Field and laboratory electroantennographic measurements of pheromone plume structure correlated with oriental fruit moth behaviour

Abstract. Peak-to-trough electroantennogram amplitudes (bursts), caused by the individual filaments of a plume of female pheromone, diminish as high-emission-rate sources are approached by male Grapholita molesta , and this reduction is correlated with in-flight arrestment (ceasing to advance upwind). These findings are consistent with the hypothesis that one cause of in-flight arrestment in response to high-concentration point sources is the attenuation of the peak-to- trough amplitudes close to the source. High burst frequency, high pheromone flux, or low levels of continuous neuronal activity all are less well correlated with arrestment. Rather, arrestment appears due to a reduction of chemosensory input to the CNS during flight up the plume, even though the actual molecular concentration continues to increase. In a laboratory wind tunnel, upwind flight initiation by more than 20% of males was elicited only by pheromone source concentrations evoking significant fluctuations in EAG amplitudes at downwind release points. The burst frequencies that evoked high levels of upwind flight initiation ranged from a mean of 0.4-2.2 bursts/s. Because a previous study revealed that flying male G. molesta change their course angle within 0.15 s of losing or contacting pheromone, these EAG burst frequencies indicate that during flight in a pheromone plume, many manoeuvres are probably made in response to contact with individual plume filaments. Thus, upwind flight tracks may be shaped by hundreds of steering reactions in response to encounters with individual pheromone filaments and pockets of clean air. Field-recorded EAGs reveal that burst amplitudes diminish from 3 to 30 m downwind of the source, whereas burst frequencies do not, averaging c. 1/s at 3, 10 and 30 m downwind.     

Influence of plume structure and pheromone concentration on upwind flight of Cadra cautella males

Abstract. The effects of pheromone plume structure and its concentration on the pheromone-mediated flight of male Cadra cautella (Lepidoptera: Phycitinae) were investigated in a laminar-flow wind tunnel. When two C. caurella males flew simultaneously along a ribbon plume of mixed smoke and pheromone, their inflight behaviour was dependent on the instantaneous structure of the plume they encountered. When a male intercepted an intact ribbon filament, he sustained a crosswind course, whereas when he intercepted a turbulent filament (created by an upwind male fragmenting the ribbon plume), he adopted a flight course more due upwind. These results indicate that C. cautella males altered their in-flight manoeuvres in response to instantaneous changes in the fine structure of the pheromone plume. We also demonstrated that differences in the fine structure of the plume had more influence on the flight pattern of C. cautella males than a 1000-fold range in pheromone dose. The size of the plume was increased by adding wind deflectors upwind of the pheromone source, independent of source dosage, males following ribbon plumes flew slow zigzag tracks, whereas males following large, turbulent plumes flew directly to the source in fast, straight tracks with less counterturning.     

Guidance of flying male moths by wind-borne sex pheromone

ABSTRACT. On passing from clean air into a homogeneous cloud of sex pheromone in a wind tunnel flying male Adoxophyes orana (F.v.R.) (Lepidoptera: Tortricidae) turned more or less upwind and reduced the time and distance between their switchings of track from one side of the wind line to the other. These responses became adapted under the constant pheromone stimulation in the cloud, thereby arresting upwind progress; but the adapted moths would now 'lock-on' to an added pheromone plume and advance upwind along it. Moths also locked-on to the border of a pheromone cloud, not by turning back on losing the scent as previously supposed but by initiating the above programme of small-amplitude, crosswind movements (reversing anemomenotaxis). The onset and cessation of the pheromone stimulus produced anemotactic responses that differed quantitatively within a continuum, not two distinct kinds of response as previously supposed. The behavioural mechanism whereby uniform permeation of an area with synthetic sex pheromone can prevent males from finding females is reconsidered.     

