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.     

Detection of short pure-tone stimuli in the noctuid ear: what are temporal integration and integration time all about?

The interception of a pheromone filament induces flying moths to surge briefly nearly straight upwind; in the absence of pheromone moths cease upwind progress and zigzag crosswind. We tested males of the almond moth, Cadra cautella (Lepidoptera, Pyralidae), in a low-turbulence wind tunnel in wind velocities of 20, 40 and 80 cm s⁻¹. A mechanical pulse generator was set to produce plumes either with same pheromone pulse frequency (pulse generation frequency of 2.9 Hz, interpulse distances from 7 cm to 28 cm) or plumes with same interpulse distance across the three wind velocities (interpulse distance of 14 ± 2 cm, pulse generation frequency of 1.7–5.0 Hz). In plumes of similar pulse frequency, the faster the speed of the wind the slower the ground speed of flight. However, in plumes of similar interpulse distance, ground speed remained relatively constant independent of the wind speed. A `realized' frequency of pulse interception for males flying along the various combinations of pulse frequencies and wind velocities was calculated using the males' average airspeed and the spatial distribution of pheromone pulses in the plume. Realized frequency of pulse interception ranged from 1.3- to 3.0-fold higher than the frequency of pulse generation. The flight tracks of males reflected the regime of realized pulse interception. These results suggest that upwind flight orientation of male C. cautella to pheromone in different wind velocities is determined by the flux of filament encounter. Accepted: 3 September 1997     

Influence of pheromone concentration and ambient temperature on flight of the gypsy moth, Lymantria dispar (L), in a sustained-flight wind tunnel

Abstract. The effects of pheromone concentration and ambient temperature on male gypsy moth, Lymantria dispar (L.) (Lepidoptera), flight responses to pheromone were investigated in a wind tunnel. As the pheromone dose increased from 10 ng to 1000 ng, males flew at progressively slower airspeeds and ground speeds, and reduced their wingbeat frequencies. Furthermore, the moths steered significantly smaller course angles as the pheromone concentration increased, indicating that they were adopting a more upwind heading. The overall width of the flight tracks also decreased when males flew in more concentrated pheromone plumes. Estimation of plume dimensions using a male wing-fanning assay showed that as pheromone dosage increased, the resultant active spaces became wider, indicating that an inverse relationship existed between the dimensions of the time-averaged plume and the width of track reversals and that most turns were initiated within the plume. When males were flown at cool (20°C) and warm (26°C) ambient temperatures but to equivalent pheromone emission rates, they exhibited higher airspeeds and ground speeds at the higher temperature but steered larger course angles. Track widths, and length of track legs were, however, similar at the two temperatures. The mean turning frequency was nearly the same (c. 4 turns/s) across all the concentrations and temperatures tested even though the moths' thoracic temperature differed by 5°C when the ambient temperature was varied.     

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.     

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.     

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.     

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.     

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.     

Responses of male Helicoverpa zea to single pulses of sex pheromone and behavioural antagonist

Male Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae) flying in a pheromone plume respond to the loss of pheromone when they fly into a large pocket of clean air by going into crosswind casting flight in a mean of 0.48 s; 0.62 s after re‐contacting pheromone presented as a single pulse, they surge upwind in a kind of narrow zigzagging flight. After 0.36 s of surging, they lapse into casting flight once again in the clean air following the pulse. The addition of a known behavioural antagonist (Z)‐11‐hexadecenyl acetate (Z11–16:Ac), to the pheromone significantly increases the mean latency of the response to a single pulse to 0.85 s. No other aspects of the surge were significantly changed by the presence of antagonist in the single pulse of pheromone. Thus, unlike males of the related species, Heliothis virescens, which show significant changes in track and course angles when antagonist is present in single pulses, only an increased latency of response to a filament containing antagonist occurred in H. zea males. The increased latency could act cumulatively when the male is exposed rapidly and repeatedly to filaments in a natural plume and explain the profound arrestment effect of the antagonist in such plumes. The latencies to casting and surging in response to a pulse of pheromone blend are longer than those of the smaller species, H. virescens, and may be due to size‐related differences in manoeuverability of H. zea vs. H. virescens.     

