successive years, the olfactory responses of Pterostichus melanarius Illiger (Coleoptera: Carabidae) to intact cabbage and white clover plants were studied, using a four‐arm olfactometer. 2 The first set of experiments tested the response to the odours of cabbage, white clover and the two plants grown together against potting compost; in the second and third sets of experiments, white clover and cabbage were tested, respectively. 3 The results indicate that female P. melanarius individuals are arrested by the combined odours of cabbage and white clover, spending significantly more time in the cabbage with white clover odour field than in either the cabbage, white clover or control odour fields. 4 Similar results were not found for males. 5 Cabbage plants alone did not elicit significant responses in either sex. 6 White clover alone did not elicit significant responses by females and the responses of males were not uniform in the two successive years.     

Plants with spider-mite prey attract more predatory mites than clean plants under greenhouse conditions

Although many predators and parasitoids are known to respond to odours produced by plants infested with their prey under laboratory conditions, there are actually few studies that show that this response leads to higher numbers of predators or parasitoids on the plants under natural conditions. Here we study the response of predatory mites (Phytoseiulus persimilis Athias-Henriot, Acari, Phytoseiidae) to odours from cucumber plants infested with two-spotted spider mites (Tetranychus urticae Koch, Acari, Tetranychidae) in greenhouse release experiments, where predators were released in the centre of a hexagon of cucumber plants. Forty to 57% of all predators released were recaptured on plants within 7 h. Of these, an average of 79.5% were found on infested plants, indicating that these attract about 4 times as many predators as do clean plants. Hence, the blind predatory mites were guided to the plants with prey by herbivore-induced odours produced by the plant, as was indicated by olfactometer experiments, where it was found that P. persimilis preferred odours from infested cucumber plants to odours from clean cucumber plants. The long-range searching behaviour of P. persimilis is discussed.     

Herbivore-induced emissions of maize volatiles repel the corn leaf aphid, shape Rhopalosiphum maidis

When maize plants, Zea mays L., are mechanically damaged and the damaged sites are treated with caterpillar regurgitant, the plants will release a specific blend of volatiles. It is known that these volatiles can be attractive to natural enemies of herbivores. We hypothesise that the plant volatiles constitute part of the induced plant defence and that herbivores will be affected by the odours as well. In laboratory and semi-field studies this hypothesis was tested for the aphid Rhopalosiphum maidis (Fitch) (Rhynchota, Sternorrhyncha, Aphididae).In a Y-tube olfactometer significantly more aphids chose the odour of healthy, undamaged maize seedlings when tested against clean air or plants treated with regurgitant. Clean air was chosen more often when tested next to the odour of treated plants. This apparently repellent effect of the odour of treated plants was significant for winged aphids, but not for the wingless aphids.In field experiments aphids were released in the centre of circles of eight potted maize plants. Four plants in each circle were damaged and treated with caterpillar regurgitant while the other plants were left unharmed. At different intervals after aphid release, the number of aphids was counted on each plant. Significantly fewer winged and wingless aphids were found back on treated plants than on healthy plants.We suggest that herbivores may be repelled by the odours because they could indicate that: 1) the plant has initiated the production of toxic compounds; 2) potential competitors are present on the plant; 3) the plant is attractive to parasitoids and predators. Aphids may be particularly sensitive to induced maize volatiles because one of the major compounds emitted by the plant is (E)--farnesene, which is a common alarm pheromone for aphids. Collections and analyses of the odours emitted by crushed R. maidis confirmed that it too emits (E)--farnesene when stressed. The results are discussed in context of plant defence strategies and their possible exploitation for the control of pest insects.     

Olfactometer Responses of Plum Curculio <Emphasis Type="Italic">Conotrachelus nenuphar</Emphasis> (Herbst) (Coleoptera: Curculionidae) to Host Plant Volatiles,Synthetic Grandisoic Acid,and Live Conspecifics

