FILAMENTOUS NATURE OF PHEROMONE PLUMES PROTECTS INTEGRITY OF SIGNAL FROM BACKGROUND CHEMICAL NOISE IN CABBAGE LOOPER MOTH, Trichoplusia ni

Transcription

1 Journal of Chemical Ecology, Vol. 18, No FILAMENTOUS NATURE OF PHEROMONE PLUMES PROTECTS INTEGRITY OF SIGNAL FROM BACKGROUND CHEMICAL NOISE IN CABBAGE LOOPER MOTH, Trichoplusia ni YONG-BIAO LIU* and KENNETH F. HAYNES Department of Entomology University of Kentucky Lexington, Kentucky 4O546-()(Y)l (Received August 26, 1991; accepted October 28, 1991) Abstract-(2)-7-Dodecenol failed to intenupt pheromone-mediated anemotactic responses by male cabbage looper moths, Trichoplusia ni (Hiibner) (Lepidoptera: Noctuidae) in a wind tunnel when released 5 cm crosswind on both sides of the pheromone source or 10 cm upwind of the source to create an overlapping plume downwind. Significant inhibitory effects of (2)- 7-dodecenol were observed when released with the six-component pheromone blend from the same septum or abutting septa. These results indicate that (2)-7- dodecenol needs to be received simultaneously with the pheromone blend to inhibit the anemotactic responses of males to the sex pheromone. We suggest that this feature and the filamentous nature of pheromone plumes render pheromone signals relatively protected from background chemical noise that may originate from pheromone plumes of other insect species. Unless filaments from a pheromone signal and an inhibitor arrive simultaneously, the integrity of the signal is maintained. Key Words-Trichoplusia ni, Lepidoptera, Noctuidae, sex pheromone, inhibitor, (Z)-7-dodecenol, wind tunnel, orientation. INTRODUCTION A male moth orienting upwind i~ a pheromone plume does not encounter a uniform pheromone signal, as some time-averaged models of plume structure might suggest (Wright, 1958; Bossert and Wilson, 1963; Sower et al., 1973; *To whom correspondence should be addressed Plenum Publishing Corporation

2 300 LIU AND HAYNES Nakamura, 1976). Rather, pheromone plumes are discontinuous in nature and are composed of many filaments (Wright, 1958; Murlis and Jones, 1981; Baker et al., 1985). In fact, the filamentous nature of pheromone plumes appears to be important in sustaining upwind flight (Baker et al., 1985).The filamentous nature of pheromone plumes may also render pheromone signals relatively protected from background chemical noise. Although the pheromone plumes of different species may overlap spatially, it is less likely that individual pheromone filaments within the overlapping plumes will mix to a great extent. The integrity of the signal may be maintained within some filaments. Thus the pheromonemediated communication system within a species may be relatively protected if insects can sense and respond to those intact pheromone filaments without being affected by subsequent detection of filaments that contain chemical noise or contaminated pheromone signals. This concept was tested using the cabbage looper moth, Trichoplusia (Hiibner) (Lepidoptera: Noctuidae). The pheromone blend of female T. ni consists of (2)- 7-dodecenyl acetate (27-12: Ac) (Berger, 1966), dodecyl acetate (12: AC), (2)-5-dodecenyl acetate (25-12: Ac), Il-dodecenyl acetate (11-12: Ac), (2)- 7-tetradecenyl acetate (27-14: Ac), and (2)-9-tetradecenyl acetate (29-14:Ac) (Bjostad et ai., 1984). (2)-7-Dodecenol (27-12 :OH) inhibits the anemotactic responses of male 7: ni to pheromone sources (Tumlinson et al., 1972; McLaughlin et al., 1974; Bjostad et al., 1984; Linn et al., 1984). This compound has been found in gland extracts and volatile collections from forcibly extruded pheromone glands of female T. ni (Bjostad et al., 1984; Haynes and Hunt, 1990), but apparently it does not playa role in the nonnal pheromone signal since it is not released by calling females (Haynes and Hunt, 1990). Male T. ni have specialized sensory receptors for 27-12: OH (O'Connell et al., 1983; O'Connell, 1985). It is possible that responding to 27-12:OH may playa role in preventing responses to other species. In this wind-tunnel study, we demonstrate that the inhibitor 27-12: OH must be received simultaneously with the pheromone blend to interrupt the normal anemotactic response of the moth to the pheromone source. This finding supports the concept that pheromone signals are relatively protected from background chemical noise. ni METHODS AND MATERIALS Insects. Cabbage loopers were reared on a semisynthetic diet (Shorey and Hale, 1965). Males were separated from females as pupae. Emerged adults were removed daily and provided with 8% sugar water. Pupae and adults were kept in 3.8-liter paper cartons under 16: 8 light-dark photoperiod and 27 ::t: 1 C. Chemicals. All pheromone components and Z7-12:0H were of >99% purity (Pesticide Research Institute, Wageningen, The Netherlands). The pro-

