Academic Journal of Entomology 5 (1): 41-46, 2012 ISSN 1995-8994 © IDOSI Publications, 2012 DOI: 10.5829/idosi.aje.2012.5.1.61267 Spatial Distribution of Nymphula depunctalis Guenée Larvae (Lepidoptera: Pyralidae), an Early Vegetative Pest of Oryza sativa L. 1 1 Hiren Gogoi and 2Dipsikha Bora Department of Zoology, Silapathar Science College, Silapathar-787059, Assam, India Department of Life Sciences, Dibrugarh University, Dibrugarh-786004, Assam, India 2 Abstract: Rice caseworm, Nymphula depunctalis Guenée is a serious pest of paddy that attacks young rice plants in waterlogged paddy fields. Present study was designed to determine the spatial distribution of N. depunctalis larvae in ten farmer’s rice fields of Dhemaji district, Assam, India and in 13 cultivars of Oryza sativa L. in a controlled experimental field. Results of the variance to mean ratio (S2/m), index of dispersion (ID), 2 test, Z test and Lloyd’s mean crowding indicated highly aggregated spatial distribution pattern of N. depunctalis larvae. In farmers field condition, only two occasions in booting period showed uniform distribution pattern. In controlled experimental field, degree of aggregation was comparatively low than the farmers field condition. Only one occasion in stem elongation and five occasions in booting period showed uniform distribution pattern. Key words: Nymphula depunctalis Guenée Crowding Spatial Distribution blade to construct larval case. Cut leaf piece trends to roll due to lack of turgor pressure which is further secured by the larvae with silk. A new case was constructed immediately after each moult or when it was accidently removed from the case. Each female moth laid 300-400 eggs, 200-300 eggs and 50-70 eggs in day 2, day 3 and day 4 respectively. N. depunctalis larvae need water for respiration and movement from plant to plant and the moths prefer to lay eggs on the underside of leaves that are floating on the water surface [15]. Egg desiccates if laid on aerial portion of the leaves [16]. Present study was designed to determine the spatial distribution of N. depunctalis larvae in ten farmer’s rice fields of Dhemaji district, Assam and in 13 cultivars of O. sativa in a controlled experimental field. INTRODUCTION Spatial distribution of a population that is the position that individuals occupy in environment one relative to the others at a given time is a central issue in population dynamics studies. A population may show three basic patterns of distribution: random, uniform (regular) and aggregated (clustered). Pattern may change in time, among developmental stages of a same species or even occur superposed in a given population [1, 2]. Identification of the distribution pattern is essential to develop sampling programs, especially those involving species considered pests [3]. Rice caseworm, Nymphula depunctalis Guenée (=Parapoynx stagnalis Zeller) (Lepidoptera; Pyralidae) is a serious pest of paddy that attacks young rice plants in waterlogged paddy fields [4-9] and is reported from South and South East Asia, China, Japan, Australia, South America (Argentina, Brazil, Uruguay, Venezuela), Central Africa (Madagascar, Malawi, Mozambique, Rwanda and Zaire), East Africa and several West African countries and probably occurs throughout West Africa [10-14]. Larvae crawl and grip with the aid of 6 thoracic legs, while the crochets are used to cling to its case. Second instar larvae cut leaves near the tip of the leaf Corresponding Author: Index of Dispersion (ID) and Lloyd’s Mean MATERIALS AND METHODS The study was conducted in Dhemaji district, Assam, India during the year 2008, 2009 and 2010. Study site was geographically situated between the 94° 12' 18'' E and 95° 41' 32'' E longitudes and 27° 05' 27'' N and 27° 57' 16'' N latitudes. It is basically plain area lying at an altitude of 104 m above mean sea level. Hiren Gogoi, Department of Zoology, Silapathar Science College, Silapathar-787059, Assam, India. 