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Damage of rice grains caused by the rice bug, Leptocorisa oratorius Fabricius (Heteroptera: Alydidae.
| Content Provider | Semantic Scholar |
|---|---|
| Author | Sugimoto, Atsushi Nugaliyadde, L. |
| Copyright Year | 1995 |
| Abstract | The symptoms associated with grain damage caused by the rice bug were examined using grain samples collected from farmers' paddy fields in relation to the occurrence of the insect. The erythrosine dye was used to stain the stylet sheaths produced by rice bugs on damaged grains. The main symptom associated with rice bug damage was the presence of unfilled grains. The unfilled grains showed a dark spot on the grain surface, but not when they were damaged at the very early stage of grain formation. These grains were considered to be damaged by the rice bug when they were re-stained to observe the stylet sheaths which penetrated into the grain. No damage of the ovary before the onset of grain development was observed. Grain damage mainly occurred from the very early stage of grain development to the milk-ripe stage when the rice bug infestation was severe. However, a considerable proportion of grains was damaged at the dough and hard-dough stages when the level of insect infestation was lower. Additional key words : Sri Lanka, Stylet sheath 13 The rice bug, Leptocorisa oratorius Fabricius, is one of the major pests of rice in tropical Asia. It feeds on the panicles and damages grains. However, only a few studies on the nature of the *This is a report on the collaborative studies carried out between the Tropical Agriculture Research Center, Japan, and Department of Agriculture, Ministry of Agricultural Development and Research, Sri Lanka during the period from July to September 1987. a)Present address: Minami 5-32-8, Ushiku-shi, Ibaraki 300-12, Japan 14 JIRCAS ]. No.2, 1995 feeding damage under field conditions have been · d '') Th . d . h carne out . e symptoms associate wit feeding damage have not been precisely identified. Though it has been mentioned that infested grains may remain empty, be filled partially or develop a brown spot1\ only Reissig et al. 8 ) described nonspotted partially filled grains. Morita & Dhanapala 6 l observed for the first time the presence of a dark-spot on the unfilled grain surface after removal of the glumes. However, their report did not refer to the occurrence of the rice bug. Furthermore, it is difficult to discriminate the grains with rice bug damage from those injured b h f 1,3,5) y ot er actors We investigated the symptoms associated with grain damage caused by the rice bug using grain samples collected from farmers' paddy fields in relation to the occurrence of the insect during the 1987 Yala rice cropping season in Sri Lanka. The unfilled grains with rice bug damage were precisely identified by staining the stylet sheaths of the rice bugs which penetrated into the grain with the erythrosine dye after removal of the glumes. Materials and methods 1) Occurrence of the rice bug To investigate grain damage in rice with different intensities of rice bug infestation, 8 target paddy fields were set up in three paddy areas, Galahitiyawa, Ja-ela and Pallewela, selected in the Gampaha district. The insect populations in the target fields were sampled by 20-stroke netsweepings 1 week after rice flowering. L. oratorius was the only ear-sucking hemipteran species observed predominantly in the fields. 2) Grain damage Rice panicles were harvested from the target paddy fields at the yellow-ripe stage at the rate of 10 panicles per field. For the examination of the grain damage, 200 spikelets were sampled for each field. Grain damage was examined in reference to the presence of the stylet sheaths produced by rice bugs on the spikelet surface as well as on the grains after removal of the glumes. The stylet sheaths were stained with the erythrosine dye so that they became more visible than in the natural state as they stained dark-pink. By the use of the erythrosine dye, the staining technique became simpler and faster as compared to the former use of the acid fuchsin dye 5 ). The method of examination was as follows: The sampled spikelets were immersed in the dye (0.5% erythrosine in 70% ethyl alcohol, modified from Naito & Masaki 7 l) for 5 min. and washed in tap water for a few minutes, to stain the heads of stylet sheaths (Note in Plate 1a b ove) as previously describel· 5 l. After the spikelet surface was examined for the presence of the stylet-sheath heads, the glumes were removed and the symptoms on the grain surface were observed. Then the grains after removal of the glumes were re-stained to observe the stylet sheaths which penetrated into the grain. Undeveloped ovaries and small unfilled grains were observed and re-stained while remaining intact with the palea as shown in Plate 1-above. The developmental stage of the grain at which the rice bug damage occurred and grain development ceased, was determined according to the length, width and thickness of damaged grain as follows: Grain which did not reach the optimum length and width and were entirely flattened· · · · Early stage; Grains which reached the optimum length and width and were entirely flattened, partially flattened and