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Analysis of mouthpart movements during feeding of Frankliniella occidentalis (pergande) and F. schultzei trybom (Thysanoptera : Thripidae)
Int. J. Insect Morphol. & Embryol., Vol. 18, No. 2/3, pp. 161-171, 1989 Printed in Great Britain
0020-7322/89 $3.00 + .00 Pergamon Press plc
ANALYSIS OF M O U T H P A R T MOVEMENTS D U R I N G F E E D I N G OF FRANKLINIELLA OCCIDENTALIS ( P E R G A N D E ) A N D F. SCHULTZEI TRYBOM ( T H Y S A N O P T E R A • THRIPIDAE)
WAYNE B. HUNTER and DIANE E. ULLMAN University of Hawaii at Manoa, Department of Entomology, 3050 Maile Way, Gilmore Hall, rm 310, Honolulu, Hawaii 96822
(Accepted 7 March 1989)
AbstraetmScanning electron microscopy was used to elucidate the morphology and sequential movement of thrips feeding structures in the 2 species, Frankliniella occidentalis and F. schultzei, (Thysanoptera : Thripidae). The mouthcone consists of paired paraglossae, and fringed labral pad, a single, apically fused mandible and a pair of interlocking maxillae. The maxillae are open apically and form a feeding tube. Ten pairs of sensory pegs of 3 distinct morphological types (sensilla basiconica with a cuticular collar, sensilla basiconica without a cuticular collar, and sensilla trichoidea) were found on the paraglossae. The possible function of these sensory structures in host finding and choice are discussed. No structure for rasping the leaf's surface was found on the mouthcone or the ventral surface of the insect. Live specimens were observed feeding through Parafilm on artificial media (sucrose solution) and lettuce leaf tissue. These observations support earlier findings that thrips feed by piercing leaf cells with the mandible and ingesting cell contents through the feeding tube formed by the maxillary stylets. Based on these findings, we suggest that thrips be classified as piercing-sucking rather than rasping-sucking insects.
Index descriptors (in addition to those in title): Feeding behavior, morphology, scanning electron microscopy, sensory structures. INTRODUCTION THRIPS a r e u n i q u e a m o n g t h e I n s e c t a in t h a t t h e m o r p h o l o g y of t h e p r o b o s c i s o r m o u t h c o n e a n d t h e f e e d i n g s t r u c t u r e s c o n t a i n e d t h e r e i n a r e a s y m m e t r i c a l ( L e w i s , 1973). T h e m o u t h c o n e is c o m p o s e d o f p a i r e d p a r a g l o s s a e , e n c l o s e d l a b r a l p a d , a s i n g l e , left m a n d i b u l a r s t y l e t , a n d a p a i r o f m a x i l l a r y stylets. P r e v i o u s w o r k s s h o w t h a t a p p e a r a n c e a n d structure of the m o u t h p a r t s vary a m o n g thrips families and a m o n g species ( B o r d e n , 1915; P e t e r s o n , 1915; R e y n e , 1927; A n a n t h a k r i s h n a n , 1951; J o n e s , 1954; R i s l e r , 1957; D a v i e s , 1958; M i c k o l e i t , 1963). T h y s a n o p t e r a n f e e d i n g b e h a v i o r is n o t c o m p l e t e l y u n d e r s t o o d , b u t a v a i l a b l e l i t e r a t u r e s u g g e s t s t h a t it m a y also v a r y w i d e l y . H o r t o n ' s
Abbreviations used in figures: a = mouthcone; b = left prothoracic leg; c = compound eye; d = antennae; e = maxillary palps; f = labial palps; g = sensory pegs; h = axonic sheaths; i = axons; j = paraglossae; k = labral pad; I = small palpi; m = long slender palpi; n = single mandibular stylet; o = paired maxillary stylets. 161
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(1918) description of Scirtothrips citri suggests that this species p u n c t u r e s a n d drains c y t o p l a s m f r o m i n d i v i d u a l p l a n t cells. In contrast, the feeding of Selenothrips rubrocinctus (Russell, 1912), Taeniothrips inconsequens ( C a m e r o n a n d T r e h e r n e , 1918), a n d Caliothrips indicus ( B e d f o r d , 1921) have b e e n described as rasping-sucking. R e c e n t l y , C h i s h o l m a n d Lewis (1984) characterized the feeding b e h a v i o r of Limothrips cerealium, the cereal thrips. T h e i r work d e m o n s t r a t e s that this thrips species pierces i n d i v i d u a l p l a n t cells a n d e m p t i e s their c o n t e n t s t h r o u g h a f e e d i n g t u b e f o r m e d by the the maxillary stylets. T h e y also describe, for the first time, sensory structures o n the labral pad that, b a s e d o n e x t e r n a l m o r p h o l o g y , a p p e a r to be c h e m o s e n s o r y a n d m a y be i n v o l v e d in host p r e f e r e n c e a n d feeding site selection. W i e s e n b o r n a n d M o r s e (1988) d e s c r i b e d similar structures o n the paraglossae of Scirtothrips cirri. T h e p r i m a r y goal of this p a p e r is to describe the m o r p h o l o g y of the feeding structures a n d the m o d e of f e e d i n g exhibited by 2 species of thrips, Frankliniella occidentalis a n d F. schultzei. B o t h species are e c o n o m i c a l l y significant pests in agricultural a n d o r n a m e n t a l crops w o r l d w i d e , causing direct d a m a g e owing to their feeding a n d indirect d a m a g e as vectors of t o m a t o s p o t t e d wilt virus ( T S W V ) ( S a k i m u r a , 1962; Paliwal, 1976; A m i n et al., 1981; G r e e n o u g h , et al., 1985; A l l e n a n d B r o a d b e n t , 1986; C h o et al., 1987). T h e specific f e e d i n g b e h a v i o r s g o v e r n i n g t r a n s m i s s i o n of T S W V by thrips are not yet clearly u n d e r s t o o d . O u r e x a m i n a t i o n of thrips feeding structures a n d feeding b e h a v i o r is the first a n d necessary step towards increasing o u r k n o w l e d g e of the m e c h a n i s m s u n d e r l y i n g thrips t r a n s m i s s i o n of T S W V , as well as the thrips/plant i n t e r a c t i o n s g o v e r n i n g p l a n t damage.