3 INTEGRITY OF PHEROMONE FROM BACKGROUND NOISE 301 portions of blend components in the six-component blend for rubber septa were as follows: 12:Ac (5%), 25-12:Ac (10%), 27-12:Ac (100%), 11-12:Ac (3% ), 27-14: Ac (3% ), and 29-14: Ac (2% ). This blend ratio resulted in a released blend from a loaded rubber septum similar to that emitted by female T. ni (Hunt et al., 1990). All solutions were prepared in hexane (HPLC grade) and checked by gas chromatography (30-m Carbowax 20 M capillary column in a Hewlett-Packard 5890A gas chromatograph linked to a Hewlett-Packard 5970B mass selective detector) to ensure purity and blend ratios of pheromone components. Dilutions were made for all solutions to yield 0.2 JLg!IJ :Ac or 27-l2:OH. A 50-IJ.1 aliquot of each solution (containing 10 IJ.g 27-12:Ac or 27-12: OH) was added to the wide end of a rubber septum (Thomas scientific, Swedesboro, New Jersey; 5 x 9 mm, preextracted with hexane). The loaded rubber septa were kept in a laboratory exhaust hood for 24 hr before being used in w_ind-tunnel flight experiments. Test Procedures. Male 7: ni (3 to 4 days old) were confined individually in 5-cm-diam. X 10-cm-high hardware cloth cages during the last hour of the photophase and placed in an environmental chamber under the same conditions as above. They were tested in a wind tunnel during the fifth through seventh hours of the scotophase under conditions of about 0.3 lux light intensity, 50 cm!sec wind velocity, and C temperature. A moth was released from the cage on a flat metal mesh surface 20 cm above the tunnel floor and 1.5 m downwind from the pheromone source, which was positioned 20 cm above the tunnel floor. Each moth was allowed about 1 min to respond after it left the cage. Behaviors of moths were scored as: wing fanning, taking flight, locking on (initiation of counter-turning), and source contact. In addition, the distance of closest approach to the pheromone source was recorded. Experiment 1. Effects of Z 7-12: OH Released 5 cm Crosswind from Pheromone Source on Male Responses. The four treatments of this experiment included: (I) the full six-component blend, (2) the full blend and 27-12: OH in the same septum, (3) the full blend and 27-12: OH in different but abutting septa, and (4) the full blend with septa containing 27-12:OH 5 cm from it across the wind line on both sides. A randomized complete-block design was used in both experiments. The rubber septa containing the pheromone blend or 27-12:OH were fixed at the downwind edge of a 15 x 15-cm metal plate positioned 20 cm above the tunnel floor. A total of 75 males was tested for each treatment. Ryan's (1960) test was used to compare proportions of behavioral responses among treatments in both experiments. Experiment 2. Effects ofz 7-12: OH Released 10 cm Upwind of Pheromone Source on Male Responses. The four treatments included: (1) the full blend, (2) the full blend and : OH in the same septum, (3) the full blend and 27-12:OH in different but abutting septa, and (4) the full blend with a 27-12:OH

