Visual ground pattern modulates flight speed of male Oriental fruit moth Grapholita...

Physiological Entomology (2014), DOI: 10.1111/phen.12072

Visual ground pattern modulates flight speed of maleOriental fruit moth Grapholita molesta

L O D E W Y K P . S . K U E N E N 1 and C O L E G I L B E R T 2

1Commodity Protection and Quality, USDA, ARS, Parlier, California, U.S.A. and 2Department of Entomology, Cornell University,

Ithaca, New York, U.S.A.

Abstract. Insects flying in a horizontal pheromone plume must attend to visual cuesto ensure that they make upwind progress. Moreover, it is suggested that flying insectswill also modulate their flight speed to maintain a constant retinal angular velocityof terrestrial contrast elements. Evidence from flies and honeybees supports such ahypothesis, although tests with male moths and beetles flying in pheromone plumesare not conclusive. These insects typically fly faster at higher elevations above ahigh-contrast ground pattern, as predicted by the hypothesis, although the increase inspeed is not sufficient to demonstrate quantitatively that they maintain constant visualangular velocity of the ground pattern. To test this hypothesis more rigorously, theflight speed of male oriental fruit moths (OFM) Grapholita molesta Busck (Lepidoptera:Tortricidae) flying in a sex pheromone plume in a laboratory wind tunnel is measuredat various heights (5–40 cm) above patterns of different spatial wavelength (1.8–90∘)in the direction of flight. The OFM modulate their flight speed three-fold over differentpatterns. They fly fastest when there is no pattern in the tunnel or the contrast elementsare too narrow to resolve. When the spatial wavelength of the pattern is sufficientlywide to resolve, moths fly at a speed that tends to maintain a visual contrast frequencyof 3.5± 3.2 Hz rather than a constant angular velocity, which varies from 57 to 611∘ s−1.In addition, for the first time, it is also demonstrated that the width of a contrast patternperpendicular to the flight direction modulates flight speed.

Key words. Flight height, moth, optomotor, sex pheromone, vision, visual texture.

Introduction

Flying insects, including male moths flying in an odour plume,must use visual cues to regulate their speed to ensure that theymake progress over the ground in the face of varying headwindspeeds (Kennedy, 1940; Kennedy & Marsh, 1974). The currentalgorithm for sustained upwind flight toward an odour source(Vickers, 2006; Cardé & Willis, 2008) indicates that malemoths also have an endogenous counterturning generator in thecentral nervous system that is engaged when in contact withairborne sex pheromone (Kennedy et al., 1980, 1981). Upwindmovement combined with vertical and horizontal movementsacross the wind line (Rutkowski et al., 2009) leads to fairlycharacteristic flight paths that appear to zigzag when viewed intwo dimensions. Although the cadence of crosswind zigzagging

Correspondence: L. P. S. Kuenen, Commodity Protection and Quaran-tine, USDA, ARS, 9611 S. Riverbend Avenue, Parlier, California 93648,U.S.A. Tel.: +1 559 596 2762; e-mail: [email protected]

is not as regular as it sometimes appears (Willis & Arbas, 1991),the nature of the zigzagging, once observed, will not readilybe mistaken for anything other than odour-mediated upwindorientation (Porter et al., 2007). Zigzagging flight is polarizedtoward the source by the presence of wind (Baker et al., 1984)and continues for some time in the absence of pheromone as well(Kennedy et al., 1980, 1981; Baker & Kuenen, 1982; Vickers& Baker, 1994), being primarily documented as nonprogressing(i.e. no net movement toward or away from the odour source)crosswind casting (Kennedy, 1983), although Kuenen & Cardé(1994) report upwind and downwind casting.