MATERIALS AND METHODS The thrips used in this study, F. occidentalisand F. schultzei, were reared on pesticide-free green bean pods, Phaseolus vulgaris L., Greencrop cultivar, in the laboratory of Dr R. F. L. Mau at the University of HawaiiManoa. Thrips cultures were maintained at 24.5°C, in white plastic containers, photo period 16 : 8 L : D. Two methods were used to prepare insects for viewing with SEM. First live thrips were placed on bean pods for 2 hr. Then, the bean pods and thrips were quickly immersed in liquid nitrogen, freezing the thrips during feeding. While still frozen the thrips were lyophilized (Virtis Co., Inc., Gardiner, New York 12515, U.S.A.) and then mounted on SEM stubs, sputter coated (Hummer II, Technics, 5510 Vine Street, Alexandria, Virginia 22310, U.S.A.) and examined at 20 kV with a Cambridge Stereoscan 150, SEM (Cambridge Instruments, U.K.). The second method involved fixing live adult thrips removed from culture, by placing in Bouin-Dubosco solution for 30 min (Humason, 1967) and dehydrating in an ethanol series consisting of two 30-min changes in 95% EtOFI and three 30-min changes in 100% EtOH. Specimens were then critical point dried (Autosamdri 180, Tousimis Research Corp., Rockville, Maryland, 20852, U.S.A.) and then mounted on SEM stubs with either double sticky tape (Scotch brand) or conductive silver paint (Sigmund-Cohn Hed Wire Inc. Mt Vernon, New York 10553, U.S.A.). Specimens were then plasma etched ("Plasma Prep", Structure Probe Inc., Tegal Co. California, U.S.A.) by Dr Kuzirian, Woods Hole, Massachusetts 02543, U.S.A. Etching exposure times ranged from 1.5-3 min at 75 W. Vacuum pressure was 800 mtorr and all samples were placed directly in the main vacuum vent. After etching, specimens were sputter coated and examined with SEM. Eighty individuals of each species were viewed and photographed. Because there were few differences between species, only photographs of F. occidentalis are shown. Thrips feeding was observed on a sucrose solution and on lettuce, Lactuca sativa L. Observation chambers were constructed of 2 depression slides taped together with the concave surfaces facing one another to create a chamber. When a sucrose Solution was used one slide was first covered with stretched Parafilm and a syringe was used to inject a 5% sucrose solution (w : v) into the well of the slide. Two adult thrips were then placed onto the Parafilm over the well and the second slide placed over the thrips. The 2 slides were then taped together. The thrips could be observed either from the top or bottom during feeding. Observations were made with a Leitz light microscope with 12.5x and 25× oculars with 10× to 40x objectives. When observations were made on lettuce, 2 thrips were placed into the well of one slide, then quickly covered with a lettuce leaf which was subsequently covered with a glass coverslip taped in place. Transmitted light made tissues translucent such that feeding could be observed.
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RESULTS
Morphology of the feeding structures There were no major morphological differences between species, therefore, figures show features of F. occidentalis. The feeding structures of both F. occidentalis and F. schultzei are housed within a ventrally located mouthcone (Fig. la). Paired labial palps and maxillary palps (Fig. lb) are located on the mouthcone and possess structures which appear to be sensory. Each of the paired paraglossae possesses 10 sensory pegs (Fig. 2a) of 3 distinct, morphologically different types: sensilla basiconica lacking a cuticular collar (1, 3 Fig. 2a), s. basiconica with cuticular collar (2, 4, 5, 7, 8 Fig. 2a) and s. trichoidea (6, 9, 10 Fig. 2a) (terminology from Chisholm and Lewis, 1984). The axons and axonic sheaths of the neurons innervating the sensory pegs on the paraglossae are shown in a lateral view of the mouthcone (Fig. 2b). When the paraglossae are closed, as they are prior to initiation of thrips probing, these sensory pegs cover the tip of the mouthcone (Fig. 3a). As probing is initiated the paraglossae move apart and extrusion of the labral pad begins (Fig. 3b). The labral pad is omega shaped with the ends meeting ventrally, The pad is fringed with many small palpi, as well as having thinner, longer palpi-like extensions toward the center (Fig. 4a). The stylets, which emerge from the opening in the center of the labral pad, consist of the single left mandible, which is closed apically and fused to the exoskeleton (Fig. 4b) and the paired maxillary stylets. The latter are interlocked in a tongue and groove fashion and open at the tip to form a feeding tube (Figs 4b; 5a, 5b). The maxillary stylets can be extended at least 30 p~m beyond the tip of the mouthcone (Fig. 5b) and are under muscular control.
Feeding behavior Figure 6 shows the typical feeding posture observed for both F. occidentalis and F. schultzei with the head pressed downward and the legs and antennae spread. Thrips placed on feeding sachets containing sucrose solution moved about randomly, stopping frequently to scrape the membrane with their forelegs. The tip of the mouthcone was not in contact with the membrane during these scraping movements. Typically, thrips scraped first with one proleg, reaching beyond the head and other proleg. Then the leg was drawn across the surface diagonally back to its original position. This behavior was sometimes repeated several times before scraping with the second leg began. The insect then performed similar movements with the other proleg. Following scraping with the forelegs, the thrips scraped the antenna with each leg. In addition, the thrips dragged the antennae in an "S"-like pattern, a movement from side to side, across the membrane. After these tarsal and antennal movements, the mouthcone was placed on the substrate. Then the paraglossae separated and the labral pad was rapidly extruded. As the insect pushed down with its head, compressing the mouthcone against the substrate surface, the mandibular stylet pierced the membrane. The maxillary stylets then followed the mandible and movement of fluids into the feeding tube was observed. When thrips were given access to lettuce leaves, the mouthcone was obscured by the plant cell contents. However, stylet insertion and the rapid ingestion of cell contents was observed on more translucent cells.