4 302 LIU AND HA YNES septum placed 10 cm upwind. Rubber septa corresponding to different treatments were supported with 2.5-mm-diam. copper wire 8 cm above the downwind edge of a 15 x 15-cm metal plate placed on a metal mesh surface 12 cm above the tunnel floor. In the last treatment, a rubber septum loaded with 27-12:OH solution was supported with the copper wire 10 cm upwind of the pheromone source. For all treatments, a 15 x 15-cm metal plate was positioned further upwind with the downwind edge angled up 15 using a metal stand. The downwind edge was 1 cm above the septa level and 17 cm from pheromone sources. This created turbulence, which improved the behavioral responses to the sixcomponent blend. Using cigarette smoke, we demonstrated that this arrangement resulted in converging of the two smoke plumes about cm downwind of the source. The two smoke plumes were generated 10 cm apart along the wind line at the positions of the pheromone source and 27-12: OH as in the last treatment. The plumes appeared to completely overlap further downwind. A total of 65 male T. ni was tested for each treatment. RESULTS AND DISCUSSION 27-12: OH reduced locking-on rates to pheromone plumes when it was released from the same septum as the pheromone or when the two septa, one containing : OH and another with the full blend, were placed together. This resulted in significantly lower source contact rates than for the full blend (Figures I and 2). When released 5 cm crosswind on both sides of the pheromone source, 27-12: OH failed to interrupt pheromone-mediated anemotactic flight. There were no significant differences in locking-on rate or source contact rate between the full blend with 27-12: OH 5 cm crosswind and the full blend alone. The locking-on rate and upwind flight for the treatment with 27-12: OH and the full blend from different but abutting septa were significantly those for the treatment with 27-12: higher than OH and the full blend from the same septum, although the difference in source contact rate was not significant. treatments, For all four most moths that locked on to plumes succeeded in contacting the pheromone sources (Figure 1).Similar results were achieved in experiment 2 in which 27-12:OH, when released 10 cm upwind of the pheromone source, failed to inhibit upwind flight of male 7: ni (Figure 2). The ineffectiveness of 27-12: OH in interrupting anemotactic responses to the pheromone source by male T. ni when it was released only 5 cm crosswind on both sides of or 10 cm upwind of the source indicated that 27-12:OH, as an inhibitor, must be detected simultaneously with the pheromone to influence the behavioral responses of the moth. Witzgall and Priesner (1991) drew the same conclusion based on a wind-tunnel study with Coleophora laricella. The. j I ii ~ t Vi i isj

5 INTEGRITY OF PHEROMONE FROM BACKGROUND NOISE ,,-.. ~ '-" QI U! c 0 0- U! QI "- c "Ō BO "> o.r:. QI [IJ 20 0 Take flight Lock-on 120 9o Distance from source (cm) source contact FIG. 1. Effects of Z7-12 : OH released 5 cm crosswind on both sides of the pheromone source on responses of male T. ni. The four treatments were: (1) the full six-component blend (.), (2) the full blend and Z7-12 : OH in the same septum (..), (3) the full blend and Z7-12:OH released from different but abutting septa (.), and (4) the full blend with Z7-12:OH septa 5 cm apart crosswind on both sides <"'). Vertical lines represent standard error of means. For each behavioral category, values with the same letter were not significantly different based on Ryan's multiple comparison test on proportions (P > 0.05) (Ryan, 1960). inhibitor (2)-5-decenyl acetate failed to intemlpt responses of male moths to the pheromone (2)-5-decenol when released 5 cm crosswind on one side of the pheromone source. Several field studies support the conclusion that an inhibitor and a pheromone must be detected simultaneously to intemlpt behavioral responses of moths : OH failed to inhibit the attractant responses by male T. ni when released in the proximity of a pheromone source (McLaughlin et al., 1974) :Ac, 29-14:Ac, or (2,E)-9,12-tetradecadienyl acetate inhibits attractant responses of male T. ni to 27-12: Ac when they were released separately within the same trap at a comparable release rate. When evaporated into the atmosphere surrounding pheromone traps, none of the three chemicals had a significant influence on the capture of male T. ni (Mitchell, 1976). In Rhyacionia buoliana, the geometrical isomer of its pheromone, (E)-9-dodecenyl acetate, inhibited attractant responses of males to the pheromone when they were released together. When dispersed around pheromone- or virgin female-baited traps, the inhibitory isomer did not suppress male orientation to the pheromone source (Daterman et