Upwind progress is independent of wind speed across a largerange of speeds (Marsh et al., 1978; Kuenen & Baker, 1982a;Willis & Cardé, 1990; Willis & Arbas, 1991), although thespeed of upwind flight toward a pheromone source may beinfluenced by several factors, such as dosage of the pheromonein the source formulation (Cardé & Hagaman, 1979; Kue-nen & Baker, 1982b; Charlton et al., 1993), plume structure(Mafra-Neto & Cardé, 1994; Vickers & Baker, 1994) and size

Published 2014. This article is a U.S. Government work and is in the public domain in the USA. 1

2 L. P. S. Kuenen and C. Gilbert

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Fig. 1. Diagrammatic representations of distanceand angle measurements used in the present studyof the flight speed of Oriental fruit moth Grapholitamolesta in response to changing ground pattern.(A) Distances ‘d’ that would have to traversed tomaintain constant angular velocity of image motionat each height ‘h’. (B) calculation of vertex angle ateye surface for each stripe width ‘w’ along the windline. (C) calculation of lateral vertex angle at eyesurface for each stripe width across the wind line,25, 50 and 100 cm.

of the male moth (Bartholomew & Casey, 1978; Kuenen &Cardé, 1993).

Effects of visual cues on flight performance of male mothsreceive only limited attention, even though such cues areessential for the moths’ upwind progress. Kennedy (1951)reports that locust swarms appear to move slower when flyingclose to the ground and faster when flying at greater heights,leading to an optomotor hypothesis of behaviour in relation toheight. Kennedy (1951) hypothesized a servo mechanism ofspeed control, whereby the insect maintains a constant preferredvisual angular velocity of image motion over the surface of theeye. Thus, higher flying insects would need to have a fasterground speed to see terrestrial features move across their retinaat the preferred angular velocity compared with the groundspeed of insects flying closer to the ground (Fig. 1). Thisservo mechanism is demonstrated qualitatively in studies offree-flying male moths by employing moving floor patterns:downwind (down tunnel) movement of the floor pattern slows

the insects’ flight toward the pheromone source (Kennedy &Marsh, 1974; Cardé & Hagaman, 1979; Kuenen & Baker,1982a) and upwind movement of the pattern increases theinsect’s flight speed toward the odour source (Kennedy & Marsh,1974; Marsh et al., 1978; Miller & Roelofs, 1978; David, 1979).

In a quantitative test of Kennedy’s (1951) optomotor servomechanism hypothesis to regulate flight speed (Kuenen & Baker,1982a), free-flying male Oriental fruit moths (OFM) Grapholitamolesta Busck (Tortricidae) and Heliothis virescens (Noctuidae)flying upwind toward a pheromone source at different heightsabove the ground pattern are observed to increase their netground speed with increased flight height as predicted, althoughthe moth’s flight speed does not increase sufficiently to maintaina constant visual angular velocity of image motion across theirretinas. Similar results are reported in other studies in whichmale insects were flown in a pheromone plume at two differentheights over a constant ground pattern: lightbrown apple mothsTortricidae: Epiphyas postvittana (Foster & Howard, 1999) and

Published 2014. This article is a U.S. Government work and is in the public domain in the USA., Physiological Entomology, doi: 10.1111/phen.12072

Visual texture affects moth flight speed 3

larger grain borer beetles Bostrichidae: Prostephanus truncatus(Fadamiro et al., 1998). In both cases, the insects fly faster athigher elevations, although the image of the ground pattern is notmaintained at a constant visual angular velocity. As with moths(Kuenen & Baker, 1982a), grain beetles are less responsiveto ventral floor pattern movements when they fly higher up,possibly indicating that the insects shift to other visual cuespresent in the tunnel or assay-room for regulating their flighttoward the pheromone source. Thus, these insects regulate theirflight speed in a pheromone plume either by using some aspectof the visual pattern other than angular velocity or they integratemore visual information than simply the ground pattern on thefloor of the flight tunnel seen in their ventral visual field.