41 Acad. J. Entomol., 5 (1): 41-46, 2012 To study the spatial distribution pattern of N. depunctalis larvae, population density was estimated both in farmer’s field condition and in controlled field condition. Larvae were identified by their larval case; presence of branched tracheal gills, shorter than segment’s length; and thorax with dorsal gill group [17]. Study was conducted during tillering, stem elongation and booting stage in the months July and August; September and October respectively. For the studies in farmer’s field condition, ten number of farmer’s paddy fields were selected in the district and study was conducted irrespective of hill density of rice plants and water level of the field. Controlled experimental field was situated in Bakal Gaon Pathar of the district. were selected for each cultivar in the controlled experimental field. Spatial distribution of N. depunctalis larvae was determined by index of dispersion and Lloyd's mean crowding. Index of Dispersion: Dispersion of a population can be classified through calculation of the variance to mean ratio; namely: S2/m = 1 random, < 1 regular and > 1 aggregated. Departure from a random distribution can be tested by calculating the index of dispersion (I D), where n denotes the number of samples: ID= (n-1) S2/m ID is approximately distributed as 2 with n-1 degrees of freedom. Values of ID which fall outside a confidence interval bounded with n-1 degrees of freedom and selected probability levels of 0.95 and 0.05, for instance, would indicate a significant departure from a random distribution. This index can be tested by Z value as follows: Z = (2ID1/2-(2 -1)1/2 = n-1 Farmer’s Field Condition: In farmer’s fields, three types of hill density viz. 16 hills per m2, 20 hills/m2 and 25 hills/m2 and water level of range 0-30 cm were considered at random. Eight to nine cultivars of O. sativa were recorded in each farmer’s field. Controlled Field Condition: In controlled experimental field, seedlings 25 DAS (days after sowing) were transplanted at a hill density 20 hills per m2 with 4 seedlings per hill. Water level of 10-15 cm for tillering stage, 5-10 cm for stem elongation stage and 0-5 cm for booting stage was maintained during the experimental period. Thirteen cultivars of O. sativa were transplanted in the controlled experimental field in an area of 100m2 per cultivar. If 1.96 Z -1.96, the spatial distribution would be random but if Z <-1.96 or Z > 1.96, it would be uniform and aggregated, respectively [19]. Lloyd's Mean Crowding X*: Mean crowding (x*) was proposed by Lloyd to indicate the possible effect of mutual interference or competition among individuals. Theoretically mean crowding is the mean number of other individuals per individual in the same quadrate: Environmental Parameters: In both farmer’s field condition and controlled field condition environmental parameters, rainfall, relative humidity and air temperature were same. Mean rainfall of 20.67-26.85 mm/day, 4.56-15.45 mm/day and 2.31-5.19 