An analysis of anemotactic flight in female moths stimulated by host odour and comparison with the males' response to sex pheromone

ABSTRACT. The flight pattern of mated female navel orangeworm moths, Amyelois transitella (Walker), responding to odour from potential larval hosts is zigzagging upwind flight. However, at times these moths are capable of flying nearly directly upwind towards the odour source (track angles near 0). This response indicates that these females are capable of very accurate anemotactic control of their heading or course angle, since small angular errors in this measure would translate into larger deviations from direct upwind flight. Males of this species exhibit flight patterns similar to those of females, including track angles clustered about 0 when flying upwind to a source of the female-produced pheromone, but under these experimental conditions they flew with a higher average airspeed than the females. When females lose contact with an odour plume they initiate a well-defined programme of cross-wind counterturning or casting, which may normally increase their chances of retrieving contact with that plume when the wind direction shifts. The resultant track angles of females increase significantly by 0.8 s after plume loss, indicating that the female has initiated changes in both her course angle and airspeed. By 1 s after plume loss the females' track angles are no longer unimodally distributed about 0, but are bimodally distributed about -90 and +90. Males responded more rapidly to the loss of a pheromone plume, demonstrating a significant change in track angle 0.4 s after plume loss. Overall, female and male A.transitella exhibited remarkably similar anemotactic flight manoeuvres during upwind flight to odour sources as well as after plume loss.     

Flight of male Cadra cautella along plumes of air and pheromone superimposed on backgrounds of pheromone

Male Cadra cautella were presented with five heterogeneous pheromone clouds (created from source doses of 0, 0.01, 1, 100, and 10 000 ng) with and without superimposed plumes of either clean air or sex pheromone in a wind tunnel. Moths provided with the lowest doses of background clouds without a superimposed plume did not fly upwind. Moths provided with higher doses of background clouds, with or without superimposed air plumes, increased their track, course, and drift angles (i.e., their zigzags headed more towards crosswind) with increased dose, but slowed their velocity. No differences in flight track parameters were observed for moths provided with a superimposed pheromone plume, regardless of the background cloud dose. Moreover, moths were able to locate the source of superimposed air plumes in the highest background dose, and of superimposed pheromone plumes in any background dose. The significance of these results is discussed in the context of mating disruption.     

Flight behaviour of Cadra cautella males in rapidly pulsed pheromone plumes

Abstract Airborne pheromone plumes in wind comprise filaments of odour interspersed with gaps of clean air. When flying moths intercept a filament, they have a tendency to surge upwind momentarily, and then fly crosswind until another filament is intercepted. Thus, the moment-to-moment contact with pheromone mediates the shape of a flight track along the plume. Within some range of favourable interception rates, flight tracks become straighter and are headed more due upwind. However, as the rate of interception increases, there comes a point at which the moth should not be able to discern discreet filaments but, rather, should perceive a 'fused signal'. At the extreme, homogeneous clouds of pheromone inhibit upwind progress by representative tortricids. In a wind tunnel, Cadra cautella (Walker) (Lepidoptera: Pyralidae) were presented with 10 ms pulses of pheromone at a repetition rate of 5, 10, 17 and 25/s and a continuous, internally turbulent plume. Pulse size and concentrations were verified with a miniature photoionization detector sampling surrogate odour, propylene, at 100 Hz. Male moths maintain upwind progress even at plumes of 25 filaments/s. Furthermore, moths exhibited greater velocities and headings more due upwind at 17 and 25 Hz than at the lower frequencies or with the continuous plume. It is hypothesized that either C. cautella possesses a versatile sensory system that allows the resolution of these rapidly pulsed pheromone plumes, or that this species does not require a 'flickering' signal to fly upwind.     