6 304 L.IU AND HA YNES ~ '-' 4) III C O 0. III 4) L- "0 L- O '> 0.c4) [D Take Lock-on Source flight contact Distance from source (cm) FiG. 2. Effects of Z7-12:OH released 10 cm upwind of the pheromone source on responses of male T. ni. The four treatments were: (I) the full blend (.), (2) the full blendandz7-12: OH in the same septum ("'), (3) the full blend andz7-12:oh released from different but abutting septa (.), and (4) the full blend with Z7-12 : OH septum placed 10 cm upwind (..). Vertical lines represent standard error of means. For each behavioral category, values with the same letter were not significantly different based on Ryan's multiple comparison test on proportions (P > 0.05) (Ryan, 1960). al., 1975). In Coleophora laricella, anemotactic flight of males towards the pheromone (Z)-5-decenol was not fully suppressed even with the same amount of the inhibitor (Z)-5-decenyl acetate placed only 2-5 cm apart. However, male responses to (Z)-5-decenol were totally suppressed when it contained only 3 % (Z)-5-decenyl acetate (Priesner and Witzgall, 1984). In the wind tunnel, plumes ofz7-12:oh and the pheromone overlapped each other as demonstrated with smoke. However, a plume is composed of many entangled filaments (Murlis and Jones, 1981; Baker et al., 1985). Although some mixing may occur between filaments containing pheromone and Z7-12: OH, respectively, especially far downwind from sources, large portions of these individual pheromone filaments may maintain their integrity. Thus, male moths may still detect intact pheromone filaments and detect Z7-12 : OH filaments or Z7-12 : OH-contaminated pheromone filaments sequentially as they fly upwind towards the pheromone source. Studies have revealed that moths are able to respond rapidly to individual pheromone filaments at the sensory level (Kaissling, 1986; Baker and Haynes, 1989). This suggests that a moth is able to detect closely spaced filaments of its pheromone and chemical noise as dis-

7 INTEGRITY OF PHEROMONE FROM BACKGROUND NOISE 305 crete signals contained within individual filaments rather than as a signal which is averaged. There is probably a threshold on the time interval within which two sequentially arriving signals would be perceived as one signal. The threshold is likely dependent in part on the speed of biochemical processes related to receiving and disposing of pheromone molecules in the insect antenna. When pheromone components are released from separated points to form an overlapping plume downwind, it is likely that some filaments may mix to such an extent that insects perceive them as a signal of the pheromone blend rather than discrete signals of individual pheromone components. This may explain the increased responses by male T. ni when 12: Ac was released close to 27-12: Ac resulting in an overlapping plume downwind (Linn and Gaston, 1981). Our own study (unpublished) also revealed that responses to Z7-12:Ac when minor pheromone components were released near 27-12: by male T. ni were enhanced Ac. As long as a complete signal is perceived within the plume, the moth will exhibit appropriate behavioral responses. The fact that an inhibitor such as 27-12: OH intemlpts the response by male T. ni to the pheromone only if detected simultaneously protects the integrity of the pheromone signal. This mechanism may help to ensure species specificity in T. ni as well as other lepidopteran species. In natural habitats, it is common that many lepidopteran species have spatially overlapping distributions and temporally overlapping diurnal calling rllythms. A moth that orients upwind within a pheromone plume of its own species may encounter pheromones produced by other species. On a time-averaged scale, a pheromone plume may be contaminated by these chemical noise signals. However, a moth is likely to detect an intact and uncontaminated pheromone.blend contained in individual filaments. Because individual filaments within a plume are less likely to overlap and sequential reception of chemical noise such as Z7-l2 : OH in different filaments does not intemlpt normal anemotactic responses of a moth to its pheromone, pheromone signals should be relatively protected from background chemical noise. Acknowledgments-We thank S.E. Boettcher, J.T. Collins, and J.-Z. Zhao for rearing insects. This material is based on work supported by the Cooperative State Research Service, U.S. Department of Agriculture, under agreement Nos and We acknowledge the support of the University of Kentucky Major Research Instrumentation Bond Program in the purchase ofequipments used in this study (ID #s. 7E-8164-II, L-250-8AO80, L-251-8AO80, and L-252-8A080). This investigation (paper No ) was conducted in connection with a project of the Kentucky Agricultural Experiment Station. REFERENCES BAKER, T.C., and HAYNES, K.F Field and laboratory electroantennographic measurements of pheromone plume structure correlated with oriental fruit moth behaviou r. Physiol. Entomol. 14:1-12.