To examine more thoroughly which aspects of the visualstimulus may be used by flying moths, male OFM, G. molestaare flown in a pheromone plume over a series of high-contrasttransverse stripes in a wind tunnel at various heights and with‘up tunnel’ flight speed recorded. Because the lateral extentof the floor pattern also changes with flight height and mayinfluence flight speed, the lateral extent of the stripes are alsovaried in separate experiments. It is shown that, at a given height,the moths’ upwind/up tunnel velocities increase as the spatialfrequency of high-contrast stripes increases (narrower stripes).Up tunnel velocities also increase as the cross-tunnel length ofthe floor stripes (i.e. the lateral width of the floor pattern) isdecreased. With stripes of a given spatial frequency, the moths’flight speed increases with height in the tunnel, although this isnot sufficient to maintain a constant angular velocity. The datasupport an alternative hypothesis that the moths regulate theirflight speed to maintain a constant visual contrast frequency inthe ‘up tunnel’ direction.

Materials and methods

Insects

The OFM G. molesta was reared on small green thinningapples (Baker et al., 1981). All life stages were maintained underan LD 16 : 8 h photocycle at 25± 2 ∘C. Males were separatedfrom females in the pupal stage and adults had continuous accessto an 8% w/v sucrose solution. Adult males were kept isolatedfrom females at all times and were segregated daily by age.Four- to 6-day-old males were tested during the last hour ofphotophase and the first hour of scotophase, at the time of theiroptimal responsiveness to pheromone (Baker & Cardé, 1979a).The moths were acclimated to tunnel wind and light conditionsfor at least 15 min before their individual release from a smallaluminium screen cone hand-held in the centre of the pheromoneplume.

Pheromones

The female sex pheromone of G. molesta consists of 5.9%(E)-8-dodecenyl acetate with 3.8% (Z)-8-dodecyl alcohol(Cardé et al., 1979) in (Z)-8-dodecenyl acetate (Roelofs et al.,1969). Ten micrograms of this mixture in 10 μL of hexane

was applied to the inside bottom of the large end of a rubberseptum (#8753-022, sleeve type, 7× 12 mm; Thomas Scientific,Swedesboro, New Jersey). The ratio of the pheromone solutions(Baker & Cardé, 1979b) was verified by gas chromatographywith a flame ionization detector (Baker & Roelofs, 1981). Septawere aged in a fume hood overnight before use, and held in aglass vial at −20 ∘C between uses.

Wind tunnel

Male G. molesta were allowed to fly in a wind tunnel (length3.6 m, height 1 m, width 1 m) at the floor level (Kuenen & Baker,1982a). Light from eight 40-W fluorescent lamps in four ceilinglight fixtures was diffused through a 1.25-cm thick layer of whiteexpanded polystyrene that covered the entire room ceiling andprovided a light intensity of 1.32 W m−2 (converted from luxas measured by conventional light meters; Young et al., 1987).The 65 cm s−1 airflow was generated by a variable speed fan andair turbulence was dampened as the air passed through threelayers of cheese cloth. A titanium tetrachloride smoke source(emanating from a rubber septum, as above) was used to createhorizontal plumes that were approximately 10 cm wide halfwaydown the tunnel and 15 cm wide at the downwind end of thetunnel. The plumes were almost circular in cross-section andthus the bottoms of the 5- and 10-cm high plumes (below)touched the tunnel floor sporadically without apparent effect onthe structure of the rest of the plume. This was likely a result ofthe high uniformity of air speed throughout the cross-section ofthe tunnel (Kuenen & Baker, 1982a).

High-contrast ground patterns of alternating black and whitestripes of equal width were laid on the floor on the inside ofthe tunnel (Table 1). Striped patterns were made by taping blackposter board strips on white poster board or by placing tape stripsof white cloth on a black poster board.

Moths’ flight heights were controlled by positioning thepheromone septum on a height-adjustable platform (Kuenen &Baker, 1982a) that was positioned on the longitudinal midlineof the tunnel 40 cm from the upwind end. The moths maintainedtheir flight height close to the centre of the horizontal plumeaxis at all test heights and this ‘centring’ was not affected bythe floor test patterns; however, a few moths flying 5 cm above20-cm wide stripes did touch the tunnel floor, although they werenot included in the data. The moths’ vertical movements wererarely seen to exceed the plume boundaries and their lateral flightmovements rarely exceeded 15–20 cm from the plume axis.