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FIG. la. Lateral view of thrips head showing mouthcone (a), left prothoracic leg (b), compound eye (c) and antennae (d). FIG. lb. Frontal view of thrips head. Square includes: tip of mouthcone (a), with labral pad (k) partially extruded, paired maxillary palps (e) and paired labial palps (f).
Analysis of Mouthpart Movements
FIG. 2a. Tip of mouthcone showing 10 sensory pegs on left paragiossa. Three morphologically distinct types are shown. Basiconica lacking a distinct cuticular collar (1, 3), basiconica with a distinct cuticular collar (2, 4, 5, 7, 8), and sensilla trichoidea (6, 9, 10). FIG. 2b. A lateral view of mouthcone with a portion of surface removed by plasma etching reveals neuron cell bodies and axons of sensilla on paraglossae. External tip of some sensory pegs remain (g) and outer portions of axonic sheaths (h) and axons (i) can be seen.
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FI6. 3a. Tip of mouthcone showing paraglossae (j) closed prior to feeding. Arrows denote point at which paired paraglossae meet when fully closed. FI6.3b. Tip of mouthcone showing separation of paraglossae and extrusion of labral pad (k) during initiation of feeding process. Arrows denote movement of paraglossae separation.
Analysis of Mouthpart Movements
Fro. 4a. Tip of mouthcone showing further extrusion of iabral pad (k) as paraglossae (j) open. Dorsal edge of labral pad is fringed with many small paipi (1), while longer, more slender palpi (m) are present ventrally. FIG. 4b. Single mandibular styler (n) is closed apically and is used to puncture leaf's surface creating an entry point for insertion of paired maxillary stylets (o). Arrow denotes subapical opening.
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Ftc. 5a. Maxillary stylets (o) are shown emerging from extruded labral pad (k). They are interlocked at tip and can be moved independently in a tongue and groove fashion. F16.5b. Longest observed extension of maxillary stylets (o) was approximately 30 ~m. Small arrow denotes the subapical opening.
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FIG. 6. Typical feeding posture of F. occidentalis with head thrust downward pressing mouthcone onto substrate.
DISCUSSION Our study shows that, like L. cerealium, Haplothrips verbasci, and S. citri (Chisholm and Lewis, 1984; Heming, 1978; Wiesenborn and Morse, 1988), F. occidentalis and F. schultzei do not have any structure on the mouthcone or ventral surface of the body capable of rasping plant cells. Based on external morphology the stylets are piercingsucking structures, the paraglossae and labral pad being smooth, lacking teeth or ridges capable of rasping. Our observations of F. occidentalis and F. schultzei, feeding from artificial media and from lettuce leaves, strongly support Chisholm and Lewis (1984) in their conclusion that thrips feed by piercing plant cells with their mandible, following the mandible with their maxillae and ingesting cell contents through the feeding tube formed by the pair of interlocked maxillae. They are also capable of ingesting fluids from the plant surface by extending the labial pad and sucking up surface fluids through the food canal in the maxillary stylets. In this manner, they may also suck up plant fluids exuded following scraping of the leaf surface with their tarsal claws. Our observations suggest that such behavior is rare, although it may potentially be of importance to host selection and warrants further study. The information from this and other recent studies (Mound, 1971; Chisholm and Lewis, 1984; Heming, 1985) strongly suggests that the feeding strategy of the Thysanoptera is not rasping-sucking, as suggested previously (Metcalf et al., 1962), but is more appropriately classified as piercing-sucking. This finding is
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significant to our understanding of how specific events in the feeding process govern thrips acquisition and inoculation of TSWV, as well as to understanding how thrips directly damage plants as a result of their feeding. Our investigation shows that the morphology of the feeding structures of F. occidentalis and F. schultzei are similar to other Thysanoptera studied (Chisholm and Lewis, 1984; Heming, 1985; Wiesenborn and Morse, 1988) in that they have a single mandible that is closed apically and fused to the exoskeleton, a pair of interlocking maxillary stylets that are open apically forming a feeding tube and an extrudable labral pad that is fringed with m a n y small palpi. Chisholm and Lewis (1984) suggest that the labral pad palpi m a y serve a sensory function. In our investigation, we did not find any evidence of innervation of these palpi and therefore suggest that the function of the labral pad and palpi is to provide structural support for the stylets and to stabilize the mouthcone on the plant surface. In addition, the labral pad may serve to prevent loss of plant fluids during feeding. Like L. cerealium and S. citri (Chisholm and Lewis, 1984; Wiesenborn and Morse, 1988), both thrips species studied have structures on the paraglossae that, based on external m o r p h o l o g y and location, appear to be sensory. O u r data, showing that the sensory pegs on the paraglossae are innervated, support this hypothesis (Fig. 2b). Furthermore, our behavioural observations indicate that these sensory structures may play an important role in the chain of sensory events leading thrips to ingest from a particular host plant. F. occidentalis and F. schultzei differ from other thrips studied in that they have 10 distinct pairs of sensory pegs on the paraglossae rather than 9 (Chisholm and Lewis, 1984). However, the same 3 morphologically distinct types of sensory pegs were found. Preliminary studies also suggest that thrips have sensilla located in their precibarial canal, similar to those reported in aphids, (Wensler and Filshie, 1969) leafhoppers, (Backus and McLean, 1982, 1983, 1985; McLean and Kinsey, 1984) and psyllids, (Ullman and McLean, 1986). We plan to further examine precibarial sensilla in thrips and work toward elucidating the function of external paraglossa sensilla and internal precibarial sensilla in thrips host finding and choice. Acknowledgements--We are indebted to Dr A. Alvarez, Dr M. Dunlap, Dr Kurzurian and Robin Lipp for the
facilities and guidance they made available. This research was supported in part by the Governors Agricultural Coordinating Committee contract No. P-280-982, Biomedical Research Support Grant, University of HawaiiManna and a Research Centers in Minority Institutions Grant RR-03061, Division of Research Resources, National Institutes of Health. This is Journal Series No. 3305 of the Hawaii Institute of Tropical Agriculture and Human Resources. REFERENCES ALLEN, W. R. and A. B. BROADBENT.1986. Transmission of the tomato spotted wilt virus in Ontario greenhouses by the western flower thrips, Frankliniella oecidentalis Pergande (Thysanoptera : Thripidae). Can. J. Plant Patkol. 8: 33-8. AMIN, P. W., D. V. R. REDDYand A. M. GHANEKAg.1981. Transmission of tomato spotted wilt virus, the causal agent of bud necrosis of peanut, by Scirtothrips dorsalis and Frankliniella schultzei. Plant Dis. 65: 663-65. ANANTHAKVaSm~AN,T. N. 1951. Variations in the mouth parts of some Thripidae (Thysanoptera-Terebrantia). Indian J. Entomol. 12: 237-39. BACKUS,E. A. and D. L. MCLEAN. 1982. The sensory systems and feeding behavior of leafhoppers. I. The aster leafhopper, Macrostelesfascifrons Stal (Homoptera : Cicadellidae). J. Morphol. 172: 359-78. BACKUS,E. A. and D. L. MCLEAN. 1983. The sensory systems and feeding behavior of leafhoppers. II. A comparison of the sensillar morphologies of several species (Homoptera : Cicadellidae). J. Morphol. 176: 3-14.