8 306 LIU AND HA YNE~ BAKER, T.C., WILUS, M.A., HAYNES, K.F., and PHELAN, P.L A pulsed cloud of se~ pheromone elicits upwind flight in male moths. Physiol. Entomol. 10: BERGER, R.S Isolation, identification and synthesis of the sex attractant of the cabbage looper. J. Econ. Entomol. 59: BJOSTAD, L.B., LINN, C.E., Du, J.W., and ROELOFS, W.L Identification of new sex pheromone components in Trichoplusia ni, predicted from biosynthetic precursors. J. Chem. Ecol. 10: BOSSERT, W.H., and WILSON, E.O The analysis of olfactory communication among animals. J. 1heor. BioI. 5: DATERMAN, G.E., DAVES, G.D., JR., and SMITH, R.G Comparison of sex pheromone versus an inhibitor for disruption of pheromone communication in Rhyacionia buoliana. Environ. Entomol.4: HAYNES, K.F., and HUNT, R.E IntelpOpulational variation in emitted pheromone blend of cabbage looper moth, Trichoplusia ni. J. Chem. Ecol. 16: HUNT, R.E., ZHAO, B.-G., and HAYNES, K.F Genetic aspects of interpopulational differences in pheromone blend of cabbage looper moth, Trichoplusia ni. J. Chem. Ecol. 16: KAISSLING, K.-E Temporal characteristics of pheromone receptor cell responses in relation to orientation behaviour of moths, pp , in T.L. Payne, M.C. Birch and C.E.J. Kennedy (eds.). Mechanisms in Insect Olfaction. Clarendon Press, Oxford. LINN, C.E., JR., and GASTON, L.K Behavioral function of the components and the blend of the sex pheromone of the cabbage looper, Trichoplusia ni. Environ. Entomol. 10: LINN, C.E., JR., BJOSTAD, L.B., Du, J.W., and ROELOFS, W.L Redundancy in a chemical signal: Behavioral responses of male Trichoplusia ni to a 6-component sex pheromone blend. J. Chem. Ecol. 10: McLAUGHLIN, J.R., MITCHELL, E.R., CHAMBERS, D.L., and TUMLINSON, J.H Perception of Z- 7-dodecen-I-o1 and modification of the sex pheromone response of male loopers. Environ. Entomol. 3: MITCHELL, E.R Inhibition of pheromone perception by male cabbage loopers and beet armyworms: Proximity vs. atmospheric permeation. Environ. Entomol. 5: MURLIS, J., and JONES, C.D Fine-scale structure of odour plumes in relation to insect orientation to distant pheromone and other attractant sources. Physiol. Entomol. 6: NAKAMURA, K The effect of wind velocity on the diffusion of Spodoptera litura (F.) sex pheromone. Appl. Entomol. Zool. 11: O'CONNELL, R.J Responses to pheromone blends in insect olfactory receptor neurons. J. Comp. Physiol. AI56: O'CONNELL, R.J., GRANT, A.J., MAYER, M.S., and MANKIN, R.W Morphological correlates 'of differences in pheromone sensitivity in insect sensi1la. Science 220: PRIESNER, E., and WITZGALL, P Modification of pheromonal behaviour in wild Coleophora laricella male moths by (Z)-5-decenyl acetate, an attraction-inhibitor. Z. Angew. Entomol. 98: RYAN, T.A Significance tests for multiple comparison of proportions, variances, and other statistics. Psychol. Bull. 57: SHOREY, H.H., and HALE, R.L Mass-rearing of the larvae of nine noctuid species on a simple artificial medium. J. Econ. Entomol. 58: SOWER, L.L., LoNG, J.S., VICK, K.W. and COFFELT, J.A Sex pheromone of the angoumois grain moth: Effects of habituation on the pheromone response of the male. Ann. Entomol. Am. 66: Soc.

9 INTEGRITY OF PHEROMONE FROM BACKGROUND NOISE 307 TUMLINSON, J.M., MITCHELL, E.R., BROWNER, S.M., MAYER, M.S., GREEN, N., MINES, R., and LINDQUIST, D.A cis-i-dodecen-1-ol, a potent inhibitor of the cabbage looper sex pheromone. Environ. Entomol. 1 : WITZGALL, P., and PRIESNER, E Wind-tunnel study on attraction inhibitor in male Coleophora laricella Mbn. (Lepidoptera: Coleophoridae). J. Chem. Ecol. 17: WRIGHT, R.M The olfactory guidance of flying insects. Can. Entomol. 80:81-89.