A non-anemotactic mechanism used in pheromone source location by flying moths

ABSTRACT. Male oriental fruit moths, Grapholitha molesta (Busck) (Tortricidae), continue to zigzag along a pheromone plume to the source in zero wind, if they have started flight with wind on. If the pheromone source is removed and the plume is hence truncated, moths flying in zero wind out of the end of the plume into clean air increase the width of their reversals and the angles of the straight legs of the tracks so they are more directly across the former wind line. Such moths reach the source less often than do those flying along a continuous plume. The males continue to zigzag up a plume in zero wind, apparently by a combination of sequential sampling of concentration along their path and the performance of an internal, self-steered programme of track reversals (zigzags) whose frequency increases with concentration. Visual feedback may aid in the still-air performance of the zigzags. We propose that both the sequential sampling (longitudinal klinotaxis) and self-steered counter-turning programme also are used in wind as well; anemotaxis apparently polarizes the direction of the zigzags to result in upwind displacement, and the narrow zigzags caused by the higher concentration in the plume keep the male 'locked on' to the odour.     

Flight behaviour of males of two moths, Cadra cautella and Pectinophora gossypiella, in homogeneous clouds of pheromone

Abstract It is thought that orientation by male moths along pheromone plumes is guided by interception of filaments of pheromone along that plume and that clean air gaps are required for upwind progress. Given that several investigations have determined that cells sensitive to pheromone can resolve only low rates of encounter with pheromone filaments, generally up to 10 pulses/s, it would appear that individual filaments encountered at higher rates would not be resolved by the insects' sensory system and therefore the stimulus would be perceived as a non-flickering signal. Behaviourally, this has been thought to be expressed as the cessation of upwind progress. Previous studies with Cadra cautella (Walker) (Lepidoptera: Pyralidae) demonstrated that upwind flight by these males is not inhibited in rapidly pulsed plumes. Therefore, to determine whether a flickering signal is required for upwind progress by C. cautella , males were introduced to homogeneous clouds of pheromone in a wind tunnel and their behaviour recorded. For comparison, Pectinophora gossypiella (Saunders) (Lepidoptera: Gelechiidae), a species from a long-diverged lineage, was also used. Upwind progress by C. cautella is not impeded by the constant olfactory signal provided by a homogeneous cloud of pheromone, but this is not true for P. gossypiella . Furthermore, although C. cautella directs its flight upwind in a homogeneous cloud, its heading is not always due upwind. Potential mechanisms are discussed. It is suggested that C. cautella does not require a flickering signal to progress upwind.     

Impact of (Z)-7-dodecenol and turbulence on pheromone-mediated flight manoeuvres of male Trichoplusia ni

Abstract. Turbulence and chemical noise are two factors which may influence pheromone-mediated flight manoeuvres of a moth in natural habitats. In this study, the effects of turbulence and the behavioural antagonist (Z)-7-dodecenol on flight manoeuvres of male Trichoplusia ni (Hübner) were evaluated in a wind tunnel. Male moths increase airspeed and course angles when turbulence is increased. This leads to significant increases in the length of flight tracks, but significant reductions in the time taken to reach a pheromone source. In less disturbed pheromone plumes, distributions of course angles and track angles of male T.ni show a prominent peak centred about 0° relative to the upwind direction, indicating that moths can temporarily steer directly upwind toward a pheromone source.
When (Z)-7-dodecenol is released 10 cm upwind of a pheromone source to form an overlapping plume downwind, course angles, airspeeds and ground-speeds of male T.ni are reduced significantly compared with those in uncon-taminated pheromone plumes. This results in a longer flight time to reach a pheromone source. The decrease in flight speed would decrease the rate of contact with filaments, and thereby perhaps allow the moth to detect uncon-taminated pheromone filaments independently from filaments containing the behavioural antagonist.     

Pre-exposure modulates attraction to sex pheromone in a moth

In behavioural experiments we investigated the influence of previous short exposure to sex pheromone on subsequent response of male Spodoptera littoralis moths to sex pheromone. We found that pre-exposed males showed increased sensitivity to female sex pheromone after a single exposure to a pheromone plume compared to that found in na?ve males. The increased responsiveness lasted for at least 27 h after the exposure, showing that it was not just a short-term sensitization of the males. Exposure to the odour source without upwind movement towards the source was enough to increase the responsiveness. Physical activation without exposure to odour did not affect responsiveness. The increase in responsiveness after exposure was higher when the males were pre-exposed to natural female pheromone gland extract than when they were exposed to a higher dose of the main component, even though both odour sources elicited similar upwind attraction in na?ve males. Thus, the quality of the pheromone mixture to which males were exposed influenced the subsequent response.     