mm/day; relative humidity of 89.55-90.84 %, 83.63-91.10 % and 81.58-85.32% and air temperature of 27.59-28.66°C, 27.46-29.29°C and 25.69-26.51 °C were recorded during tillering, stem elongation and booting period. X* = m + (S2/m)-1 As an index, mean crowding is highly dependent upon both the degree of clumping and population density. To remove the effect of changes in density, Lloyd introduced the index of patchiness, expressed as the ratio of mean crowding to the mean. As with the variance-to-mean ratio, the index of patchiness is dependent upon quadrate size x* / m= 1 random, <1 regular and >1 aggregated [20]. Population Density in Farmer’s Field: Population density of N. depunctalis larvae was estimated in ten number of farmer’s paddy fields. For this, 45-55 quadrats [18] were selected in each field depending on the number of rice cultivars, considering five quadrats for each cultivar. RESULT AND DISCUSSION Population Density in Different Cultivars of O. sativa: Density was also estimated in 13 cultivars of O. sativa traditionally cultivated by the farmers of Dhemaji district, Assam (India) in an experimental field. For this, 5 quadrats Population density, considered as the mean number of larvae per square meter (m), variance to mean ratio (S2/m), chi square value ( 2), index of dispersion (ID) and Lloyd mean crowding are shown in Table 1-6. 42 Acad. J. Entomol., 5 (1): 41-46, 2012 Table 1: Mean population density of N. depunctalis larvae per square meter (m), variance to mean ratio (S2/m), chi square value ( 2), index of dispersion (ID), Z value and Lloyd mean crowding (x*) in farmer’s rice fields during tillering period of O. sativa Rice field n 2008 ------------------------------------------------------------2 2 ID Z X* m S /m Akajan Patiri Arsi Lason Balikata Miri Fulbari Asomia Galowa Jalakia Suti Kulajan Mesu Sila Gaon Sissi Borgaon 45 55 45 45 50 55 55 55 45 45 29.42 18 21.76 61.24 42.35 14.45 24.87 27 16.58 15.69 P value of 2 101.35 91.93 182.03 116.99 161.74 90.22 89.44 162.22 42.73 50.12 238.71 299.55 245.00 145.00 86.80 191.27 114.53 216.36 277.44 129.07 4459.25 85.11 4964.11 89.30 8009.46 117.24 5147.67 92.14 7925.13 116.05 4871.94 88.37 4829.95 87.94 8759.70 122.02 1880.29 52.00 2205.49 57.09 129.77 108.93 202.79 177.23 203.09 103.67 113.31 188.22 58.31 64.81 2009 ------------------------------------------------------------2 2 m S /m ID Z X* 9.69 22.97 18.60 52.72 2.24 8.83 41.47 81.25 8.19 20.87 16.69 50.06 9.65 40.93 14.76 107.44 22.72 69.09 21.16 90.41 86.67 358.58 132.80 183.80 285.24 146.22 172.73 82.20 215.40 229.89 1010.81 2847.05 388.65 3575.09 1022.64 2703.25 2210.20 5801.55 3040.08 3978.15 35.64 65.12 18.55 75.23 35.38 63.18 56.14 97.37 68.65 79.87 31.66 70.32 10.07 121.72 28.06 65.75 49.58 121.20 90.81 110.57 2010 -------------------------------------------------------2 2 m S /m ID Z X* 21.07 49.69 17.95 80.05 10.27 91.43 38.33 84.16 32.86 74.71 13.71 47.20 16.62 38.98 30.02 102.21 30.89 102.55 13.38 58.20 114.04 276.53 181.60 106.58 121.36 269.73 119.80 209.00 111.93 177.44 2186.15 4322.92 4023.00 3703.15 3660.86 2548.63 2105.10 5519.15 4512.19 2560.86 56.80 82.64 80.37 76.73 75.72 61.05 54.54 94.72 85.67 62.24 69.76 97.00 100.70 121.49 106.57 59.91 54.60 131.23 132.44 70.58 < 0.01 in all cases; n = number of quadrats Table 2: Mean population density of N. depunctalis larvae per square meter (m), variance to mean ratio (S2/m), chi square value ( 2), index of dispersion (ID), Z value and Lloyd mean crowding (x*) in