How locusts separate pattern flow into its rotatory and translatory components (Orthoptera: Acrididae)

The ability of desert locusts,Schistocerca gregaria, to separate pattern flow within the lateral visual fields into its rotatory and translatory components was studied in tethered flight under open-loop conditions. The optomotor turning behavior results from the sum of compensatory steering and upwind/downwind turning induced by the rotatory and translatory component of pattern flow, respectively. Thereby, the analysis of the visual stimulus is supposedly achieved by linear binocular interaction, i.e., by summation and subtraction of the optomotor effectiveness of the pattern flow on either side. Our results indicate that, in addition, locusts take into account the relative contribution of the rotatory and the translatory stimulus component to the sum total of pattern flow. This yields a factor which modifies the gain of the control loop of either of the response components to give a nonlinear response. It results in a weakening of the behavior upon stimuli composed of rotatory and translatory components. We discuss our results as an adaptation by which an animal avoids inappropriate behavior upon ambiguous stimulus situations.     

Adjustment of flight speed of gregarious desert locusts (Orthoptera: Acrididae) flying side by side

In tethered flying locusts, optomotor thrust responses induced by translatory pattern motion within the lateral visual fields were studied under closed-loop conditions. By modulating thrust in a compensatory manner, locusts counteracted a bias motion superposed on the thrust-related motion. This way, pattern speed was kept at 0° s^–1, indicating the set point of the respective optomotor control circuit. Though the quality of bias compensation varied greatly, it was largely independent from pattern characteristics. It might indicate that the gain of behavior not only is controlled by an automatic mechanism but also is affected by spontaneous modulations. Compensation of bias motion was critically dependent on the relation between self- and bias-generated motion: Locusts did not take control over pattern motion if self- and bias-generated motion differed greatly. Instead, locusts adopted a constant, supposingly preferred, thrust value. Therefore, flight speed is assumed to be controlled by two systems: the optomotor and a preferred thrust system. In free flight, an equalization of the flight speed of locusts within a swarm might result from similar behavior. In combination with a presumed coordination of the locusts' course direction, this may explain the continued cohesion of swarms in the field.     

Control of hindlimb posture by wind-sensitive hairs and antennae during locust flight

Steering movements of tethered, flying locusts, Schistocerca gregaria, subjected to simulated yaw were examined under open-loop conditions. Lateral movements of hindlimbs or curling of the abdomen were monitored with a capacitive movement transducer and were interpreted as indicating the tendency of the animal to turn. Three responses to simulated yaw were noted: Yaw-correcting upwind turning tendencies (Figs. 1, 2, 3). Downwind turning tendencies (Figs. 2, 3, 4, 5), and transient adjustments of hindlimb position consistent with an upwind turning tendency occurred in animals that made either no sustained postural adjustments of hindlimbs, or that exhibited sustained downwind turning tendencies (Figs. 4, 5). Ablations of certain mechanoreceptors tested their roles in wind detection and wind angle determination. The expression of upwind turning tendencies, whether sustained or transient, depends on inputs from cephalic mechanosensory hairplates (Figs. 2, 3, 4, 5). With hairplates occluded, all locusts exhibited downwind turning tendencies. All downwind turning tendencies depend on inputs from the antennae (Figs. 2, 3). Antennae and hairplates operate in an apparent antagonism in the steering responses they produce, which may provide the control flexibility required for complex flight maneuvering.     

Dynamic properties of large-field and small-field optomotor flight responses in <Emphasis Type="Italic">Drosophila</Emphasis>

Optomotor flight control in houseflies shows bandwidth fractionation such that steering responses to an oscillating large-field rotating panorama peak at low frequency, whereas responses to small-field objects peak at high frequency. In fruit flies, steady-state large-field translation generates steering responses that are three times larger than large-field rotation. Here, we examine the optomotor steering reactions to dynamically oscillating visual stimuli consisting of large-field rotation, large-field expansion, and small-field motion. The results show that, like in larger flies, large-field optomotor steering responses peak at low frequency, whereas small-field responses persist under high frequency conditions. However, in fruit flies large-field expansion elicits higher magnitude and tighter phase-locked optomotor responses than rotation throughout the frequency spectrum, which may suggest a further segregation within the large-field pathway. An analysis of wing beat frequency and amplitude reveals that mechanical power output during flight varies according to the spatial organization and motion dynamics of the visual scene. These results suggest that, like in larger flies, the optomotor control system is organized into parallel large-field and small-field pathways, and extends previous analyses to quantify expansion-sensitivity for steering reflexes and flight power output across the frequency spectrum.     