approaching optimum thickness· · · · Milk-ripe, dough and hard-dough stages, respectively. Filled grains with a partial discolouration with distinct margin were found to be pecky grains caused by the rice bug infestation. Results and Discussion 1) Identification of grains damaged by the rice bug The unfilled grains with stylet-sheath heads showed a characteristic dark-bluish discolouration around a spot on the grain (Plate 1-below) as observed earlie/l. These grains were considered to be damaged by the rice bug, since they occurred in the spikelets with stylet-sheath heads, and Atsushi SUGIMOTO & Lionel NUGALIYADDE: Damage of Rice Grains caused by the Rice Bug 15 Plate 1 . Damage of rice grains caused by the rice bug , Leptocorisa oratorius. Above: Stylet sheath that penetrated into the grain, stained with erythrosine dye after removal of the lemma of the spikelet. Note that the head of sty let sheath is located at the junction of the glumes as observed earlie/ 5 ). Below: Unfilled grains into which the stylet sheaths were found to penetrate. furthermore, the stylet sheaths were found to penetrate into the grain (Plate 1-below). However, the grains whose development had ceased at the very early stage of grain formation only showed flattening and a ligh-brownish discolouration, and were considered to be damaged by the rice bug only when they were re-stained to observe the stylet sheaths which penetrated into the grain (grain on the far left in Plate 1-below). Grains showing such symptoms were also detected among the spikelets without stylet-sheath heads (Figures in the column with parenthese in Table 1). Therefore it is considered that the stylet-sheath heads had been detached from the spikelets after the insect attack. The percentage of unfilled spikelets, mainly with undeveloped ovaries without any symptoms, ranged from 1.5 to 17.5% in the spikelet samples analysed. However, a smaller number of unfilled grains were considered to be affected by unidentified diseases. These grains showed a dark-discolouration at the base and/ or apex or the entire grain. Morita & Dhanapala 6 l diagnosed the bug damage on the basis of the presence of a dark-spot on the unfilled grain surface. On the other hand, Litsinger et al. 5 ) reported that the technique for staining the heads of stylet sheaths of rice bugs on the spikelet surface enabled to distinguish the unfilled grains with rice bug damage from those affected by fungi, bacteria, etc. According to the present study some of the damaged grains did not show conspicuous symptoms and the stylet-sheath heads may drop. Therefore, it is essential to stain the stylet sheath with the grain opened for precisely identifying the unfilled grains with rice bug damage. 2) Characteristics of grain damage by the rice bug As shown in Table 1, grain damage was found to be generally proportional to the intensity of rice bug infestation of paddy fields, whereas Ito et al. 3 ) did not observe a clear relationship between the rice bug population density and grain damage due to the difficulty in discriminating the grains with rice bug damage from those injured by other factors. The main symptom associated with rice bug damage was the presence of spikelets with unfilled grains whose development ceased at different stages of development (Plate 1-below, Table 1). Lower percentages of pecky grains were observed irrespective of the level of grain damage. Since the stylet sheath never penetrated into an undeveloped ovary, it is thus considered that the rice bug hardly attacks the ovary before the onset of grain development. So far, the rice bug has been considered to prefer grains at the milk-ripe stage. It has been observed in cage tests that the insect adults and nymphs fed on panicles at the milk-ripe, dough and hard-dough stages ZJ. However, it has also been observed that the adults damaged a larger number of grains when panicles were attacked at the early post-flowering stage as compared to the attack at 16 JIRCAS ]. No.2, 1995 Table 1. Number of rice bugs collected by 20-stroke net-sweepings 1 week after rice flowering and percentage of unfilled and pecky rice grains with rice bug damage in panicles harvested at yellow-ripe stage in paddy fields surveye. Paddy area Date of No. of rice Unfilled grains Pecky Totalbl Collection Nymphs Adults Total E MR D HD grains Galahitiyawa Jul.14 2 6 8 1.0 4.5 2.0 1.5 0.5 9.5( 6.5) Pallewela Jul.15 10 3 13 0.5 0.5 2.5 3.5 1.0 8.0( 5.5) Pallewela Jul.IS 23 6 29 6.5 0.5 1.5 9.0 3.0 20.5( 6.0) Ja-ela Jul.14 27 38 65 7.5 8.0 3.5 13.0 2.5 34.5( 7.0) Pallewela Jul.IS 55 18 73 9.5 12.5 9.0 22.5 2.0 55.5( 9.5) Ja-ela Aug.11 0 35 35 10.5 5.0 5.5 6.0 1.5 28.5(16.0) Ja-ela Aug.11 0 48 48 17.5 12.5 9.0 11.0 0.5 50.5(28.5) Ja-ela Aug.II 37 192 229 39.5 25.5 8.5 6.5 0.5 80.5( 6.0) a):E:MR, ~, and HD: Developmental stages of grain: early, milk-ripe, dough and hard-dough stages, respectively. b):F1gures m parentheses indicate the percentages of damaged grains detected among s |
| Starting Page | 13 |
| Ending Page | 17 |
| Page Count | 5 |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://www.jircas.go.jp/sites/default/files/publication/jircas_journal/jircas_journal2-_13-17.pdf |
| Volume Number | 2 |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