farmer’s rice fields during stem elongation period of O. sativa Rice field n 2008 ------------------------------------------------------------2 ID Z X* m S2/m Akajan Patiri Arsi Lason Balikata Miri Fulbari Asomia Galowa Jalakia Suti Kulajan Mesu Sila Gaon Sissi Borgaon 45 55 45 45 50 55 55 55 45 45 3.27 4.8 8 3.09 4.67 8.91 3.41 7.13 8.38 4.87 P value of Table 3: 2 22.45 29.56 21.61 18.80 14.05 44.87 11.10 44.03 23.45 35.51 156.40 276.66 40.07 132.71 231.60 185.58 73.95 284.31 54.11 168.89 987.80 1596.00 951.00 827.37 688.61 2422.76 599.54 2377.64 1031.85 1562.30 35.12 46.15 34.28 31.35 27.26 59.27 24.28 58.61 36.10 46.57 24.72 33.36 28.61 20.89 17.72 52.78 13.51 50.16 30.83 39.38 6.82 4.42 22.80 4.38 11.73 2.96 5.7 7.05 13.04 11.18 42.40 18.60 81.23 14.54 26.49 13.87 27.80 53.43 32.12 76.14 145.89 68.27 69.33 105.27 79.60 98.80 102.66 138.09 58.60 133.00 1865.46 1004.35 3574.26 639.73 1298.01 749.06 1501.45 2885.43 1413.32 3350.09 51.75 34.47 75.22 26.44 41.10 28.36 44.45 65.62 43.84 72.53 48.22 22.02 103.03 17.92 37.22 15.83 32.50 59.48 44.16 86.32 2010 -------------------------------------------------------2 m S2/m ID Z X* 1.47 4.69 6.29 2.49 4.30 2.53 2.69 3.93 8.71 6.98 10.90 18.52 26.15 9.45 17.27 25.72 29.12 35.75 14.51 47.85 127.98 176.86 118.44 66.88 136.68 218.27 207.00 214.84 76.18 124.07 479.45 1000.18 1150.48 415.95 846.16 1388.73 1572.27 1930.53 638.41 2105.39 21.64 34.38 38.64 19.52 31.29 42.36 45.73 51.79 26.41 55.56 11.37 22.21 31.44 10.94 20.57 27.25 30.81 38.68 22.22 53.83 < 0.01 in all cases; n = number of quadrats Mean population density of N. depunctalis larvae per square meter (m), variance to mean ratio (S2/m), chi square value ( 2), index of dispersion (ID), Z value and Lloyd mean crowding (x*) in farmer’s rice fields during booting period of O. sativa Rice field n 2008 ------------------------------------------------------------2 ID Z X* m S2/m Akajan Patiri Arsi Lason Balikata Miri Fulbari Asomia Galowa Jalakia Suti Kulajan Mesu Sila Gaon Sissi Borgaon 45 55 45 45 50 55 55 55 45 45 0.13 0.05 0.42 0.4 2.46 0.8 0.6 0.11 0.18 0 P value of 2009 ------------------------------------------------------------2 m S2/m ID Z X* 2 3.27 0.96 1.78 1.98 21.64 3.31 2.01 1.59 1.35 0.00 78.40 43.66 62.11 40.13 202.00 85.09 85.09 87.31 57.73 - 144.00 52.00 78.11 87.00 1060.33 178.50 108.67 85.67 59.50 0.00 7.64 -0.15 3.17 3.86 36.20 8.55 4.40 2.75 1.58 -9.33 2.40 0.01 1.20 1.38 23.10 3.11 1.61 0.70 0.53 -1.00 2009 ------------------------------------------------------------2 m S2/m ID Z X* 1.11 1.64 8.8 2.69 8.70 1.24 1.95 6.95 11.36 3.33 3.12 7.95 33.47 9.53 55.58 4.58 7.76 55.80 40.99 11.82 100.24 121.89 163.33 162.33 135.92 74.44 107.36 117.91 152.51 49.00 137.20 429.44 1472.64 419.37 2723.28 247.44 418.84 3013.03 1803.37 520.20 7.24 18.96 44.94 19.63 63.95 11.90 18.60 67.28 50.73 22.93 3.23 8.59 41.27 11.22 63.28 4.82 8.71 61.75 51.35 14.15 2010 -------------------------------------------------------2 m S2/m ID Z X* 0.24 1.52 0.62 4.04 0.07 0.95 0.84 6.24 12.14 137.95 2.40 31.77 0.44 3.80 5.44 48.19 0.27 1.26 1.33 6.72 80.07 66.73 136.46 218.35 33.80 42.00 212.96 274.63 137.20 6759.64 152.85 1715.50 126.00 205.17 137.93 2602.39 44.40 55.50 78.36 295.50 2.22 0.76 10.55 3.66 -0.16 0.02 14.11 6.08 106.42 149.09 48.23 33.17 9.91 3.24 61.80 52.63 1.21 0.53 14.98 7.05 < 0.01 in all cases; n = number of quadrats Table 4: Mean population density of N. depunctalis larvae per square meter (m), variance to mean ratio (S2/m), chi square