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.     

Control of flight by means of lateral visual stimuli in gregarious desert locusts, Schistocerca gregaria

Abstract. Tethered flying locusts were stimulated either by a periodic grating or by a spotted 'swarm-simulating' pattern moving horizontally, parallel to their longitudinal body axis within their lateral visual fields. The direction of movement of the pattern was changed periodically from progressive to regressive and vice versa.
Both kinds of patterns induced a correlated modulation of yaw-torque and thrust. The two measured flight parameters were modulated independently of each other. Each parameter either increased with progressive and decreased with regressive pattern motion or vice versa. The characteristic curves of thrust and yaw-torque responses - i.e. response amplitude versus contrast frequency resp. angular velocity – measured upon stimulation with the periodic grating between 2 and 70 Hz were at a maximum at 10 Hz and decreased at higher and lower contrast frequencies. The shape of the curves was nearly identical. The characteristic curves measured upon stimulation with the 'swarm-simulating' pattern between 60 and 1500^o s^-1 could be simulated using the spatial wavelength content of the pattern and the characteristic curves for periodic gratings.
Therefore, we suggest that the speed and direction of locusts' flight result from the optomotor effectiveness of the pattern image formed by the neighbouring individuals under free flight. The measured responses would thus contribute to the common orientation of groups of locusts within a migrating swarm and thus to swarm cohesion.     

Visually-guided, upwind turning behaviour of free-flying tsetse flies in odour-laden wind: a wind-tunnel study

Abstract. To test the hypothesis that tsetse flies use visual input from the apparent movement of the ground to assess wind direction while in flight, Glossina morsitans morsitans Westwood females were video- recorded in a wind-tunnel as they entered, in cross-wind flight, a broad plume of simulated host odour (C0₂ at c. 0.05%). The tunnel (2.3 times 1.2 m wide) generated winds up to 0.25 m s^-1 and had a strongly patterned floor that could be moved upwind or downwind to increase or decrease the visual input due to wind drift. Flight tracks were analysed for speed, direction relative to the wind, and angle of turn. Mean groundspeeds were c. 1.8 m s^-1. In control measurements in still air (with or without odour) flies turned 50:50 'upwind': 'downwind'. With a 0.25 m s^-1 odour-perme- ated wind, 79% turned upwind, and c. 70% left view flying upwind. When the floor was moved at 0.25 m s^-1 upwind (to mimic the visual input from the ground due to a 0.5 m s_^-1 wind), the strength of this response increased. If instead the floor was moved downwind, faster than the wind speed (to mimic the visual input due to a wind from the opposite direction), 59% turned downwind and c. 70% left view flying downwind, and thus away from the source (though progressing 'upwind' in terms of the visual input from apparent ground pattern movement). Upwind turns were on average significantly larger than downwind turns. It is concluded that tsetse navigate up host odour plumes in flight by responding to the visual flow fields due to their movement over the ground (optomotor anemotaxis), even in weak winds blowing at a fraction of their groundspeed.     

The activity of pleurodorsal muscles during flight and at rest in the moth Manduca sexta (L.)

In the moth Manduca sexta, the paired mesothoracic flight steering muscle II PD2m takes part in the generation of the flight rhythm and is spontaneously active in the non-flying animal. This spontaneous activity is modulated by optomotor stimuli and directionally selective. The directional response characteristics are analyzed. Another spontaneously active steering muscle pair, the III PD2c, is situated in the metathorax. The activities of this pair and of a third muscle pair, the III PD3 are also influenced by visual stimulation.The responses of all 6 muscles to optomotor stimuli which simulate the flight situations yaw, roll, thrust and lift are analyzed. Each situation elicits a unique pattern of activation/deactivation within this set of muscles. The activity pattern in non-flying animals allows the prediction of flight steering mechanisms such as changes of wing area in flight turns and provides a useful basis for the analysis of visuo-motor pathways.     

