Life tables of yellow stem borer, Scirpophaga incertulas on resistant and susceptible varieties of rice during post rainy seasons
V.Nandagopal, Anand Prakash, J.Rao, K.Vanitha and Niraj Kumar Singh
Abstract The changes in the population density of rice yellow Stem borer, Scirpophaga incertulas and their causes were quantified on susceptible cv.T N 1 and resistant cv.Ramaboita rice varieties planted during post rainy season in 2008 and 2009 under artificial infestation conditions. The 2008 life tables showed an increasing trend index of 1.85 and 1.23 of cohort population in both the cultivars respectively. The 2009 life tables showed an decreasing trend by 0.84 times on T N 1 while the cohort showed a trend index of 0.39 on Ramaboita indicating the decline in population density. The survivorship curves fitted to type IV of Slobodkin indicating that high mortality occurred in the early stages of the insect life cycle. The mortality factors included parasitization by hymenopterous parasitoids., predation by arthropods, infertility and wind blown loss in egg stage and dispersal loss in the first instar migration and some unknown losses, bacterial infection during the other larval instars. Pupal mortality was resulted by the failure of emergence due to, parasitization and bacterial infection and unknown reasons. The K-factor analysis showed suspected arthropod predation and dispersal loss during the first instar and loss due to migration and unknown causes in other larval instars were the key mortality factors. The hymenopterous parasitoids, suspected arthropod predators and pathogens were recorded and their role was discussed.
---------------------------------------------------------------------------------------------------------------------
Division of Crop Protection, Central Rice Research Institute (Indian Council of Agricultural Research), Cuttack-753 006, Orissa, India
Introduction
Among the five economically important species of moth borers infesting rice in India, only two species viz.,YSB, Scirpophaga incertulas and Stripped stem borer Chilo suppressallis are important in Peninsular India. The YSB damages the crop in the vegetative and booting stage causing dead heart and white ear head respectively (Sain and Prakash, 2008). The damage due to YSB infestation resulted in yield loss to an extent of from 22 to 50% in different cultivars tested (Panigrahi and Rajamani, 2009). The life cycle is completed in 35 to 45 days and the pest has four to six overlapping generations (Tripathy and Senapati, 1995). The borer makes a modest beginning by third week of January and progressed up to 3rd week of March with the growth of the crop (Anon, 2008-09). In order to predict the population build-up of the pest and to determine the role of natural mortality factors, the present investigation was taken up with a susceptible and resistant varieties of rice.
Materials and methods
The investigations were undertaken in the experimental fields at the Central Rice Research Institute, Cuttack, India. During 2008, two trials were conducted with the susceptible variety TN 1, and the resistant variety Ramaboita (Anon, 2008-09) planted during January (early main season planting) and in July (late main season planting) (Prakash et al., 2007).
Experimental field
The experiments were conducted during 2008 and 2009 in an extent of 0.08 ha in randomized block design. There were 4 replications. Each plot measured 1.1 x 1.1 m and was surrounded by a vacant space of 1.5 m to prevent larval migration from plot to plot. Totally 32 such plots were there in each trail. All the 8 growth developmental stages (as treatments ) were accommodated in each trail. Each plot had 10 rows with a spacing of 10 cm between plants and 15 cm between rows. Normal cultivation practices were followed.
Infestation
Since the population density of the pest was low in nature, infestation was created artificially in each plot. The gravid moths collected between 6 to 8 am from fields were introduced at the rate of 5 females and 5 males per plot on the day of emergence inside a cloth bag cage (100 cm diameter x 60 cm height) which was open at the bottom. The tops of 3 or 4 rice hills were introduced inside the cage and the bottom portion of the cage closed. The cage was held in position with bamboo sticks. The moth releases were done during the evening hours, and after 36 to 48 hrs after release the cages were removed and the leaves were observed for egg masses. The egg masses were individually labeled and the number of eggs in each mass was counted after they hatched in the laboratory on their possible desiccation, infertility, egg parasitisation, loss due to wind blown etc.
Sampling
The egg masses were observed daily until hatching for predation and parasitization. After hatching, the egg masses were brought to the laboratory and the number of egg shells and dead eggs were determined under a binocular microscope. The first census was made on the following day when all eggs had hatched. Subsequently, census were made on the 4th, 7th, 10th, 13th and 16th 19th 21st day after larval hatching. Since the sampling procedure was destructive, one plot in each replication was sampled at random during each census. All the tillers in a plot were cut and the number of total tillers counted and the tillers with dead heart or white Ear head were separated. The bored tillers were split open and the live and dead larvae were counted along with the number of bore holes. Subsamples of the larvae were preserved in 70% alcohol for head capsule measurements. Another subsample of larvae were reared in the laboratory for possible parasit emergence and disease occurrence. Dead larvae were also examined for the same.
The number of eggs hatched was recorded as the initial number of first instar larvae. The total number of living and dead larvae found within the tiller in the first census on the 4th day gave the number of first instar larvae that successfully bored into the tiller; the living larvae alone gave the number of larvae that successfully moulted into the second instar. Since cohorts of a known number of larvae of same age were involved, the number of naturally occurring larvae could be accounted separately by head capsule measurements. Similar observations were made in the subsequent census. The pupae collected during the fifth census were kept in the laboratory for moth emergence after sexing. The females upon emergence were dissected out and the number of eggs counted.