Fine-scale resolution of closely spaced pheromone and antagonist filaments by flying male Helicoverpa zea

The limits of a male moth's ability to resolve closely spaced odor filaments have been investigated. Male Helicoverpa zea normally respond to their conspecific sex pheromone blend by exhibiting an upwind flight, which culminates in source contact by at least 50% of the bioassayed individuals. When loaded onto the same filter paper source containing this hitherto attractive pheromone blend, or onto a separate filter paper and co-emitted from the same pipette source with pheromone, (Z)-11-hexadecenyl acetate severely reduced upwind flight and source contact by male H. zea. A similar level of upwind flight inhibition was recorded when the antagonist (Z)-11-hexadecenyl acetate was emitted from its own point source placed 1 mm upwind of the pheromone point source, both plumes being simultaneously emitted in a continuous mode to form a confluent strand. However, (Z)-11-hexadecenyl acetate was less effective in reducing upwind flight and source contact when it was isolated and pulsed from its own source, placed 1 mm either upwind, downwind or cross-wind of a pipette source from which pheromone was simultaneously being pulsed, such that both filaments were separated in time by 0.001–0. 003 s. These results suggest that male H. zea are able to distinguish between odor sources separated by as little as 1 mm in space and 0.001 s in time. Accepted: 31 March 1999     

Adaptation of male moth antennal neurons in a pheromone plume is associated with cessation of pheromone-mediated flight

Recordings of the firing rates of single antennal neurons whenAgrotis segetum antennac were placed 70 cm downwind of a pheromonesource revealed that cells sensitive to the most volatile componentadapted rapidly in a plume from a high–concentration sourceknown from previous studies to cause in-flight arrestment ofprogress towards the source. No adaptation was found in responseto lower-concentration plumes known to promote high levels ofsustained flight to the source with little premature arrestment.Adaptation was not observed in antennal neurons of a secondspecies, Heliothis virescens, when they were placed in plumesof this species' sex pheromone blend, regardless of the concentration.In flight-tunnel tests these same pheromone sources evoked highlevels of source location with little arrestment. These resultsindicate that adaptation or attenuation of antennal neuronalburst frequencies in response to rapidly arriving pheromonefilaments in a plume may be important peripheral determinantsof whether or not prolonged upwind flight and successful pheromonesource location occurs.     

Pheromone-mediated optomotor anemotaxis and altitude control exhibited by male oriental fruit moths in the field

Abstract. In the field over short grass, pheromone-stimulated oriental fruit moth males, Grapholita molesta (Busck), flying under high windspeeds tended to steer courses more into the wind and to increase their airspeeds compared with those flying in low windspeeds.Thus, optomotor anemotaxis enabled the males to steer relatively consistent upwind track angles and to maintain an upwind progress of between c. 50–100 cm/s despite variable wind velocities.Zigzagging flight tracks were observed at both 10 m and 3 m from the source, as were tracks with no apparent zigzags.Transitions from casting to upwind flight or vice-versa were observed.The durations of the intervals between reversals during both upwind zigzagging flight and casting were consistent with those observed in previous wind-tunnel experiments.The control of altitude was more precise during upwind zigzagging flight than during casting.In general, the side-to-side deviations in the tracks were greater than the up-and-down deviations, with both the side-to-side and vertical distances and their ratios being consistent with previous wind-tunnel studies of pheromone-mediated flight.One difference between the field and laboratory flight tracks was that males in the field exhibited much higher airspeeds than in the wind tunnel.Males occasionally were observed to progress downwind faster than the wind itself, and further analysis showed that they were steering a downwind course in pheromone-free air following exposure to pheromone, which is the first time this has been recorded in moths.We propose that such downwind flight may aid in the relocation of a pheromone plume that has been lost due to a wind-shift, by enabling the moth to catch up to the pheromone as it recedes straight downwind away from the source.     