Three-dimensional analysis of aphid landing behaviour in the laboratory and field

The final second of the landing approach of black bean aphids, Aphis fabae, was analysed in three dimensions using video techniques. A yellow landing platform was placed upwind or downwind from aphids aggregating under a ceiling light in a laboratory wind tunnel with 10, 20, 30, 40 or 50 cm s^–1 wind speeds, and up-tunnel or down-tunnel in still air. As individual aphids flew to the platform, body orientation (assessed by direct observation) was predominantly into-wind whether the initial flight direction to the landing platform was upwind or downwind. A greater proportion showed into-wind body orientation as wind speed increased. Flight track parameters which differed significantly between wind speeds were the track length, linear start to finish distance, linearity index, horizontal ground speed, speed vertical to the ground, vertical turning rate, and horizontal turning rate. The position of the landing platform was important for track length, linear start to finish distance, horizontal ground speed, three-dimensional turning rate, horizontal turning rate, vertical turning rate, and sinuosity. As wind speed increased above 30 cm s^–1 the ground speed became more consistent and indicated considerable variation in air speed to adjust for ground speed. For the majority of aphids there was a strong preference (88%) for into-wind landings with initial upwind directed flight, while for downwind flights a significant number (55%) of insects reversed initial flight direction and landed into-wind. Field recorded landings showed that 66% of aphids landed into-wind and there was a mean bearing to the wind of 71 ± 42°, a similar finding to wind-tunnel studies.     

Factors influencing the landing of maleEpiphyas postvittana (Walker) exhibiting pheromone-mediated flight (Lepidoptera: Tortricidae)

The effects of changes in various visual and olfactory properties of a white card surface on the landing position of male Epiphyas postvittanaexhibiting pheromone-mediated flight were studied in a wind tunnel. Males landed predominantly at the most downwind position of a surface in line with the pheromone source, regardless of the strength of the source. The position on the surface that males landed was strongly influenced by visual factors. The landing position of males appeared to be influenced by visual cues along all three axes of the surface. Decreases in either the dimension horizontally perpendicular to the wind direction or the vertical dimension resulted in greater numbers of males landing farther upwind on the surface than the downwind edge. Visual changes in the axis along the wind direction also affected the position at which males landed. For example, when presented with two white card surfaces with a 4- cm gap between them, males tended to land on the downwind edge of the upwind surface (on which the source was located). When the gap was bridged with clear Mylar, the landing pattern was significantly different, with the greater proportion of males landing on the downwind surface. However, when Mylar was placed on the plexiglass floor of the tunnel (in addition to bridging the gap), the landing pattern on the surface was not significantly different from that on the two surfaces without the Mylar bridge. It is suggested that during the prelanding and landing phases of pheromone-mediated flight, male moths orient to visual features of the surface containing the pheromone source rather than to visual features of the source (conspecific female moth) itself.     

Visual afferences to flight steering muscles controlling optomotor responses of the fly

Summary In tethered flying house-flies (Musca domestica) visually induced turning reactions were monitored under open-loop conditions simultaneously with the spike activity of four types of steering muscles (M.b1, M.b2, M.I1, M.III1). Specific behavioral response components are attributed to the activity of particular muscles. Compensatory optomotor turning reactions to large-field image displacements mainly occur when the stimulus pattern oscillates at low frequencies. In contrast, turning responses towards objects are preferentially induced by motion of relatively small stimuli at high oscillation frequencies. The different steering muscles seem to be functionally specialized in that they contribute to the control of these behavioral responses in different ways. The muscles I1, III1 and b2 are preferentially active during small-field motion at high oscillation frequencies. They are much less active during small-field motion at low oscillation frequencies and large-field motion at all oscillation frequencies which were tested. M.b2 is most extreme in this respect. These steering muscles thus mediate mainly turns towards objects. In contrast, M.b1 responds best during large-field motion at low oscillation frequencies and, thus, is appropriate to control compensatory optomotor responses. However, the activity of this muscle is also strongly modulated during small-field motion at high oscillation frequencies and, therefore, may be involved also in the control of turns towards objects. These functional specializations of the different steering muscles in mediating different behavioral response components are related to the properties of two parallel visual pathways that are selectively tuned to large-field and small-field motion, respectively.Abbreviations FD (cell) figure detection (cell) - HS (cell) horizontal (cell)     

Odor-modulated upwind flight of the sphinx moth,Manduca sexta L.