The plum curculio Conotrachelus nenuphar (Herbst) (Coleoptera: Curculionidae) is a major pest of pome and stone fruit, but will also attack other fruits. Males produce the aggregation pheromone grandisoic acid; emitting only the (+)-enantiomer which is attractive to both sexes of the univoltine and multivoltine strains, while the synthetic racemic mixture contains optical isomers with equal amounts of (+)- and (?)-enantiomers. Synergy between odours can increase trap captures and improve monitoring techniques, therefore tests were performed in a dual-choice olfactometer with odours attractive to plum curculios according to literature to determine 1) under what physiological conditions (mating status, age, starvation period) these odours are attractive or repulsive, 2) if the (+)-enantiomer or the odour of live males synergizes with host plant volatiles, and 3) if there is a difference in response between plum curculio strains. Females were exposed to: benzaldehyde; trans-2-hexenal; apples; extracts of: plums, apples, blueberries; grandisoic acid; and live males. Plum essence was found to be the most attractive host-plant odour for both immature and mature virgin females, and immature whole apples were attractive to starved females, while trans-2-hexenal, McIntosh apple essence, benzaldehyde along with the combination of benzaldehyde and plume essence was found to be repulsive. Starvation, age, and mated status all influence response to odours. No synergistic or additive affects were observed between any of the odour combinations tested, including the combination of both the natural and synthetic pheromone and plum essence or apples.     

Flight behaviour of the black bean aphid, Aphis fabae, and the cabbage aphid, Brevicoryne brassicae, in host and non-host plant odour

Abstract. Walking alate virginoparae of Brevicoryne brassicae (L.) and Aphis fabae Scopoli were presented with odours of steam-distilled extracts of the non-host plants summer savoury ( Satureja hortensis L.) and tansy ( Tanacetum vulgare L.) in an olfactometer. No effects of the extracts were observed on B. brassicae. However, A.fabae were repelled by summer savoury and tansy odour; both extracts also masked an attractant response to bean (host plant) odour. In a flight chamber, air permeated with odour from host or non-host plants was blown over flying alates of both species, with a green, plant-mimicking target presented once a minute. The incidence of targeted (host-responsive) flight of B.brassicae was not affected by odour from a growing host plant ( Brassica oleoracea ) or a non-host plant tansy. Host plant ( Vicia faba ) odour did not affect the initial rate of climb or the incidence of targeted flight of A.fabae , but when the bean odour was alternated with odour from non-host tansy plants a greater number of targeted flights occurred in the host plant odour. The volatile extracts of tansy and summer savoury were also presented to flying A.fabae. Aphids flying in air permeated with tansy odour at 450g plant equivalents initiated fewer targeted flights than when flying in clean air. However, no differences in flight behaviour were observed with summer savoury extract. In a horizontal wind tunnel the tansy extract at 90 g plant equivalents blown across the surface of yellow targets reduced the numbers of alate A.fabae landing. The results indicate that plant odours can affect flight and landing of aphids.     

The effect of repellents Ocimum forskolei and deet on the response of Anopheles stephensi to host odours

The behavioural response of Anopheles stephensi Liston (Diptera: Culicidae) to incubated host odours (from human and goat) and to human odour in combination with a repellent plant, Ocimum forskolei (Labiatae), or deet (N, N, diethyl-toluamide) (20%) was tested in a dual-port olfactometer. An. stephensi was significantly attracted to both host odours compared with air alone, but showed no preference when given a choice between both host odours simultaneously. In choice tests, the addition of deet to human odour did not significantly divert mosquitoes to human odour alone, but did divert them to goat odour. O. forskolei combined with human odour diverted mosquitoes to goat or human odour alone. Combinations of human odour and O. forskolei, and human odour and deet were both as unattractive as air alone, and attracted mosquitoes equally when offered simultaneously. The results indicate that use of O. forskolei as a repellent would be beneficial in reducing vector biting if used in communities in areas with partially zoophilic mosquito species such as An. stephensi, and where animals are present.     

Hunting for moving hosts: Cephalonomia tarsalis, a parasitoid of free-living grain beetles

The parasitic wasp Cephalonomia tarsalis parasitizes larvae of the saw-toothed grain beetle Oryzaephilus surinamensis, which feed on wheat grains. In contrast to most other host–parasitoid systems studied so far, the grain beetles are highly mobile within their habitat, bulk of grains in grain stores. This should increase the wasps’ problem to locate the hosts. To study the host-finding strategy of C. tarsalis females, the reaction of wasps to different grain and host-derived odour sources was tested in a four-chamber-olfactometer. These experiments revealed that wasps were attracted by healthy grains and mechanically damaged grains. In direct comparison, healthy and mechanically damaged grains are equally attractive. Both potential sources of host-derived odours, host faeces and trail-traces of larvae on filter paper were attractive to the wasps. The response to trail-traces vanished 30 min after larvae had been removed from the filter paper. With respect to the specificity of the odours, it turned out that wasps were attracted to odours from the seed–host complexes from O. surinamensis and Oryzaephilus mercator and the non-host complex of larvae of the granary weevil Sitophilus granarius in wheat grains. Odours from the seed–host complex were preferred. From these results, we hypothesize that host habitat location in C. tarsalis is achieved by using grain-derived odours. Within the habitat, wasps search for kairomones from host faeces and host trails. Following these larval trails finally leads wasps to their hosts.     