Chemical communication in heliothine moths. VII. Correlation between diminished responses to point-source plumes and single filaments similarly tainted with a behavioral antagonist

Addition of (Z)-11-hexadecenyl acetate (Z11-16:Ac) into a normally attractive binary blend of Heliothis virescens pheromone components resulted in a suppression of upwind flight and source location by males. Male response was reduced even at the lowest dosages of Z11-16:Ac tested but upwind flight and source location were most clearly reduced when the loading of Z11-16:Ac reached 10% or more of the (Z)-11-hexadecenal (Z11-16:Ald) loading (the major component present in the binary blend). Similar patterns of suppression in response were noted when Z11-16:Ac was added to binary blends of pheromone components at both 10 and 100 μg loadings of Z11-16:Ald. Males in casting flight following upwind flight in a mechanically generated pulsed plume, responded to the interception of a subsequent, single binary-blend filament by making a toward-source upwind surge. Responses of males to a single filament that was tainted by a level of Z11-16:Ac that had allowed some reduced level of upwind flight and source location to occur in the previous plume experiments were diminished compared with their control counterparts. Analysis of the flight tracks revealed that the surges in response to single tainted filaments were stunted because males made fewer significant changes in course angles steered, airspeeds generated, and in the tempo of counterturns executed. Accepted: 28 December 1996     

Effects of pheromone plume structure and visual stimuli on the pheromone-modulated upwind flight of male gypsy moths (Lymantria dispar) in a Forest (Lepidoptera: Lymantriidae)

The pheromone-modulated upwind flight ofLymantria dispar males responding to different pheromone plume structures and visual stimuli designed to mimic trees was video recorded in a forest. Males flying upwind along pheromone plumes of similar structure generated tracks that were similar in appearance and quantitatively similar in almost all parameters measured, regardless of the experimentally manipulated visual stimuli associated with the pheromone source. Net velocities, ground speeds, and airspeeds of males flying in point-source plumes were slower than those of males flying in the wider, more diffuse plumes issuing from a cylindrical baffle. The mean track angle of males flying in plumes issuing from a point source was greater (oriented more across the wind) than that of males flying in plumes issuing from a transparent cylindrical baffle. Males flying in point-source plumes also turned more frequently and had narrower tracks overall than males responding to plumes from a cylindrical baffle. These data suggest thatL. dispar males orienting to pheromone sources (i.e., calling females) associated with visible vertical cylinders (i.e., trees) use predominantly olfactory cues to locate the source and that the structure of the pheromone plume markedly affects the flight orientation and the resultant track.     

Investigations of Mechanisms of Pheromone Communication Disruption of Choristoneura rosaceana (Harris) in a Wind Tunnel

Although atmospheric treatment with synthetic pheromone is used commercially to control several lepidopteran pests, little is known about how mate-finding behavior is altered by this procedure. Mechanisms of disruption of the mate-finding behavior of Choristoneura rosaceana (Lepidoptera: Tortricidae) were examined in wind tunnel bioassays. Male moths were exposed to synthetic sex pheromone or pheromone components prior to or during upwind flight to a calling virgin female. The proportion of males successfully contacting a calling female was reduced only when the synthetic pheromone treatment was presented simultaneously. A synthetic source of the attractive four-component pheromone placed upwind of a calling female was more effective than a less attractive two-component blend in achieving disorientation. Habituation of the central nervous system does not appear to be a significant factor in disruption of mate-finding behavior of male C. rosaceana, as exposure to pheromone prior to the bioassay did not alter the proportion of males that achieved subsequent upwind flight. Disruption of mate-finding behavior of C. rosaceana in a wind tunnel is probably the result of a combination of mechanisms including adaptation of antennal receptors, camouflage of the female-produced plume, and false-trail following which contributed to the additive disruption effect observed with the most attractive four-component pheromone.     