1. Male and female Manduca sexta flew upwind in response to the odor of female sex-pheromone gland extract or fresh tobacco leaf respectively, and generated very similar zigzagging tracks along the odor plume. 2. After loss of odor during flight, males and females alike: (1) first flew slower and steered their flight more across the wind, then (2) stopped moving upwind, and finally (3) regressed downwind. 3. Males flying upwind in a pheromone plume in wind of different velocities maintained their ground speed near a relatively constant 'preferred' value by increasing their air speed as the velocity of the wind increased, and also maintained the average angle of their resultant flight tracks with respect to the wind at a preferred value by steering a course more precisely due upwind. 4. The inter-turn duration and turn rate, two measures of the temporal aspects of the flight track, were maintained, on average, with remarkable consistency across all wind velocities and in both sexes. The inter-turn durations also decreased significantly as moths approached the odor source, suggesting modulation of the temporal pattern of turning by some feature of the odor plume. This temporal regularity of turning appears to be one of the most stereotyped features of odor-modulated flight in M. sexta.     

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.     

Olfactory orientation responses of the eucalyptus woodborer, Phoracantha semipunctata, to host plant in a wind tunnel

The eucalyptus woodborer, Phoracantha semipunctata Fabricius (Coleoptera: Cerambycidae), attacks mainly species of Eucalyptus (Myrtaceae). This study investigated walking and flight behaviour of P. semipunctata males and females exposed to an odour plume originating from a log of E. globulus placed vertically in the upwind end of a wind tunnel. In control experiments, beetles were exposed to a PVC drainpipe in the same position as the log, providing a visual stimulus without host‐tree odour. No statistical differences were found between behavioural responses of either sex when exposed to the log or PVC pipe. No beetles landed on the PVC pipe, whereas 49% of the beetles exposed to host‐tree odour plume landed on the log. Beetles aged over 24 days after emergence from the host tree were more responsive than beetles aged 20–24 days, and accounted vor 86% of the beetles that landed on the log. While walking, host‐tree odour affected the behaviour of the beetles that landed on the log as follows: upwind movement and path linearity increased, whereas turning rate, stopping frequency, mean stopping time and time to take‐off flight decreased. During flight, host‐tree odour affected the behaviour of the beetles that landed on the log as follows: increased upwind flight, turning rate, flight time, flight distance, and decreased flight speed. For beetles that never lost contact with the odour plume, flight progressed upwind with narrow zigzags, and showed higher directedness upwind, path linearity, faster flight speed and lower turning rate than for beetles that lost contact with the odour plume. After loosing contact with the plume, beetles tended to decrease their upwind progression, exhibiting a sharp turn or quick counterturns followed by crosswind or downwind excursions. This led to regaining contact with the odour plume and resumed upwind progression at higher speed provided they flew within the boundaries of the plume. The results showed that host‐tree odour affects both walking and flight behaviour of P. semipunctata beetles, inducing a more directed upwind movement and landing on the visual stimulus of a tree trunk.     

Flight behaviour of tsetse flies in host odour plumes: the initial response to leaving or entering odour

ABSTRACT. Free-flying, wild male and female Glossina pallidipes Aust. and G. m. morsitans Westw. were video-recorded in the field in Zimbabwe as they entered or left the side of a host-odour plume in cross-wind flight, or as they overshot a source of host odour in upwind flight (camera 2.5 m up looking down at a 3 times 2.5 m field of view at ground level). 80% of cross-wind odour leavers turned sharply ( turns 95^o), but without regard to wind direction (overshooters behaved essentially the same except that nearly 100% turned). Many fewer flies entering a plume cross wind turned ( c . 60%), and when they did they made much smaller turns ( 58^o); these turns were, however, significantly biassed upwind ( c . 70%). All three classes of fly had similar groundspeeds ( 5.5–6.5 m s^_1) and angular velocities ( 350–400^o s^-1). Clear evidence was obtained of in-flight sensitivity to wind direction: significantly more flies entering odour turned upwind than downwind, and odour losers turning upwind made significantly larger turns than average. The main basis for the different sizes of turn was the different durations of the turning flight, rather than changes in angular velocity or speed. No evidence was found of flies landing after losing contact with odour.     