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BACKUS, E. A. and D. L. McLEAN. 1985. Behavioral evidence that the precibarial sensilla of leafhoppers are chemosensory and function in host discrimination. Entomol. Exp. Appl. 37: 219-28. BEDFORD, H. W. 1921. The cotton thrips (Heliothrips indicus Bagn.) in the Sudan. Bull. Wellcome Trop. Res. Labs. Entomol. Sect. 18. BORDEN, A. D. 1915. The mouthparts of the Thysanoptera and the relation of thrips to the non-setting of certain fruits and seeds. J. Econ. Entomol. 8: 354-60. CAMERON, A. E. and R. C. TREHERNE. 1918. The pear thrips (Taeniothrips inconsequens Uzel) and its control in British Columbia. Bull. Dep. Agric. Can. Entomol. 15. CHISHOLM, I. F. and T. LEwis. 1984. A new look at thrips (Thysanoptera) mouthparts, their action and effects of feeding on plant tissue. Bull. Entomol. Res. 74: 663-75. CHO,J. J., W. C. MITCHELL,R. F. L. MAu and K. SAKIMURA.1987. Epidemiology of tomato spotted wilt virus disease on crisphead lettuce in Hawaii. Plant Dis. 71: 505-08. DAVIES, R. G. 1958. Observations on the morphology of the head and mouthparts in the Thysanoptera. Proc. R. Entomol. Soc. Lond. 33(A): 97-106. GREENOUGH, D. R., L. L. BLACK, R. N. STORY, L. D. NEWSOM and W. P. BOND. 1985. Occurrence of FranklinieUa occidentalis in Louisiana: a possible cause for the increased incidence of tomato spotted wilt virus. (Abstr.) Phytopathology 75: 1362. HEMING, B. S. 1985. Thrips (Thysanoptera) in Alberta. Agric. For. Bull. 8: 19-24. HEMING, B. S. 1978. Structure and function of mouthparts in larvae of Haplothrips verbasci (Osborn) (Thysanoptera, Tublifera, Phlaeothripidae). J. Morphol. 156: 1-37. HORTON, J. R. 1918. The citrus thrips. Bull. U.S. Dep. Agric.616. HUMASON, G. L. 1967. Fixation, pp. 14. In R. EMERSON, D. KENNEDY and R. B. PARK. (eds) Animal Tissue Techniques. Freeman, San Francisco. JONES, T. 1954. The external morphology of Chirothrips hamatus (Trybom) (Thysanoptera). Trans. R. Entomol. Soc. Lond. 105: 163-87. LEwis, T. 1973. Thrips, Their Biology, Ecology and Economic Importance. Academic Press, London. MCLEAN, O. L. and M. G. KINSEY. 1984. The precibarial valve and its role in the feeding behavior of the pea aphid, Acyrthosiphon pisum. Bull. Entomol Soc. Amer. 30: 26-31. METCALF, C. L., W. P. FLINT, and R. L. METCALE. 1962. Destructive and Useful Insects. McGraw-Hill, New York. MICKOLEIT, E. 1963. Untersuchungen zur Kopfmorphologie der Thysanopteren. Zool. Jahrb. Anat. 81: 101-50. MOUND, L. A. 1971. The feeding apparatus of thrips. Bull. Entomol. Res. 60: 547-48. PALIWAL, Y. C. 1976. Some characteristics of the thrips vector relationship of tomato spotted wilt virus in Canada. Can. J. Bot. 54: 402-05. PETERSON, A. 1915. Morphological studies of the head and mouthparts of the Thysanoptera. Ann. Entomol. Soc. Amer. 8: 20--66. REYNE, A. 1927. Untersuchungen uber die Mundteile der Thysanopteren. Zool. Jahrb. Anat. 49: 391-500. RISEER, H. 1957. Der Kopf von Thrips physapus L. (Thysanoptera, Terebrantia). Zool. Jahrb. Anat. 76: 251302. RUSSELL, H. M. 1912b. The red-banded thrips. Bull. Bur. Entomol. U.S. Dep. Agric. 99: 17-29. SAKIMURA,K. 1962. The present status of thrips-borne viruses, pp. 1-192. In K. MARAMOROSCH(ed.) Biological Transmission of Disease Agents. Academic Press, New York. ULLMAN,D. E. and D. L. MCLEAN. 1986. Anterior alimentary canal of the pear psylla, Psylla pyricola Foerster (Homoptera, Psyllidae. J. Morphol. 189: 89-98. WENSLER,R. J. and B. K. FILSHIE. 1969. Gustatory sense organs in the food canal of aphids. J. Morphol. 129: 473-92. WIESENBORN,W. D. and J. G. MORSE. 1985. Feeding rate of Thysanoptera estimated using C14 inulin. J. Econ. Entomol. 78: 151-18.