value ( 2), index of dispersion (ID), Z value and Lloyd mean crowding (x*) in cultivars of O. sativa during tillering period Rice cultivar n 2008 ------------------------------------------------------------2 ID Z X* m S2/m Bahadur Bogilahi Bor Jahingia Ijong Kati Neoli Kekua Kola Joha Ranjit Ronga Bora Saru Jahingia Solpuna Suagmoni Toraboli 5 5 5 5 5 5 5 5 5 5 5 5 5 66.8 7 34 135.5 111 225.6 16 112.4 17.6 34.8 7.6 7 13.4 P value of 2 30.67 5.83 10.15 19.15 23.01 69.13 8.17 40.56 5.59 24.31 10.53 7.28 4.25 0.00 0.00 0.00 0.00 0.60 0.00 0.00 0.00 0.00 0.00 0.00 0.60 0.00 153.36 29.15 50.74 95.77 115.05 345.63 40.86 202.81 27.97 121.57 52.67 36.39 21.27 14.87 1013.50 4.99 40.00 7.43 136.00 11.19 505.80 12.52 639.50 23.65 4977.30 6.39 81.70 17.49 1756.70 4.83 47.90 12.95 622.30 7.62 117.80 5.89 55.70 3.88 29.90 2009 ------------------------------------------------------------2 m S2/m ID Z X* 2010 -------------------------------------------------------2 m S2/m ID Z X* 118.6 12.7 4 30.8 31.6 69.2 58 120.4 103.5 6.2 3.27 20.6 5.4 179 86 62.4 40 12.4 149 76.6 121 82 20.6 3.4 151.6 9.8 5.46 2.25 3.91 16.18 5.59 20.87 13.49 7.37 32.32 5.36 2.51 10.92 5.50 0.60 0.60 0.60 0.60 0.60 0.60 0.00 0.60 0.60 0.00 0.60 0.60 0.40 > or = 0.5 in all cases; n = number of quadrats 43 27.29 11.26 19.56 80.89 27.97 104.37 67.45 36.84 161.61 26.79 12.55 54.61 27.52 4.74 2.10 3.61 10.07 4.83 11.80 8.97 5.94 15.33 4.67 2.36 7.81 4.77 147.40 16.78 17.70 291.50 61.90 503.90 242.20 173.70 1139.80 33.90 8.57 138.90 34.70 32.06 47.22 39.18 9.97 8.20 15.31 6.31 26.08 21.97 10.09 3.21 9.04 8.14 0.00 0.00 0.00 0.00 1.60 0.00 0.00 0.60 0.00 0.60 0.00 0.60 0.00 160.31 236.11 195.91 49.87 41.02 76.57 31.54 130.38 109.83 50.45 16.05 45.22 40.68 15.26 19.09 17.15 7.34 6.41 9.73 5.30 13.50 12.18 7.40 3.02 6.86 6.37 1206.00 2315.00 1596.70 138.50 78.70 382.50 115.40 800.00 563.50 121.40 12.70 232.40 75.00 Acad. J. Entomol., 5 (1): 41-46, 2012 Table 5: Mean population density of N. depunctalis larvae per square meter (m), variance to mean ratio (S2/m), chi square value ( 2), index of dispersion (ID), Z value and Lloyd mean crowding (x*) in cultivars of O. sativa during stem elongation period Rice cultivar n 2008 ------------------------------------------------------------2 2 ID Z X* m S /m Bahadur Bogilahi Bor Jahingia Ijong Kati Neoli Kekua Kola Joha Ranjit Ronga Bora Saru Jahingia Solpuna Suagmoni Toraboli 5 5 5 5 5 5 5 5 5 5 5 5 5 0 2 67 0 3.2 0 57.4 0 1.8 3.8 1.6 4.2 26.8 P value of 2 2.83 12.04 2.39 16.70 3.49 2.77 2.30 1.30 4.82 1.6 0.00 0.00 0.00 1.6 0.00 1.60 0.60 0.00 14.14 60.21 11.94 83.49 17.46 13.87 11.51 6.52 24.08 2.67 8.33 2.24 10.28 3.26 2.62 2.15 0.97 4.29 9.00 211.00 7.90 335.20 13.00 10.50 5.90 4.90 49.00 2009 ------------------------------------------------------------2 2 m S /m ID Z X* 2010 -------------------------------------------------------2 2 m S /m ID Z X* 127.2 7.30 21.4 43.5 18.2 61.2 66.5 22 91.8 32.4 4.8 29 16.2 92 62 20.4 7.4 7.6 70.2 40 3.4 56.4 4.4 5.2 22.8 4.2 6.14 3.27 7.83 30.21 8.20 9.18 13.41 21.08 13.45 11.76 0.78 20.02 9.63 0.00 0.00 0.60 0.00 0.00 0.00 0.00 0.60 0.00 0.60 0.40 0.00 0.60 30.70 16.36 39.15 151.07 40.99 45.88 67.04 105.42 67.25 58.80 3.89 100.12 48.14 5.19 3.07 6.20 14.74 6.41 6.93 8.93 11.87 8.95 8.20 0.14 11.51 7.17 163.90 17.00 81.70 948.80 84.40 144.40 245.25 465.50 276.00 169.70 4.50 429.00 107.90 34.39 21.97 20.18 10.29 7.92 41.12 9.97 3.21 14.57 1.52 3.27 21.25 1.30 0.00 0.00 0.60 1.60 0.00 0.60 0.00 0.00 0.60 