Evidence for velocity-tuned motion-sensitive descending neurons in the honeybee.

Behavioural experiments suggest the existence of two functionally distinct movement-sensitive pathways in honeybees: one mediates optomotor behaviour, consisting of reflexive turning responses preventing deviations from course, and the other controls flight speed. The first consists of direction-selective neurons responding optimally to a particular temporal frequency of motion, regardless of the pattern's spatial structure. The temporal frequency dependence matches the temporal tuning of the optomotor output. Behavioural experiments suggest the second pathway contains velocity-tuned cells, which generate equal-sized responses for any given image velocity, for patterns with a range of spatial structures. Here, recordings were made from direction-selective neurons in the honeybee's ventral nerve cord. Neurons were tested for responses to motion at velocities of 40-1000 deg s(-1) using four gratings with spatial periods of 11-76 degrees. In addition to temporal frequency-dependent optomotor neurons, direction-selective cells were found that had the same shaped velocity-response functions for all four patterns. The velocity-tuning properties of these cells suggest a possible role in monitoring flight speed because their velocity tuning matches the image velocities encountered during free flight and landing behaviour.     

The three-dimensional optomotor torque system ofDrosophila melanogaster

Summary The well known optomotor yaw torque response in flies is part of a 3-dimensional system. Optomotor responses around the longitudinal and transversal body axes (roll and pitch) with strinkingly similar properties to the optomotor yaw response are described here forDrosophila melanogaster. Stimulated by visual motion from a striped drum rotating around an axis aligned with the measuring axis, a fly responds with torque of the same polarity as that of the rotation of the pattern. In this stimulus situation the optomotor responses for yaw, pitch and roll torque have about the same amplitudes and dynamic properties (Fig. 2). Pronounced negative responses are measured with periodic gratings of low pattern wavelengths due to geometrical interference (Fig. 3). The responses depend upon the contrast frequency rather than the angular velocity of the pattern (Fig. 4). Like the optomotor yaw response, roll and pitch responses can be elicited by small field motion in most parts of the visual field; only for motion below and behind the fly roll and pitch responses have low sensitivity.The mutantoptomotor-blind ^H31 (omb ^H31) in which the giant neurones of the lobula plate are missing or severely reduced, is impaired in all 3 optomotor torque responses (Fig. 5) whereas other visual responses like the optomotor lift/thrust response and the landing response (elicited by horizontal front-to-back motion) are not affected (Heisenberg et al. 1978).We propose that the lobula plate giant neurons mediate optomotor torque responses and that the VS-cells in particular are involved in roll and pitch but not in lift/thrust control. This hypothesis accommodates various electrophysiological and anatomical observations about these neurons in large flies.Abbreviation EMD elementary movement detector     

Comparison between the movement detection systems underlying the optomotor and the landing response in the housefly

Flies evaluate movement within their visual field in order to control the course of flight and to elicit landing manoeuvres. Although the motor output of the two types of responses is quite different, both systems can be compared with respect to the underlying movement detection systems. For a quantitative comparison, both responses were measured during tethered flight under identical conditions. The stimulus was a sinusoidal periodic pattern of vertical stripes presented bilaterally in the fronto-lateral eye region of the fly. To release the landing response, the pattern was moved on either side from front to back. The latency of the response depends on the stimulus conditions and was measured by means of an infrared light-beam that was interrupted whenever the fly lifted its forelegs to assume a preprogrammed landing posture (Borst and Bahde 1986). As an optomotor stimulus the pattern moved on one side from front to back and on the other side in the opposite direction. The induced turning tendency was measured by a torque meter (Götz 1964). The response values which will be compared are the inverse latencies of the landing response and the amplitude of the yaw torque.