0020-7322/89 $3.00 + .00 Pergamon Press plc
ANALYSIS OF M O U T H P A R T MOVEMENTS D U R I N G F E E D I N G OF FRANKLINIELLA OCCIDENTALIS ( P E R G A N D E ) A N D F. SCHULTZEI TRYBOM ( T H Y S A N O P T E R A • THRIPIDAE)
WAYNE B. HUNTER and DIANE E. ULLMAN University of Hawaii at Manoa, Department of Entomology, 3050 Maile Way, Gilmore Hall, rm 310, Honolulu, Hawaii 96822
(Accepted 7 March 1989)
AbstraetmScanning electron microscopy was used to elucidate the morphology and sequential movement of thrips feeding structures in the 2 species, Frankliniella occidentalis and F. schultzei, (Thysanoptera : Thripidae). The mouthcone consists of paired paraglossae, and fringed labral pad, a single, apically fused mandible and a pair of interlocking maxillae. The maxillae are open apically and form a feeding tube. Ten pairs of sensory pegs of 3 distinct morphological types (sensilla basiconica with a cuticular collar, sensilla basiconica without a cuticular collar, and sensilla trichoidea) were found on the paraglossae. The possible function of these sensory structures in host finding and choice are discussed. No structure for rasping the leaf's surface was found on the mouthcone or the ventral surface of the insect. Live specimens were observed feeding through Parafilm on artificial media (sucrose solution) and lettuce leaf tissue. These observations support earlier findings that thrips feed by piercing leaf cells with the mandible and ingesting cell contents through the feeding tube formed by the maxillary stylets. Based on these findings, we suggest that thrips be classified as piercing-sucking rather than rasping-sucking insects.
Index descriptors (in addition to those in title): Feeding behavior, morphology, scanning electron microscopy, sensory structures. INTRODUCTION THRIPS a r e u n i q u e a m o n g t h e I n s e c t a in t h a t t h e m o r p h o l o g y of t h e p r o b o s c i s o r m o u t h c o n e a n d t h e f e e d i n g s t r u c t u r e s c o n t a i n e d t h e r e i n a r e a s y m m e t r i c a l ( L e w i s , 1973). T h e m o u t h c o n e is c o m p o s e d o f p a i r e d p a r a g l o s s a e , e n c l o s e d l a b r a l p a d , a s i n g l e , left m a n d i b u l a r s t y l e t , a n d a p a i r o f m a x i l l a r y stylets. P r e v i o u s w o r k s s h o w t h a t a p p e a r a n c e a n d structure of the m o u t h p a r t s vary a m o n g thrips families and a m o n g species ( B o r d e n , 1915; P e t e r s o n , 1915; R e y n e , 1927; A n a n t h a k r i s h n a n , 1951; J o n e s , 1954; R i s l e r , 1957; D a v i e s , 1958; M i c k o l e i t , 1963). T h y s a n o p t e r a n f e e d i n g b e h a v i o r is n o t c o m p l e t e l y u n d e r s t o o d , b u t a v a i l a b l e l i t e r a t u r e s u g g e s t s t h a t it m a y also v a r y w i d e l y . H o r t o n ' s
Abbreviations used in figures: a = mouthcone; b = left prothoracic leg; c = compound eye; d = antennae; e = maxillary palps; f = labial palps; g = sensory pegs; h = axonic sheaths; i = axons; j = paraglossae; k = labral pad; I = small palpi; m = long slender palpi; n = single mandibular stylet; o = paired maxillary stylets. 161
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(1918) description of Scirtothrips citri suggests that this species p u n c t u r e s a n d drains c y t o p l a s m f r o m i n d i v i d u a l p l a n t cells. In contrast, the feeding of Selenothrips rubrocinctus (Russell, 1912), Taeniothrips inconsequens ( C a m e r o n a n d T r e h e r n e , 1918), a n d Caliothrips indicus ( B e d f o r d , 1921) have b e e n described as rasping-sucking. R e c e n t l y , C h i s h o l m a n d Lewis (1984) characterized the feeding b e h a v i o r of Limothrips cerealium, the cereal thrips. T h e i r work d e m o n s t r a t e s that this thrips species pierces i n d i v i d u a l p l a n t cells a n d e m p t i e s their c o n t e n t s t h r o u g h a f e e d i n g t u b e f o r m e d by the the maxillary stylets. T h e y also describe, for the first time, sensory structures o n the labral pad that, b a s e d o n e x t e r n a l m o r p h o l o g y , a p p e a r to be c h e m o s e n s o r y a n d m a y be i n v o l v e d in host p r e f e r e n c e a n d feeding site selection. W i e s e n b o r n a n d M o r s e (1988) d e s c r i b e d similar structures o n the paraglossae of Scirtothrips cirri. T h e p r i m a r y goal of this p a p e r is to describe the m o r p h o l o g y of the feeding structures a n d the m o d e of f e e d i n g exhibited by 2 species of thrips, Frankliniella occidentalis a n d F. schultzei. B o t h species are e c o n o m i c a l l y significant pests in agricultural a n d o r n a m e n t a l crops w o r l d w i d e , causing direct d a m a g e owing to their feeding a n d indirect d a m a g e as vectors of t o m a t o s p o t t e d wilt virus ( T S W V ) ( S a k i m u r a , 1962; Paliwal, 1976; A m i n et al., 1981; G r e e n o u g h , et al., 1985; A l l e n a n d B r o a d b e n t , 1986; C h o et al., 1987). T h e specific f e e d i n g b e h a v i o r s g o v e r n i n g t r a n s m i s s i o n of T S W V by thrips are not yet clearly u n d e r s t o o d . O u r e x a m i n a t i o n of thrips feeding structures a n d feeding b e h a v i o r is the first a n d necessary step towards increasing o u r k n o w l e d g e of the m e c h a n i s m s u n d e r l y i n g thrips t r a n s m i s s i o n of T S W V , as well as the thrips/plant i n t e r a c t i o n s g o v e r n i n g p l a n t damage.