0.60 0.60 0.00 0.60 171.94 109.83 100.91 51.43 39.62 205.62 49.87 16.05 72.85 7.58 16.36 106.27 6.52 15.90 12.18 11.56 7.50 6.26 17.63 7.34 3.02 9.43 1.25 3.07 11.93 0.97 1273.50 543.50 426.70 112.20 69.40 1760.40 138.50 12.70 267.70 5.70 14.90 473.50 4.90 > or = 0.5 in all cases; n = number of quadrats 2 Table 6: Mean population density of N. depunctalis larvae per square meter (m), variance to mean ratio (S2/m), chi square value ( ), index of dispersion (ID), Z value and Lloyd mean crowding (x*) in cultivars of O. sativa during booting period Rice cultivar n 2008 ------------------------------------------------------------2 2 ID Z X* m S /m Bahadur Bogilahi Bor Jahingia Ijong Kati Neoli Kekua Kola Joha Ranjit Ronga Bora Saru Jahingia Solpuna Suagmoni Toraboli 5 5 5 5 5 5 5 5 5 5 5 5 5 0 1.2 3.2 0 0 0.8 1.2 0 0 0.6 0 0.8 1.2 P value of 2 1.79 3.42 0.84 0.84 0.55 0.84 1.30 1.60 0.60 0.40 0.20 0.40 0.60 8.94 17.10 4.18 4.18 2.74 4.18 6.52 1.58 3.20 0.25 0.25 -0.31 0.25 0.97 3.40 13.90 0.50 0.90 -0.10 0.50 1.90 2009 ------------------------------------------------------------2 2 m S /m ID Z X* 2010 -------------------------------------------------------2 2 m S /m ID Z X* 23 11.87 5.13 2.75 5.1 6.59 19.33 7.11 12.8 5.81 50.8 7.66 68.14 7.64 9.6 2.30 75.8 96.99 7.8 9.63 4.4 1.40 13 4.69 3 2.74 0 0 22.8 21.25 7.4 6.35 3 2.74 0 54.4 15.24 4.6 4.56 106.6 89.22 0.8 0.84 3.8 3.83 3 3.24 0.6 0.55 0.00 0.00 0.60 0.00 0.00 0.00 0.00 0.60 0.00 0.60 0.60 0.00 0.00 59.37 13.75 32.97 35.57 29.03 38.31 38.20 11.51 484.93 48.14 7.01 23.45 13.69 8.25 2.60 5.47 5.79 4.97 6.11 6.10 2.15 28.50 7.17 1.10 4.20 2.59 163.00 10.63 45.30 67.13 45.50 108.50 123.84 13.90 1224.30 99.50 5.70 34.00 9.50 0.00 0.00 0.00 0.60 0.00 0.40 0.40 0.60 0.20 106.27 31.74 13.69 76.21 22.80 446.09 4.18 19.17 16.20 2.74 11.93 5.32 2.59 9.70 4.11 27.22 0.25 3.55 3.05 -0.31 473.50 46.70 9.50 285.70 24.40 8065.40 0.50 17.50 12.50 -0.10 > or = 0.5 in all cases; n = number of quadrats During study period, 1485 quadrats were sampled from farmer’s field and 195 quadrats were sampled from the controlled experimental field. Results of the variance to mean ratio (S2/m), index of dispersion (ID), Z test and Lloyd mean crowding indicated highly aggregated spatial distribution pattern of N. depunctalis larvae. All rice fields of farmers showed aggregated pattern of distribution during tillering and stem elongation period. During booting stage, Arsi lason in the year 2008 and Balikata miri in the year 2010 showed uniform distribution. Degree of aggregation was highest during tillering stage and then decreases gradually during stem elongation and booting stage. Index of dispersion (ID), 2 value, Z test indicated a significant departure from a random distribution in all the rice fields. Degree of aggregation was comparatively low in controlled field than that of farmer’s field. All cultivars showed aggregated pattern of distribution during tillering stage. No infestation was recorded in four cultivars, Bahadur, Ijong, Kekua and Ranjit during stem elongation period in the year 2008. No population was recorded in Bahadur, Ijong, Kati Neoli, Ranjit, Ronga Bora and Solpuna in the year 2008 and in Bahadur, Bogilahi and Kekua in the year 2010 during booting period. Maximum aggregation was observed in Ronga bora during booting stage in the year 2008 and 2009. Kekua, Kola Joha, Saru Jahingia and Suagmoni in the year 2008 and Saru Jahingia and Toraboli in the year 2010 showed uniform distribution during