1. Optomotor course-control is more sensitive to pattern movement at small spatial wavelengths (10° and 20°) than the landing response (Fig. 1a and b). This suggests that elementary movement detectors (EMDs, Buchner 1976) with large detection base (the distance between interacting visual elements) contribute more strongly to the landing than to the optomotor system.
2. The optimum contrast frequencies of the different responses obtained at a comparatively high pattern contrast of about 0.6 was found to be between 1 and 10 Hz for the optomotor response, and around 20 Hz for the landing response (Fig. 2a and b). This discrepancy can be explained by the fact that the optomotor response was tested under stationary conditions (several seconds of stimulation) while for the landing response transient response characteristics of the movement detectors have to be taken into account (landing occurs under these conditions within less than 100 ms after onset of the movement stimulus). To test the landing system under more stationary conditions, the pattern contrast had to be reduced to low values. This led to latencies of several seconds. Then the optimum of the landing response is around 4 Hz. This is in the optimum range of the optomotor course-control response. The result suggests the same filter time constants for the movement detectors of both systems.
3. The dependence of both responses on the position and the size of the pattern was examined. The landing response has its optimum sensitivity more ventrally than the optomotor response (Fig. 3a and b). Both response amplitudes increase with the size of the pattern in a similar progression (Fig. 3c and d).

In first approximation, the present results are compatible with the assumption of a common set of movement detectors for both the optomotor course-control and the landing system. Movement detectors with different sampling bases and at different positions in the visual field seem to contribute with different gain to both responses. Accordingly, the control systems underlying both behaviors are likely to be independent already at the level of spatial integration of the detector output.     

Habitat cues synergize to elicit chemically mediated landing behavior in a specialist phytophagous insect,the grape berry moth

Many phytophagous insects locate their host plant using mixtures of volatile compounds produced by the plant. A key behavior in the host location process that has been the focus of decades of behavioral research is optomotor anemotaxis. Another key step in host location is landing on (or near) the odor source. In previous work, rubber septa emitting a synthetic blend of volatiles extracted from young shoots of grape plants, Vitus spp. (Vitaceae), elicited equivalent levels of oriented upwind flight by female grape berry moths (GBM), Paralobesia viteana (Clemens) (Lepidoptera: Tortricidae), as did actual (control) grape shoots. However, in contrast to the shoots, females did not land on the odor source. In this study, we used flight tunnel assays to investigate the landing response of GBM females with respect to chemical and visual stimuli, as well as differences in relative humidity. When stimuli were presented individually, only the synthetic blend of host plant volatiles elicited equivalent levels of oriented upwind flight compared to the plants. Interestingly, wet cotton strips elicited low but consistent levels of upwind flight. In paired assays, only the synthetic blend paired with wet cotton strips elicited landing, although at significantly lower levels than that elicited by grape shoots. To achieve landing rates equivalent to live grape shoots, grape berry moth females required all three stimuli we tested: host odor cues, moisture, and visual cues simulating a grape shoot. These results suggest the cues have a synergistic effect, and that landing behavior requires complex sensory processing using multiple sensory inputs. Furthermore, these results suggest that moisture plays an important role in the host plant location process.     

Upwind flight by the cabbage root fly, Delia radicum

ABSTRACT. In fourteen releases, most female D.radicum (L.) (Diptera, Anthomyiidae) flew upwind or at an angle to it of less than 77° regardless of the presence of host-plant odour. Females ready to lay eggs flew upwind without prior stimulation by odours from either a host crop or a trap releasing up to 3 ml/day of the attractant allylisothiocyanate. Upwind flight was more pronounced in flies from a diapause than from a continuous, non-diapause culture. Males from the non-diapause culture dispersed upwind and downwind in more or less equal numbers; old males flew mainly downwind. But, like the females, most males from the diapause culture flew upwind. Long-distance, odour-modulated anemotaxis did not appear to be used to locate distant host crops by either sex. It is concluded that the distances of insect orientation to plant odours recorded to date are only of intermediate range, and that long-range orientation to the odours of a host-plant still has to be proven. It is suggested that host-plant volatiles are involved not only in the final stage of host location but also in the first, and probably most important stage of host selection whilst the insect is still in flight.