Pheromone-mediated upwind flight of male gypsy moths, Lymantria dispar, in a forest

Abstract Lymantria dispar L. males flying upwind in a pheromone plume in a forest were video-recorded at 2.5, 10 and 20 m from the source of pheromone. Males flew slower and steered more across the wind as they approached the source. In concert, their ground speed decreased and track angles increased. In contrast to these changes, their drift angles were fairly constant and the transverse component of image flow, above and/or below the moths eyes, showed almost no change. The inter-turn duration (time between sequential turns), a temporal aspect of the male flight manoeuvres, showed a consistent but relatively small increase as the distance from the source increased. The flight tracks narrowed as the males approached close (2.5 m) to the source. Because of unpredicted correlations between physical variables (i.e. temperature, wind velocity) and the distance from the source, we used principal components analysis to generate a set of completely independent variables. Greater than 90% of the variability in the data could be explained by four principal factors which corresponded well with known relationships in the flight manoeuvres. All four of these factors showed a significant regression against distance to the source. Although uncontrolled factors such as temperature and wind velocity may have contributed to changes in flight behaviour, recent data indicate that, in addition to concentration, certain temporal and spatial characteristics (i.e. burst period, burst return period) of plumes in wind vary systematically with distance from the source. We propose that L.dispar males might adjust their flight manoeuvres in response to these changes.     

Cowpea weevil flights to a point source of female sex pheromone: analyses of flight tracks at three wind speeds

Abstract.  Two-day-old male cowpea weevils, Callosobruchus maculatus, fly upwind to a point source of female sex pheromone at three wind speeds. All beetles initiating flight along the pheromone plume make contact with the pheromone source. Analysis of digitized flight tracks indicates that C. maculatus males respond similarly to moths tested at several wind speeds. Beetles' mean net upwind speeds and speeds along their track are similar ( P  > 0.05) across wind speeds, whereas airspeeds increase ( P <  0.01) with increasing wind speed. Beetles adjust their course angles to fly more directly upwind in higher wind speeds, whereas track angles are almost identical at each wind speed. The zigzag flight paths are generally narrow compared with most moth flight tracks and interturn distances are similar ( P  > 0.05) at the wind speeds employed. The frequency of these counterturns across the wind line is almost constant regardless of wind speed, and there is little variation between individuals. The upwind flight tracks are more directly upwind than those typically seen for male moths flying upwind toward sex pheromone sources. Male moths typically produce a bimodal distribution of track angles to the left and right of the windline, whereas C. maculatus males' track angles are centred about 0°. Preliminary examination of two other beetle species indicates that they fly upwind in a similar fashion.     

Effects of light levels and plume structure on the orientation manoeuvres of male gypsy moths flying along pheromone plumes

The upwind zigzag flights of male gypsy moths (Lymantria dispar L.; Lepidoptera: Lymantriidae) along narrow, ribbon‐like and wide, turbulent plumes of pheromone were examined in a wind tunnel at light levels of 450 and 4 lux. Under all conditions tested males flew upwind zigzag paths. In 450 lux, males flying along turbulent plumes had the highest ground speeds and the widest crosswind excursions between counterturns, compared to slow flight and a narrow zigzag of males along a ribbon plume. In a turbulent plume, males flew more slowly and had narrower zigzags in 4 than in 450 lux. Across most treatments of plume structure and light level, the rate of transverse image flow and the frequency of counterturning remained relatively constant. The effects of light levels on orientation are not readily reconcilable with a model in which moths in low light levels would head more towards crosswind, thereby enhancing the rate of transverse image flow and the perception of wind‐induced drift.     