booting stage. Unlike farmers field condition similar trend of distribution was observed during tillering, stem elongation and booting stage in controlled experimental field. Aggregation observed in both studies may be resultant of intrinsic behavior of the individuals, response to the food and habitat resources distribution, water level and hill density of the rice field. Concentration of a suitable resource in some areas has been considered as the prevailing cause of aggregation of most organisms [21]. In case of N. depunctalis, comparatively lower aggregation pattern in different cultivars in controlled experimental field indicated the role of varietal preference 44 Acad. J. Entomol., 5 (1): 41-46, 2012 in aggregation. However, aggregation pattern within the same cultivar in the controlled experimental field indicated the intrinsic behavior. Moth prefers to lay egg on the underside of leaves floating on water [15]. Therefore, the moth aggregates in such places to oviposit. Larvae crawl and grip with the aid of 6 thoracic legs, while the crochets are used to cling to its case and are not able to migrate to distant places without the aid of water. This resulted in aggregated distribution pattern of the larvae. N. depunctalis larvae need water for respiration and movement from plant to plant [15]. Egg desiccates if laid on aerial portion of the leaves [16]. In areas without water, most of the larvae were unable to survive and these may result higher density in areas with water and lower density in areas without water. Venugopalrao et al., Heinrichs et al. and Oyediran et al. [22-24] reported higher infestation of N. depunctalis with higher hill density. This was another factor that contributed to the higher degree of aggregation in farmer’s field condition. In farmer’s field condition, there was diversity in habitat pattern due to water resource, hill density and rice cultivar. It also resulted in aggregated pattern of distribution. During stem elongation and booting period water level was almost same in the rice fields, it was also a cause for decrease of the degree of aggregation in these periods compared to tillering stage. During booting stage water resources near the rice field may results in high density of larvae and higher aggregation. Sometime low infested areas during tillering stage may be a choice for the moth and the larvae during stem elongation and booting period as observed in Ronga bora during booting stage in the year 2008 and 2009. Based upon the obtained results it may be concluded that the spatial distribution of N. depunctalis larvae was aggregated. N. depunctalis larvae distribution pattern was determined by the oviposition pattern of the adults, type of rice cultivar, water level and hill density. The results indicate that use of sampling strategy that incorporate spatial distribution information will help to model crop loss more accurately strengthening IPM decision making and that spatially targeted applications of insecticide will reduce the cost of management of N. depunctalis and will also reduce the risk of toxicity from synthetic insecticides. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. REFERENCES 1. Southwood, T.R.E., 1978. Ecological methods, with particular reference to the study of insect populations. Chapman and Hall, London, 2nd. ed., pp: 524. 15. 