Experiments

For each experiment, floor pattern panels were positionedend-to-end to cover the entire floor of the tunnel. Treatmentswere changed by removing the pheromone from the tun-nel, switching floor pattern panels and then re-installing thepheromone septum at the appropriate height. The net groundvelocities of the moths were determined by timing their flightduration through the central 1.22-m section of the flight tun-nel length with a stop watch (Kuenen & Baker, 1982a, 1982b).



Table 1. Heights and angular pattern sizes over which Oriental fruit moth(Grapholita molesta) males were tested in wind tunnels.

Individualstripewidth (cm)

Subtendedangleat 5 cmheight




Transverse black and white stripes widths0.63 7.13 3.58 1.79 0.901.25 14.04 7.13 3.58 1.792.50 26.57 14.04 7.13 3.585.00 45.00 26.57 14.04 7.1310.0 65.43 45.00 26.57 14.0420.0 75.96 65.43 45.00 26.57Lateral (10 cm) black and white stripe widths25 – 51.34 32.01 17.3550 – 68.20 51.34 32.01100 – 78.69 68.20 51.34

Subtended angle for lateral width of stripes was calculated using half thewidth of the stripe based on the assumption that the moths fly along themidline of tunnel and look only left or right with each compound eye. Thisis in contrast to the angular width of transverse stripes that the moths can seefrom ‘front to back’ with both eyes.

Net speed was considered to be a sufficient indicator of changesin behaviour because increases in flight speed of OFM maleswith increased height (Kuenen, 2013) and visual cue size(Kuenen et al., 2014) are almost exclusively the result of a pos-itive orthokinetic response.

In the first experiment, moths flew over floor patterns withstripe widths of 5, 10 or 20 cm at flight heights of 5, 10, 20or 40 cm above the tunnel floor. Noting only a small decreasein speed over the 20-cm wide stripes at low flight elevations,a second experiment was conducted, in which moths flew overstripes of 0.625, 1.25, 2.5 or 5 cm width at heights of 5, 10, 20 or40 cm. Finally, moths were flown over a blank white floor patternas a control. The subtended angle of the stripes was calculatedas the vertex angle of the right triangle made by extending a linevertically down from the insects’ flight height to the downwindedge of a floor stripe and the line extending from the flight heightdown and forward to the upwind edge of a stripe (Fig. 1B).

As moths fly higher above the ground pattern, not only doesthe angular width of the pattern become narrower along theflight axis (i.e. in the up tunnel direction), but also the angularwidth of the pattern in all other directions became narrower.To test whether male moths might also use pattern informationperpendicular to the flight (tunnel) axis, flight speeds of maleswere examined as the lateral (cross-tunnel) width of the stripepattern was changed from wide, to medium or narrow (100, 50and 25 cm, respectively) (Fig. 1C) by masking the edges of theground pattern symmetrically with white poster board along theentire length of the tunnel (Table 1). All ground pattern stripeswere 10-cm wide in the up tunnel dimension. The lateral angularwidth of the floor pattern was calculated as the vertex angle ofthe right triangle made by a line extending vertically down fromthe insects’ flight height to the left–right centre of the floor stripeand the line extending from the flight height down to the lateraledge of a floor stripe (Fig. 1C). Moths flew at 10, 20 and 40 cmabove these ground patterns.

Statistical analysis

In all experiments, tunnel floor pattern and height combina-tions were tested in a randomized complete block design withfive moths tested per treatment per day for 10 days. Flight speeddata were analyzed by two-way analysis of variance with procglm (SAS, 2004) and mean separations were conducted withTukey’s honestly significant difference test (SAS, 2004). Treat-ments were analyzed by the height over each floor pattern/sizeand over each floor pattern/size at each height. Data were trans-formed by log(x+ 1) when needed, as indicated by Bartlett’s testfor homogeneity of variances (Sokal & Rohlf, 1981).