MATERIALS AND METHODS The thrips used in this study, F. occidentalisand F. schultzei, were reared on pesticide-free green bean pods, Phaseolus vulgaris L., Greencrop cultivar, in the laboratory of Dr R. F. L. Mau at the University of HawaiiManoa. Thrips cultures were maintained at 24.5°C, in white plastic containers, photo period 16 : 8 L : D. Two methods were used to prepare insects for viewing with SEM. First live thrips were placed on bean pods for 2 hr. Then, the bean pods and thrips were quickly immersed in liquid nitrogen, freezing the thrips during feeding. While still frozen the thrips were lyophilized (Virtis Co., Inc., Gardiner, New York 12515, U.S.A.) and then mounted on SEM stubs, sputter coated (Hummer II, Technics, 5510 Vine Street, Alexandria, Virginia 22310, U.S.A.) and examined at 20 kV with a Cambridge Stereoscan 150, SEM (Cambridge Instruments, U.K.). The second method involved fixing live adult thrips removed from culture, by placing in Bouin-Dubosco solution for 30 min (Humason, 1967) and dehydrating in an ethanol series consisting of two 30-min changes in 95% EtOFI and three 30-min changes in 100% EtOH. Specimens were then critical point dried (Autosamdri 180, Tousimis Research Corp., Rockville, Maryland, 20852, U.S.A.) and then mounted on SEM stubs with either double sticky tape (Scotch brand) or conductive silver paint (Sigmund-Cohn Hed Wire Inc. Mt Vernon, New York 10553, U.S.A.). Specimens were then plasma etched ("Plasma Prep", Structure Probe Inc., Tegal Co. California, U.S.A.) by Dr Kuzirian, Woods Hole, Massachusetts 02543, U.S.A. Etching exposure times ranged from 1.5-3 min at 75 W. Vacuum pressure was 800 mtorr and all samples were placed directly in the main vacuum vent. After etching, specimens were sputter coated and examined with SEM. Eighty individuals of each species were viewed and photographed. Because there were few differences between species, only photographs of F. occidentalis are shown. Thrips feeding was observed on a sucrose solution and on lettuce, Lactuca sativa L. Observation chambers were constructed of 2 depression slides taped together with the concave surfaces facing one another to create a chamber. When a sucrose Solution was used one slide was first covered with stretched Parafilm and a syringe was used to inject a 5% sucrose solution (w : v) into the well of the slide. Two adult thrips were then placed onto the Parafilm over the well and the second slide placed over the thrips. The 2 slides were then taped together. The thrips could be observed either from the top or bottom during feeding. Observations were made with a Leitz light microscope with 12.5x and 25× oculars with 10× to 40x objectives. When observations were made on lettuce, 2 thrips were placed into the well of one slide, then quickly covered with a lettuce leaf which was subsequently covered with a glass coverslip taped in place. Transmitted light made tissues translucent such that feeding could be observed.
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RESULTS
Morphology of the feeding structures There were no major morphological differences between species, therefore, figures show features of F. occidentalis. The feeding structures of both F. occidentalis and F. schultzei are housed within a ventrally located mouthcone (Fig. la). Paired labial palps and maxillary palps (Fig. lb) are located on the mouthcone and possess structures which appear to be sensory. Each of the paired paraglossae possesses 10 sensory pegs (Fig. 2a) of 3 distinct, morphologically different types: sensilla basiconica lacking a cuticular collar (1, 3 Fig. 2a), s. basiconica with cuticular collar (2, 4, 5, 7, 8 Fig. 2a) and s. trichoidea (6, 9, 10 Fig. 2a) (terminology from Chisholm and Lewis, 1984). The axons and axonic sheaths of the neurons innervating the sensory pegs on the paraglossae are shown in a lateral view of the mouthcone (Fig. 2b). When the paraglossae are closed, as they are prior to initiation of thrips probing, these sensory pegs cover the tip of the mouthcone (Fig. 3a). As probing is initiated the paraglossae move apart and extrusion of the labral pad begins (Fig. 3b). The labral pad is omega shaped with the ends meeting ventrally, The pad is fringed with many small palpi, as well as having thinner, longer palpi-like extensions toward the center (Fig. 4a). The stylets, which emerge from the opening in the center of the labral pad, consist of the single left mandible, which is closed apically and fused to the exoskeleton (Fig. 4b) and the paired maxillary stylets. The latter are interlocked in a tongue and groove fashion and open at the tip to form a feeding tube (Figs 4b; 5a, 5b). The maxillary stylets can be extended at least 30 p~m beyond the tip of the mouthcone (Fig. 5b) and are under muscular control.
Feeding behavior Figure 6 shows the typical feeding posture observed for both F. occidentalis and F. schultzei with the head pressed downward and the legs and antennae spread. Thrips placed on feeding sachets containing sucrose solution moved about randomly, stopping frequently to scrape the membrane with their forelegs. The tip of the mouthcone was not in contact with the membrane during these scraping movements. Typically, thrips scraped first with one proleg, reaching beyond the head and other proleg. Then the leg was drawn across the surface diagonally back to its original position. This behavior was sometimes repeated several times before scraping with the second leg began. The insect then performed similar movements with the other proleg. Following scraping with the forelegs, the thrips scraped the antenna with each leg. In addition, the thrips dragged the antennae in an "S"-like pattern, a movement from side to side, across the membrane. After these tarsal and antennal movements, the mouthcone was placed on the substrate. Then the paraglossae separated and the labral pad was rapidly extruded. As the insect pushed down with its head, compressing the mouthcone against the substrate surface, the mandibular stylet pierced the membrane. The maxillary stylets then followed the mandible and movement of fluids into the feeding tube was observed. When thrips were given access to lettuce leaves, the mouthcone was obscured by the plant cell contents. However, stylet insertion and the rapid ingestion of cell contents was observed on more translucent cells.