45 Elliott, J.M., 1983. Some methods for the statistical analysis of sampling of benthic invertebrates. Freshwater Biological Association, Cumbria, pp: 176. Davis, P.M., 1993. Statistics for describing populations. In: L.P. Pedigo and G.D. Buntin, Handbook of sampling methods for arthropods in agriculture. CRC Press, Boca Raton. Shroff, K.D., 1919. Notes on miscellaneous pests in Burma. Proceedings of 3rd entomological meeting, Pusa, pp: 341-354. Sison, P.L., 1938. Some observations on the life history and control of the rice caseworm, Nymphula depunctalis Guenee. Philippines Journal of Agriculture, 9: 272-301. Alum, A.Z., 1967. Insect pest of rice in East Pakistan. Major insect pests of rice plant. John Hopkins press, pp: 633-655. Grist, D.H. and R.J.A.W. Lever, 1969. Pest of rice. London: Longmans, Greens and Co. Ltd., pp: 520. Chi, T.T.N., B.T.T. Tam, H.X. Dau, N.T. Khoa, N.T.P. Lan and T.R. Paris, 1995. Current status of rice pest management by farmers in direct-seeded rice and transplanted rice area. Omonrice, 4: 42-50. Vromant, N., A.J. Rothuis, N.T.T. Cuc and F. Ollevier, 1998. The effect of fish on the abundance of rice caseworm Nymphula depunctalis (Guenee) (Lepidoptera: Pyralidae) in direct-seeded, concurrent rice-fish fields. Biocont. Sc. and Techn., 8: 539-546. Pathak, M.D., 1975. Insect pests of rice. Manila (Philippines): International Rice Research Institute. Brenière, J., 1983. The principal insect pests of rice in West Africa and their control. Monrovia (Liberia): West Africa Development Association. Reissig, W.H., E.A. Heinrichs, J.A. Litsinger, K. Moody, L. Fiedler, T.W. Mew and A.T. Barrion, 1985. Illustrated guide to integrated pest management in rice in tropical Asia. Los Banos, the Philippines: IRR, pp: 411. Hill, D.S., 1987. Agricultural insect pests of the tropics and their control (2nd ed.). Cambridge University Press, Cambridge, pp: 746. Dale, D., 1994. Insect pests of the rice plant-their biology and ecology. In: E.A. Heinrichs, editor. Biology and management of rice insects. New Delhi: Wiley Eastern, pp: 363-485. Bandong, J.P. and J.A. Litsinger, 1981. Ovicidal activity of insecticides on the rice caseworm Nymphula depunctalis. Int. Rice Res. Newsl., 6(1): 17-18. Acad. J. Entomol., 5 (1): 41-46, 2012 16. Patgiri, P., 1997. Bioecological studies of Nymphula depunctalis (Guenee) (Pyralidae: Lepidoptera) and its management. Dissertation, Ph.D. Assam Agricultural University, Jorhat. 17. Zeller, 1852. Lepidoptera microptera quae, J.A. Wahlberg in cafrorum terra collegit, Lepid. micr. Caff. 18. Pound, R. and F.E. Clements, 1898. A method of determining the abundance of secondary species. Minn. Bot. Studies, 2: 19-24. 19. Patil, G.P. and W.M. Stiteler, 1974. Concepts of aggregation and their quantification: A critical review with some new results and applications. Res. Pop. Ecol., 15: 238-254. 20. Lloyd, M., 1967. Mean crowding. J. Anim. Ecol., 36: 1-30. 46 21. Ricklefs, R.E. and G.L. Miller, 2000. Ecology. W.H. Freeman and company, New York, 4th edition, pp: 822. 22. Venugopalrao, N., B.H.K. Rao and P.S. Reddy, 1982. Note on the effect of cultural practices on the incidence of insect pests of rice. Indian J. Agric. Sci., 52(1): 48-50. 23. Heinrichs, E.A., D.E. Johnson and A.A. Sy, 1993. Seedling age and plant spacing effects on insect pests in lowland irrigated rice. In WARDA annual report 1993. Bouaké (Côte d’Ivoire), West Africa Rice Development Association, pp: 43-44. 24. Oyediran, I.O. and E.A. Heinrichs, 1999. Seasonal abundance of rice feeding insects and spiders in continuously cropped lowland rice in West Africa. Insect Science. Appl., 19: 121-129.