Results

Effect of pattern angular width in the upwind direction

Consistent with earlier results using a single ground pattern(Kuenen & Baker, 1982a), increasing flight height elicitedhigher net flight speeds when moths flew above all stripe widths(P< 0.05; Fig. 2). The moths flew twice as fast at a 40-cmheight compared with speeds recorded at the 5-cm flight height.Although the increase in flight speed was generally monotonicwith flight height over any given pattern size, significance levelsvaried with the treatment. When moths flew over 5-, 10- or20-cm stripes (Fig. 2), males flew more slowly upwind above20-cm wide stripes at 5- and 10-cm flight heights (P< 0.05) but,at 20- and 40-cm flight heights, this disparity decreased to non-significant levels (P> 0.05; Fig. 2). When moths flew over nar-rower stripes, the narrowest of these (0.625 and 1.25 cm) elicitedsignificantly higher upwind speed at the 5-cm flight height thandid wider stripes (P< 0.05; Fig. 3) and males flew faster stillover the blank white floor, 0-cm stripe width (P< 0.05). At the10-cm flight elevation, the three narrower stripe patterns elicitedgreater upwind flight speeds compared with the 5-cm stripes(P< 0.05) and, again, males flew faster still over the blank floorpattern (P< 0.05). At the 20-cm flight height, the same trendsexisted, although only the blank floor and narrowest stripes(0.625 cm) elicited significantly faster speeds (P> 0.05; Fig. 3).At the 40-cm flight height all trends disappeared (P> 0.05;Fig. 3) and, except for the no contrast and 0.625-cm stripeground patterns, flight speeds were faster than flight at a lowerelevation (P< 0.05), with the difference by height exhibitedmost strongly when moths flew over the widest (5 cm) stripes.

Effect of pattern angular width perpendicular to the netdirection of movement

Because the lateral visual angular width of the stripe patternalso decreases at higher flight heights in the wind tunnel, theeffect of laterally shorter stripes was examined with moths flyingover 10-cm stripes extending for three different widths acrossthe tunnel. There was no effect of stripe width when the mothsflew at the lowest elevation (10 cm) above the stripes. However,at flight elevations of 20 and 40 cm, there was a linear increasein flight speed with decreasing stripe width and moths flew



OFM Flights at 4 Heights Over 3 Floor Patterns

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Fig. 2. Net ‘up tunnel’ ground speed of Oriental fruit moth (OFM) Grapholita molesta males at four heights over wide stripes. Values are the mean±SE.Columns with no letters in common are significantly different; lowercase letters are for mean comparison tests within the column groups, uppercaseletters are for mean comparisons of each height & pattern combination between column groups. Tukey’s honestly significant difference test, P< 0.05.

OFM flights at 4 heights over 5 floor patterns

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Fig. 3. Net ‘up tunnel’ ground speed of Oriental fruit moth (OFM) Grapholita molesta males at four heights over narrow stripes. Values are themean± SE. Columns with no letters in common are significantly different; lowercase letters are for mean comparison tests within the column groups,uppercase letters are for mean comparisons of each height & pattern combination between column groups. Tukey’s honestly significant difference test,P< 0.05.

significantly faster over the narrowest stripes (25 cm) at bothheights (P< 0.05; Fig. 4). Thus, moths also can use the lateralextent of a visual contrast pattern to regulate flight speed similarto their use of the upwind extent of the pattern, as shown above.

What parameter(s) of the visual stimulus do the mothsregulate?