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FIG. la. Lateral view of thrips head showing mouthcone (a), left prothoracic leg (b), compound eye (c) and antennae (d). FIG. lb. Frontal view of thrips head. Square includes: tip of mouthcone (a), with labral pad (k) partially extruded, paired maxillary palps (e) and paired labial palps (f).
Analysis of Mouthpart Movements
FIG. 2a. Tip of mouthcone showing 10 sensory pegs on left paragiossa. Three morphologically distinct types are shown. Basiconica lacking a distinct cuticular collar (1, 3), basiconica with a distinct cuticular collar (2, 4, 5, 7, 8), and sensilla trichoidea (6, 9, 10). FIG. 2b. A lateral view of mouthcone with a portion of surface removed by plasma etching reveals neuron cell bodies and axons of sensilla on paraglossae. External tip of some sensory pegs remain (g) and outer portions of axonic sheaths (h) and axons (i) can be seen.
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FI6. 3a. Tip of mouthcone showing paraglossae (j) closed prior to feeding. Arrows denote point at which paired paraglossae meet when fully closed. FI6.3b. Tip of mouthcone showing separation of paraglossae and extrusion of labral pad (k) during initiation of feeding process. Arrows denote movement of paraglossae separation.
Analysis of Mouthpart Movements
Fro. 4a. Tip of mouthcone showing further extrusion of iabral pad (k) as paraglossae (j) open. Dorsal edge of labral pad is fringed with many small paipi (1), while longer, more slender palpi (m) are present ventrally. FIG. 4b. Single mandibular styler (n) is closed apically and is used to puncture leaf's surface creating an entry point for insertion of paired maxillary stylets (o). Arrow denotes subapical opening.
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Ftc. 5a. Maxillary stylets (o) are shown emerging from extruded labral pad (k). They are interlocked at tip and can be moved independently in a tongue and groove fashion. F16.5b. Longest observed extension of maxillary stylets (o) was approximately 30 ~m. Small arrow denotes the subapical opening.
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FIG. 6. Typical feeding posture of F. occidentalis with head thrust downward pressing mouthcone onto substrate.
DISCUSSION Our study shows that, like L. cerealium, Haplothrips verbasci, and S. citri (Chisholm and Lewis, 1984; Heming, 1978; Wiesenborn and Morse, 1988), F. occidentalis and F. schultzei do not have any structure on the mouthcone or ventral surface of the body capable of rasping plant cells. Based on external morphology the stylets are piercingsucking structures, the paraglossae and labral pad being smooth, lacking teeth or ridges capable of rasping. Our observations of F. occidentalis and F. schultzei, feeding from artificial media and from lettuce leaves, strongly support Chisholm and Lewis (1984) in their conclusion that thrips feed by piercing plant cells with their mandible, following the mandible with their maxillae and ingesting cell contents through the feeding tube formed by the pair of interlocked maxillae. They are also capable of ingesting fluids from the plant surface by extending the labial pad and sucking up surface fluids through the food canal in the maxillary stylets. In this manner, they may also suck up plant fluids exuded following scraping of the leaf surface with their tarsal claws. Our observations suggest that such behavior is rare, although it may potentially be of importance to host selection and warrants further study. The information from this and other recent studies (Mound, 1971; Chisholm and Lewis, 1984; Heming, 1985) strongly suggests that the feeding strategy of the Thysanoptera is not rasping-sucking, as suggested previously (Metcalf et al., 1962), but is more appropriately classified as piercing-sucking. This finding is
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significant to our understanding of how specific events in the feeding process govern thrips acquisition and inoculation of TSWV, as well as to understanding how thrips directly damage plants as a result of their feeding. Our investigation shows that the morphology of the feeding structures of F. occidentalis and F. schultzei are similar to other Thysanoptera studied (Chisholm and Lewis, 1984; Heming, 1985; Wiesenborn and Morse, 1988) in that they have a single mandible that is closed apically and fused to the exoskeleton, a pair of interlocking maxillary stylets that are open apically forming a feeding tube and an extrudable labral pad that is fringed with m a n y small palpi. Chisholm and Lewis (1984) suggest that the labral pad palpi m a y serve a sensory function. In our investigation, we did not find any evidence of innervation of these palpi and therefore suggest that the function of the labral pad and palpi is to provide structural support for the stylets and to stabilize the mouthcone on the plant surface. In addition, the labral pad may serve to prevent loss of plant fluids during feeding. Like L. cerealium and S. citri (Chisholm and Lewis, 1984; Wiesenborn and Morse, 1988), both thrips species studied have structures on the paraglossae that, based on external m o r p h o l o g y and location, appear to be sensory. O u r data, showing that the sensory pegs on the paraglossae are innervated, support this hypothesis (Fig. 2b). Furthermore, our behavioural observations indicate that these sensory structures may play an important role in the chain of sensory events leading thrips to ingest from a particular host plant. F. occidentalis and F. schultzei differ from other thrips studied in that they have 10 distinct pairs of sensory pegs on the paraglossae rather than 9 (Chisholm and Lewis, 1984). However, the same 3 morphologically distinct types of sensory pegs were found. Preliminary studies also suggest that thrips have sensilla located in their precibarial canal, similar to those reported in aphids, (Wensler and Filshie, 1969) leafhoppers, (Backus and McLean, 1982, 1983, 1985; McLean and Kinsey, 1984) and psyllids, (Ullman and McLean, 1986). We plan to further examine precibarial sensilla in thrips and work toward elucidating the function of external paraglossa sensilla and internal precibarial sensilla in thrips host finding and choice. Acknowledgements--We are indebted to Dr A. Alvarez, Dr M. Dunlap, Dr Kurzurian and Robin Lipp for the
facilities and guidance they made available. This research was supported in part by the Governors Agricultural Coordinating Committee contract No. P-280-982, Biomedical Research Support Grant, University of HawaiiManna and a Research Centers in Minority Institutions Grant RR-03061, Division of Research Resources, National Institutes of Health. This is Journal Series No. 3305 of the Hawaii Institute of Tropical Agriculture and Human Resources. REFERENCES ALLEN, W. R. and A. B. BROADBENT.1986. Transmission of the tomato spotted wilt virus in Ontario greenhouses by the western flower thrips, Frankliniella oecidentalis Pergande (Thysanoptera : Thripidae). Can. J. Plant Patkol. 8: 33-8. AMIN, P. W., D. V. R. REDDYand A. M. GHANEKAg.1981. Transmission of tomato spotted wilt virus, the causal agent of bud necrosis of peanut, by Scirtothrips dorsalis and Frankliniella schultzei. Plant Dis. 65: 663-65. ANANTHAKVaSm~AN,T. N. 1951. Variations in the mouth parts of some Thripidae (Thysanoptera-Terebrantia). Indian J. Entomol. 12: 237-39. BACKUS,E. A. and D. L. MCLEAN. 1982. The sensory systems and feeding behavior of leafhoppers. I. The aster leafhopper, Macrostelesfascifrons Stal (Homoptera : Cicadellidae). J. Morphol. 172: 359-78. BACKUS,E. A. and D. L. MCLEAN. 1983. The sensory systems and feeding behavior of leafhoppers. II. A comparison of the sensillar morphologies of several species (Homoptera : Cicadellidae). J. Morphol. 176: 3-14.