Helicoverpa zea males (Lepidoptera: Noctuidae) respond to the intermittent fine structure of their sex pheromone plume and an antagonist in a flight tunnel

Abstract. We investigated the behavioural response of male Helicoverpa zea (Boddie) to the fine-scale structure of an odour plume experimentally modified in a wind tunnel by using an air-pulsing device. Male H. zea flew upwind to pulsed filaments of a binary pheromone blend of (Z)-11-hexadecanal (Z11-16:Ald) and (Z)-9-hexadecanal (Z9-16:Ald) in the ratio of 20:1. Sustained upwind flight in experimentally altered intermittent plumes was dependent on concentration, as well as the frequency of generation of odour filaments. At a loading of 10μg of the major pheromone component, Zll-16:Ald, which gave an emission rate of approximately that released by a female H. zea , sustained upwind flight and source contact correlated positively with filament delivery rate, becoming significant at a minimum filament delivery rate of 2/s. Decreases in upwind progress and source location were recorded at a loading of 1 μg of Z11-16:Ald. At this suboptimal dosage, a high filament generation rate of 10/s was necessary for significant upwind progress and source contact. When an interspecific compound: (Z)-11-hexadecenyl acetate (Z11-16:OAc), was added to the attractive pheromone binary aldehyde blend of H. zea at a proportion of 10% of the major pheromone component, and pulsed from the same source, there was a significant reduction in sustained upwind progress and source location by males, indicating that Z11-16:OAc is antagonistic to the upwind progress of H. zea. However, Z11-16:OAc was less antagonistic when its filaments were isolated and alternated with pheromone filaments, indicating a strong effect of the synchronous arrival of odour filaments on the antenna needed for antagonism of upwind flight.     

Behaviour of flying oriental fruit moth males during approach to sex pheromone sources

Abstract. The pheromone-modulated upwind flight tracks of Grapholita molesta (Busck) males were video recorded as they approached a point-source of pheromone in a wind tunnel. The field of view of the video recording was divided longitudinally into 33 cm sections and the flight behaviour of the males in these sections was measured and compared as they approached from 233 cm to 50 cm downwind of the pheromone source. As the males approached the source, their mean ground speeds decreased. The mean values of their track angles increased with respect to due upwind (0), indicating movement more across the wind. These changes resulted mainly from the males decreasing their air speeds as they progressed up the plume toward the source. They did not change the average direction of their steering (course angle). Thus, the increase in track angles resulted from the males allowing themselves to drift more in the wind as they approached the odour source. The males also increased their average rate of counterturning as they approached the source. The net result of all these behavioural changes was a track that slowed and grew narrower, giving the impression that the males were 'homing-in' on the pheromone source as they approached. Causes of these systematic changes in behaviour are considered with respect to the known systematic changes in pheromone plume structure as the distance to the source decreases.     

Effects of intermittent and continuous pheromone stimulation on the flight behaviour of the oriental fruit moth, Grapholita molesta

ABSTRACT. When male oriental fruit moths, Grapholita molesta (Busck) (Tortricidae), casting in clean air entered an airstream permeated with pheromone their flight tracks changed immediately on initial contact with pheromone, but after a few seconds returned to casting as if in clean air. The degree of change in the flight track was directly related to the concentration of pheromone. Although little net uptunnel movement occurred in response to the continuous stimulation provided by a uniformly permeated airstream, when an intermittent stimulus provided by a point-source plume was superimposed onto the permeated airstream moths were able to 'lock on' and zigzag uptunnel in the plume. The percentage of moths doing so corresponded to the difference between the peak concentration within the plume and the background concentration of pheromone permeating the airstream. Moths also locked onto, and flew upwind along the pheromone-clean-air boundary formed along a pheromone-permeated side corridor. Because a similar response was observed along a horizontal edge between a pheromone-permeated floor corridor and clean air, we conclude that the intermittent stimulation at the edge perpetuated the narrow zigzagging response to pheromone.