Because the moths change their flight speed in a regular waywhen flying at different heights over the same ground pattern

they are likely regulating some aspect of their visual stimulation.To explore this, all of the data from the experiments in whichmoths flew at different heights over stripes that varied in theirwidth along the wind direction to look for a constant relationshipwith flight speed. Although the data confirm qualitatively theresults from previous studies that insects fly faster at higher ele-vations, they do not provide quantitative support for Kennedy’s(1951) hypothesis that insects attempt to hold visual angularvelocity constant (Fig. 5). Visual angular velocity is the angularspeed at which a contrast edge moves through the moth’s visualfield. Using the formula of Baird et al. (2005) to estimate retinal



OFM Flight Over 10 cm Stripes of 3 Lateral Widths

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Fig. 4. Net ‘up tunnel’ ground speed of Oriental fruit moth (OFM)Grapholita molesta males over 10 cm stripes of three lateral widths.Values are the mean±SE. Columns with no letters in common aresignificantly different; lowercase letters are for mean comparison testswithin the column groups, uppercase letters are for mean comparisonsof each height & pattern combination between column groups. Tukey’shonestly significant difference test, P< 0.05.

angular velocity [angular velocity= (flight speed/distance frompattern) (180/π)], the results for moths flying ‘up tunnel’ at vari-ous elevations over different stripe widths show a wide range ofvisual angular velocities resulting from the moths’ flight speeds.However, closer inspection of the data is warranted. Many of thedata points represent spatial patterns that may be too narrow forthe moths to resolve at a given height and, thus, they should beconsidered similar to the condition with the blank white floor.Four of the six fastest flight speeds from 50.8 to 60.0 cm s−1

(Fig. 5, open symbols) occurred when there was no explicit pat-tern on the floor. The data indicated by the red symbols are alsofrom very fast flights but occurred when the floor pattern sub-tended less than 3.6∘. This is certainly less than one functionalinterommatidial angle for the moths at the light levels in the tun-nel and thus the pattern would not modulate visual contrast. Themoths treat it as no pattern and fly between 50.6 and 52.6 cm s−1,which is similar to their speed when there was no experimentalpattern on the floor. Similar logic could be applied to the bluedata points, which represent floor patterns with an angular sub-tense of 7.1∘, and green data points, which represent patternswith an angular subtense of 14.2∘. Thus, if the treatments withno contrast pattern in the tunnel or a pattern too small to resolveat the set flight height are discounted for the moment, the datado not appear to show that the moths vary their flight speed tomaintain a constant visual angular velocity of the patterns theycan see. The moths vary their flight speed more than two-foldfrom 19.5 to 47.5 cm s−1 and produce retinal angular velocitiesof the stripe patterns that vary from 57.3 to 611.6∘ s−1.

Failure to support Kennedy’s (1951) hypothesis promptedan investigation of whether the moths might vary their flightspeed to control the visual contrast frequency of the groundpattern. The contrast frequency of a visual pattern is the timing

of how fast light and dark edges move across the retina andseveral species of insects in flight have been shown to attendto this parameter of a visual stimulus in different contexts. Thecontrast frequency was measured using the formula of Götz(1972): [contrast frequency= visual angular velocity/patternwavelength]. The four flight speeds plotted directly on the x-axis(Fig. 6) are among the fastest measured (50.8–60 cm s−1) andoccurred when there was no explicit pattern on the floor. Again,in this plotting of the data, the other fastest flight speeds occurredover stripes that the moths likely could not resolve and thus theyflew as though there was no pattern on the floor. If the data forstripe periods of 3.6, 7.1 and 14.2∘, represented by red, blue andgreen symbols, are discounted, the remaining data cluster largelyat a visual contrast frequency below 5 Hz when the moths variedtheir flight speed two-fold up to 41.7 cm s−1. The mean contrastfrequency maintained is 3.5± 3.2 Hz.

In flight tests where there was no experimental contrastpattern, or when the pattern was below the visual resolutionof the moths, they flew with the highest velocities. The mothswere likely using some other visual cue from the contours of thetunnel or the laboratory to gauge their upwind progress, althoughit has not yet proved possible to quantify that contrast frequency.