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BACKUS, E. A. and D. L. McLEAN. 1985. Behavioral evidence that the precibarial sensilla of leafhoppers are chemosensory and function in host discrimination. Entomol. Exp. Appl. 37: 219-28. BEDFORD, H. W. 1921. The cotton thrips (Heliothrips indicus Bagn.) in the Sudan. Bull. Wellcome Trop. Res. Labs. Entomol. Sect. 18. BORDEN, A. D. 1915. The mouthparts of the Thysanoptera and the relation of thrips to the non-setting of certain fruits and seeds. J. Econ. Entomol. 8: 354-60. CAMERON, A. E. and R. C. TREHERNE. 1918. The pear thrips (Taeniothrips inconsequens Uzel) and its control in British Columbia. Bull. Dep. Agric. Can. Entomol. 15. CHISHOLM, I. F. and T. LEwis. 1984. A new look at thrips (Thysanoptera) mouthparts, their action and effects of feeding on plant tissue. Bull. Entomol. Res. 74: 663-75. CHO,J. J., W. C. MITCHELL,R. F. L. MAu and K. SAKIMURA.1987. Epidemiology of tomato spotted wilt virus disease on crisphead lettuce in Hawaii. Plant Dis. 71: 505-08. DAVIES, R. G. 1958. Observations on the morphology of the head and mouthparts in the Thysanoptera. Proc. R. Entomol. Soc. Lond. 33(A): 97-106. GREENOUGH, D. R., L. L. BLACK, R. N. STORY, L. D. NEWSOM and W. P. BOND. 1985. Occurrence of FranklinieUa occidentalis in Louisiana: a possible cause for the increased incidence of tomato spotted wilt virus. (Abstr.) Phytopathology 75: 1362. HEMING, B. S. 1985. Thrips (Thysanoptera) in Alberta. Agric. For. Bull. 8: 19-24. HEMING, B. S. 1978. Structure and function of mouthparts in larvae of Haplothrips verbasci (Osborn) (Thysanoptera, Tublifera, Phlaeothripidae). J. Morphol. 156: 1-37. HORTON, J. R. 1918. The citrus thrips. Bull. U.S. Dep. Agric.616. HUMASON, G. L. 1967. Fixation, pp. 14. In R. EMERSON, D. KENNEDY and R. B. PARK. (eds) Animal Tissue Techniques. Freeman, San Francisco. JONES, T. 1954. The external morphology of Chirothrips hamatus (Trybom) (Thysanoptera). Trans. R. Entomol. Soc. Lond. 105: 163-87. LEwis, T. 1973. Thrips, Their Biology, Ecology and Economic Importance. Academic Press, London. MCLEAN, O. L. and M. G. KINSEY. 1984. The precibarial valve and its role in the feeding behavior of the pea aphid, Acyrthosiphon pisum. Bull. Entomol Soc. Amer. 30: 26-31. METCALF, C. L., W. P. FLINT, and R. L. METCALE. 1962. Destructive and Useful Insects. McGraw-Hill, New York. MICKOLEIT, E. 1963. Untersuchungen zur Kopfmorphologie der Thysanopteren. Zool. Jahrb. Anat. 81: 101-50. MOUND, L. A. 1971. The feeding apparatus of thrips. Bull. Entomol. Res. 60: 547-48. PALIWAL, Y. C. 1976. Some characteristics of the thrips vector relationship of tomato spotted wilt virus in Canada. Can. J. Bot. 54: 402-05. PETERSON, A. 1915. Morphological studies of the head and mouthparts of the Thysanoptera. Ann. Entomol. Soc. Amer. 8: 20--66. REYNE, A. 1927. Untersuchungen uber die Mundteile der Thysanopteren. Zool. Jahrb. Anat. 49: 391-500. RISEER, H. 1957. Der Kopf von Thrips physapus L. (Thysanoptera, Terebrantia). Zool. Jahrb. Anat. 76: 251302. RUSSELL, H. M. 1912b. The red-banded thrips. Bull. Bur. Entomol. U.S. Dep. Agric. 99: 17-29. SAKIMURA,K. 1962. The present status of thrips-borne viruses, pp. 1-192. In K. MARAMOROSCH(ed.) Biological Transmission of Disease Agents. Academic Press, New York. ULLMAN,D. E. and D. L. MCLEAN. 1986. Anterior alimentary canal of the pear psylla, Psylla pyricola Foerster (Homoptera, Psyllidae. J. Morphol. 189: 89-98. WENSLER,R. J. and B. K. FILSHIE. 1969. Gustatory sense organs in the food canal of aphids. J. Morphol. 129: 473-92. WIESENBORN,W. D. and J. G. MORSE. 1985. Feeding rate of Thysanoptera estimated using C14 inulin. J. Econ. Entomol. 78: 151-18.