Codling Moth, Cydia pomonella, Captures in Monitoring Traps as Influenced by Proximately to Competing Female-Like- vs. High-Releasing Pheromone Point Sources

This research tested the hypothesis that a codling moth male, Cydia pomonella L., selects the sex pheromone plume of highest concentration when encountering overlapping plumes of differing concentrations. A plume of normal concentration generated by a female-like lure (7 ηg/h), and a plume of high concentration generated by a polyethylene sex pheromone “rope” dispenser, Isomate-C® Plus (6,000 ηg/h), were deployed at various distances and elevations from a standard monitoring trap (Pherocon® VI delta trap) deployed in an abandoned apple orchard. Moth captures in lure-baited traps in the tree canopy were significantly inhibited when the plume from the rope was nearby. However, the concentrated plume from the rope positioned below the monitoring trap at distances of 50 cm or more did not influence moth capture significantly. When the plume from the rope was positioned at the same elevation as the lure, moth captures were reduced significantly only when the separation was 50 cm or less. Our data: 1) falsify the hypothesis that codling moth males are able to select plumes of normal pheromone concentration when exposed simultaneously to plumes of high vs. normal concentrations; 2) confirm that codling moth males favor more vs. less concentrated plumes; and 3) provide further support for competitive attraction as the disruptive mechanism for the Isomate-C® Plus.     

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.     

Manoeuvres used by flying male oriental fruit moths to relocate a sex pheromone plume in an experimentally shifted wind-field

ABSTRACT. In a wind-field experimentally shifted in direction by 35d?, flying male Grapholita molesta (Busck) zigzagging upwind either maintained contact with a pheromone plume and followed it across during the shift or lost it and commenced casting at c. 90d? across the shifting windline to locate it eventually in its new position. Males accomplished both of these results by integrating the previously described systems of optomotor anemotaxis and self-steered counterturning, but with faster reaction-times to pheromone on and off than heretofore calculated for this species. We found no evidence that males following the plume across used chemotaxis as proposed for another species, Rather, the sawtoothed-shaped tracks were a result of the anemotactic and counterturning systems responding rapidly and reiteratively to each loss and gain of pheromone along the plume in the shifting wind. The response to an increase or decrease in pheromone concentration by males was to change their course angle to more upwind or more crosswind, respectively, on the very first reversal (within c. 0.15 s) after the concentration changed. Because males adjusted their airspeeds more slowly to changes in concentration, the groundspeeds along the more upwind-orientated legs were lower than those along cross-wind legs, contributing to the sawtoothed shape of tracks of plume-followers. The self-steered counterturning programme also reacted quickly to concentration changes, the reversal intervals tending to be shorter following each contact with pheromone than after each excursion into cleaner wind. Following casting after losing the plume, males relocating the pheromone plume exhibited an upwind ‘surge’ of narrow zigzagging flight because on the first leg in the plume they steered a course more directly upwind than on the previous leg and increased the frequency of counterturning to its highest value while maintaining the relatively high airspeed acquired while casting.     

Odor tracking flight of male Manduca sexta moths along plumes of different cross-sectional area

Males of the hawkmoth, Manduca sexta, track wind-borne plumes of female sex pheromone by flying upwind, while continuously turning from side-to-side and changing altitude. Their characteristic “zigzagging” trajectory has long been thought to result from the interaction of two mechanisms, an odor-modulated orientation to wind and a built-in central nervous system turning program. An interesting and as of yet unanswered question about this tracking behavior is how the cross-section of an odor plume or its clean-air “edges” affects moths’ odor tracking behavior. This study attempts to address this question by video recording and analyzing the behavior of freely flying M. sexta males tracking plumes from pheromone sources of different lengths and orientations with equal odor concentration per unit area. Our results showed that moths generated significantly wider tracks in wide plumes from the longest horizontally-oriented sources as compared to narrower point-source plumes, but had relatively unaltered tracks when orienting to plumes from the same length sources oriented vertically. This suggests that in addition to wind and the presence of pheromones, the area of the plume’s cross section or its edges may also play an important role in the plume tracking mechanisms of M. sexta.     

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.     

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.