Discussion

The present study tests the optomotor hypothesis of Kennedy(1951), which suggests that flying insects adjust their flightspeed to maintain a constant visual angular velocity of groundpattern contrast elements across their retinas, by measuring thenet flight speed of the OFM, G. molesta, flying at various heightsin a wind tunnel above ground patterns of different spatialfrequency. The two principal results obtained are that changes inthe visual angular size of the ground pattern, whether as a resultof different absolute sizes of contrast elements in the patternor different flight heights, affect flight speed not only when thespatial frequency of the pattern is changed in the net upwinddirection of flight, but also when the spatial frequency is changedperpendicular to the net direction of flight. Nevertheless, theincrease in flight speed with height does not support Kennedy’shypothesis quantitatively. OFM males do not appear to regulateflight speed to maintain a constant visual angular velocity of theground pattern.

The functional increase in speed is much shallower thanexpected if a preferred retinal angular velocity is being main-tained. This relationship is similar to that seen in the beetleP. truncatus flying in a plume of sex pheromone at differentheights (Fadamiro et al., 1998). Beetles fly faster at higher eleva-tions, although the increase in speed is not sufficient to maintaina constant retinal angular velocity of the ground pattern.

In addition to moths and beetles, other insects use visual cuesto maintain a preferred flight speed; for example, mosquitoes(Kennedy, 1940), locusts at low elevation (Kennedy, 1951), fruitflies (David, 1982; Fry et al., 2009) and honey bees (Bairdet al., 2005, 2006; Barron & Srinivasan, 2006). Honey beesare able to regulate their flight at a constant up tunnel speedagainst various wind speeds (Barron & Srinivasan, 2006) andin a no-wind flight tunnel regardless of a four-fold difference in



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Fig. 5. Estimated retinal angular velocity resulting from mean flight speed of Grapholita molesta measured flying over different stripe patterns whenmoths flew at different elevations. Although the moths flew faster at higher elevations, they did not hold retinal angular velocity constant. Data arecolour coded according to the angular subtense of the pattern width: red ≤ 3.6∘, blue= 7.1∘, green= 14.2∘, black ≥ 28∘. Open symbols indicate flightover the blank/no stripe floor.

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Fig. 6. Estimated visual contrast frequency resulting from mean flight speed of Grapholita molesta measured over different stripe patterns when mothsflew at different elevations. Data are colour coded according to the angular subtense of the pattern width: red ≤ 3.6∘, blue= 7.1∘, green= 14.2∘, black≥ 28∘. Open symbols indicate flight over the blank/no stripe floor.

the spatial frequency of the ground pattern (Baird et al., 2006).Thus, they appear to judge retinal velocity independently of thecontrast frequency and use it to regulate flight speed. Honeybeesalso fly faster over axial stripes running parallel to the tunnelwalls and the mean direction of flight than when they fly over acheckerboard pattern, which affords strong transverse cues notpresent in the axial stripes (Baird et al., 2005, 2006). Thus, theaxial pattern does not provide contrast cues in the direction offlight, and the bees’ flight speed is regulated by other visualcues in the experimental environment, similar to the results forlightbrown apple moth (Tortricidae) in a flight tunnel (Foster& Howard, 1999) and the results of the present study withmale OFM flying in the tunnel without an experimental groundpattern. Male gypsy moths, Lymantria dispar (Lepidoptera:Erebidae), flying tethered to a thrust meter in a modified ‘barberpole’ flight tunnel, also generate the maximum forward thrustwhen they view the surroundings through an insufficient regionof the eye (Preiss, 1987).

The fruit fly Drosophila hydei maintains flight speed whenthe spatial frequency of the pattern changes in the middleof a ‘barber pole’ tunnel (David, 1982; Fig. 8), providingsupport for a speed control mechanism based on retinal imagevelocity. However, when this tunnel design is configured withtwo different diameters (David, 1982; Fig. 6), such that theflies fly at different distances from patterns of identical spatialwavelength in different portions of the tunnel, the D. hydeimaintain flight speed but fly as though they are controlling theirspeed to maintain a constant retinal contrast frequency, ratherthan image velocity. Fry et al. (2009) report testing regulationof flight speed by another species of fruit fly, D. melanogaster,with 3D real-time tracking and a virtual reality display to presentstripes to the lateral visual field of the fly. In this set-up, fliesmodulate flight speed using pattern velocity in the face of aneight-fold difference in spatial frequency of the stripes on thewall of the flight tunnel. Thus, fruit flies likely regulate flightspeed by maintaining pattern angular velocity, although there



is some evidence to support a mechanism based on contrastfrequency.