Odor-mediated patch choice in the parasitoid Venturia canescens: temporal decision dynamics

Parasitoids foraging for hosts in a heterogeneous environment would greatly benefit if they could decide already from a distance in which areas search for resources would be most profitable and to avoid areas of low fitness returns. Interestingly, the temporal dynamics of the decision process in parasitoid patch choice have rarely been investigated. In a Y-tube olfactometer, we tested whether thelytokous and arrhenotokous females of the parasitoid wasp Venturia canescens (Gravenhorst) (Hymenoptera: Ichneumonidae) respond to differences in cues indicating the quality of a host-containing patch and choose more profitable patches. Special attention was given to the time it took females to make their choices (patch choice time) when differences in patch quality were either qualitative (absence vs. presence of hosts and kairomone) or quantitative (various concentrations of hosts and kairomone, and presence of competitors). We found that both thelytokous and arrhenotokous wasps only chose the higher-quality patch based on odor cues when the difference was qualitative. When patches differed only with respect to the number of hosts, or the presence or absence of competing female parasitoids, no significant preference could be found in females of either strain of the parasitoid. In contrast, both the time until females reached the junction of the Y-tube olfactometer (response time) and the time until females decided for either patch (decision time) varied with parasitoid strain and odor treatment. Thelytokous wasps were faster than arrhenotokous wasps in their response time and in their decision time. However, females of both strains responded faster with increasing number of total hosts releasing kairomone. Yet, decision time for patches did not significantly vary as a function of patch quality offered to Venturia wasps.     

Laboratory studies on the olfactory behaviour of Anopheles quadriannulatus

The host preference of Anopheles quadriannulatus Theobald (Diptera: Culicidae), the zoophilic member of the malaria mosquito complex Anopheles gambiae Giles, was investigated in a dual‐choice olfactometer. Naïve female mosquitoes were exposed to CO₂, acetone, 1‐octen‐3‐ol, and skin emanations from cows and humans in various combinations. Their behavioural responses were recorded when they had entered one of either upwind traps from where the odours were being released. The mosquitoes did not respond to CO₂ when released at human or cattle equivalent concentrations. Too few mosquitoes responded to acetone to allow for a statistical analysis. The combination of CO₂+ 1‐octen‐3‐ol was repellent. Cow odour alone was slightly attractive, but there was a synergistic attractive effect of cow odour + CO₂. Surprisingly, the mosquitoes were attracted to human odour, and in a choice situation human odour was selected above cow odour + CO₂. Anthropophilic An. gambiae Giles s.s. was repelled by cow odour + CO₂ in contrast to An. quadriannulatus. In a choice situation, both mosquito species selected human odour above cow odour + CO₂. The implications of these results are discussed in the light of recent behavioural data from the field.     

Cereal aphid responses to sex pheromones and host-plant odours in the laboratory

Abstract. The movement of Sitobion fragariae (Wlk.) (Homoptera: Aphididae) males towards odours from conspecific sexual females (oviparae) and the aphid sex pheromone component (4a S ,7 S ,7a R )-nepetalactone were demonstrated in a linear-track olfactometer. Males of the closely related Sitobion avenae (Fab.) were attracted to odours from conspecific oviparae and also oviparae of S. fragariae. These results were consistent with the recent identification of this nepetalactone isomer as the major component of the sex pheromone in these species. Males of S.fragariae were not significantly attracted to oviparae of S.avenae. indicating some qualitative or quantitative difference in the pheromones of the two species. Males did not respond to the related (1R,4aS,7S,7aR)-nepetalactol or to host-plant odours. Gynoparae of these species moved towards the nepetalactone and also to host-plant odours. A combination of both stimuli was more attractive than plant odour alone. Gynoparae of S.fragariae also responded significantly to the combined treatment when this was tested against the nepetalactone.