The present study investigates visual contrast frequency as aparameter used to regulate flight speed because OFM males donot maintain a constant retinal angular velocity of the groundpattern. The plot of contrast frequency versus flight speed(Fig. 6) does not appear to indicate that contrast frequency isconstant. However, many of the data points represent patternwavelengths (≤ 14∘) that are certainly or likely to be too smallto be resolved by the moths and thus should be treated asthough there was no pattern in the flight tunnel. Indeed, theflight speed over these patterns is in the same range as the flightspeeds over no patterns. The contrast frequency of the remainingdata points for patterns that the moth can certainly see is heldrelatively constant, with a mean of 3.5 Hz. Currently, there is noinformation about the acuity of the visual system of G. molesta.Because G. molesta is an insect that is crepuscular, it is likelyto have a refracting superposition visual system in which visualacuity is sacrificed for photon sensitivity (Warrant & Dacke,2011), which is supported by the few studies of eyes of othertortricids (Yagi & Koyama, 1963; Hämmerle & Kolb, 1987).Theobald et al. (2010) report measurements of the sensitivityto moving visual patterns in motion-sensitive interneuronesin nocturnal hawkmoths. The responses are band-pass filteredfor spatial frequency, with neurones responding maximallyto pattern wavelengths of 28.5∘ in the nocturnal sphingidDeilephila elpenor (Lepidoptera: Sphingidae) and 13.5∘ in thecrepuscular sphingid Manduca sexta. The temporal propertiesare low-pass filtered with peak responses at 3 Hz or less andsharp cut-offs. Thus, the available comparative data supportthe hypothesis that crepuscular Oriental fruit moths G. molestamay have difficulty resolving ground patterns of low spatialwavelength and will treat these as though there was no pattern inthe tunnel. Moreover, motion sensitive visual neurones could betuned to respond maximally at contrast frequencies in the regionof 3 Hz and mediate the regulation of flight speed.

The apparent divergence of mechanisms of flight speed controlrelative to height and visual pattern texture falls along lines ofinsects flying upwind toward sex pheromone (moths and beetles)versus those flying upwind to food sources (flies and bees).In addition, flights by honeybees typically contain a lengthylearning component both for training to a feeding station and,by extension, learning the wind tunnel environment. Test flightswith insects responding to sex pheromone typically use mothswith no prior experience/exposure to the sex pheromone ortunnel visual environment. Thus, there may be fundamentallydifferent algorithms underlying the flight controls during matelocation with its attendant strong selection factors versus theflight controls required to locate a food source likely with lowerselection pressures.

Despite apparent differences in speed control algorithms, itcan still be safely concluded that insects need visual cuesto allow them to steer and proceed upwind toward an odoursource (Kennedy, 1940; Kennedy & Marsh, 1974; Duistermars& Frye, 2008) and that they are clearly adept at the task. Theresults obtained in the present study demonstrate that changes inmale moths’ visual environment can influence their flight speedtoward a pheromone source. It is also clear that further work

will be required to separate the test cues from the rest of thevisual surround to determine how the moths alter their steeringand orthokinetic flight components when flying toward a sexpheromone source.

Acknowledgements

We thank Richard Vetter for rearing the insects and Sarah Hoferfor assistance with data transcription and graphs, as well as T. C.Baker for comments on an earlier draft of this manuscript. Thiswork was funded in part by NSF grant BNS-8102168 and byfunds from the California Pistachio Research Board. Mentionof trade names or commercial products in this article is solelyfor the purpose of providing specific information and does notimply recommendations or endorsement by the United StatesDepartment of Agriculture (USDA). The USDA is an equalopportunity provider and employer.

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Accepted 15 June 2014


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