The data were summarized in a life table (Harcourt, 1969). The number of eggs unfertilized, parasitized or predated and the number of larvae failed to enter the rice tillers, died disappeared or remained in the stem/clump were tabulated and k values were calculated for each category of loss based on Varley and Gradwell’s (1960) method. These values were summed over the intervals to determine the contribution of each factor to change in cohort density (Varley et al., 1973).
Results
Life tables of YSB on susceptible T N 1 during 2008
Summary of life tables of YSB in post rainy season of 2008 in both the cultivars are given in Tables 1 and 2. It can be seen that about 42.25 % of the eggs failed to hatch due to various reasons (in cv. TN 1). The egg mortality was mainly from arthropod parasitization by hymenopteran parasitoids for 5.6%. The loss of eggs due to arthropod predation and inability to establish were more in first instar accounting 12.6%.
Table 1. Life tables of YSB on susceptible cv. T N 1 and resistant during Post Rainy 2008.
Age
interval(x)
|
No. living at beginning of x (ix)
|
Factors responsible for dx (dxf)
|
Number dying during x (dx)
|
Dx as a % of Ix (100 qx)
|
k
|
Egg 1
|
2088
|
Infertility
|
64
|
3.07
|
0.014
|
Parasitization
|
117
|
5.60
|
0.026
|
||
Arthropod predation
|
263
|
12.60
|
0.064
|
||
Blown by winds
|
438
|
20.98
|
0.135
|
||
Sub total
|
882
|
42.25
|
0.239
|
||
I instar
|
1206
|
Predation inability to establish
|
263
|
12.60
|
0.107
|
Diseased
|
118
|
5.65
|
0.058
|
||
Sub total
|
381
|
18.25
|
0.165
|
||
II instar
|
825
|
unknown diseased
|
263
|
12.60
|
0.167
|
Diseased
|
49
|
2.35
|
0.040
|
||
Sub total
|
312
|
14.95
|
0.207
|
||
III instar
|
513
|
unknown
|
113
|
5.41
|
0.108
|
Sub total
|
113
|
5.41
|
0.108
|
||
IV instar
|
400
|
Migration + unknown
|
84
|
4.02
|
0.102
|
Predation
|
63
|
3.02
|
0.097
|
||
Sub total
|
147
|
7.04
|
0.199
|
||
V instar
|
253
|
Migration + unknown
|
68
|
3.26
|
0.136
|
Predation
|
54
|
2.59
|
0.150
|
||
Sub total
|
122
|
5.85
|
0.286
|
||
Pupa
|
131
|
Parasitization/Predation/ Disease
|
16
|
0.77
|
0.057
|
Failure to emerge
|
11
|
0.53
|
0.044
|
||
Sub total
|
27
|
1.3
|
0.101
|
||
Adult
|
104
|
K
|
1.305
|
Number of female moths emerged:53
Number of eggs produced :3864
Trend index (N2/N1): 1.85
Life tables of YSB on resistant cv. Ramaboita during 2008
It can be seen that about 45.67 % of the eggs laid in cv. Ramaboita failed to hatch due to various reasons. The egg mortality was mainly from arthropod parasitization (5.07%), infertility (3.25%), los due to wind blown (21.92%). Similarly, in the first instar, arthropod predation was accounted for nearly 15.43% (Table 2).
Table 2. Life tables of YSB on susceptible cv. Ramabita and resistant during Post Rainy 2008.
Age
interval(x)
|
No living at beginning of x (ix)
|
Factors responsible for dx (dxf)
|
Number dying during x (dx)
|
Dx as a % of Ix (100 qx)
|
k
|
Egg 2
|
1756
|
Infertility
|
57
|
3.25
|
0.014
|
Parasitization
|
89
|
5.07
|
0.023
|
||
Arthropod predation
|
271
|
15.43
|
0.080
|
||
Blown by winds
|
385
|
21.92
|
0.147
|
||
Sub total
|
802
|
45.67
|
0.264
|
||
I instar
|
954
|
Predation + inability to establish
|
186
|
10.59
|
0.094
|
Parasitization
|
63
|
3.59
|
0.037
|
||
Sub total
|
249
|
14.18
|
0.131
|
||
II instar
|
705
|
unknown
|
124
|
7.06
|
0.084
|
Diseased
|
106
|
6.04
|
0.087
|
||
Sub total
|
230
|
13.1
|
0.171
|
||
III instar
|
475
|
Diseased
|
89
|
5.07
|
0.090
|
unknown
|
32
|
1.82
|
0.038
|
||
Sub total
|
121
|
6.89
|
0.128
|
||
IV instar
|
354
|
Migration
|
89
|
5.07
|
0.126
|
Predation
|
71
|
4.04
|
0.135
|
||
Sub total
|
160
|
9.11
|
0.261
|
||
V instar
|
194
|
Migration + unknown
|
23
|
1.31
|
0.055
|
Predation
|
26
|
1.48
|
0.072
|
||
Sub total
|
49
|
2.79
|
0.127
|
||
Pupa
|
145
|
Parasitization
|
8
|
0.46
|
0.025
|
Failure to emerge
|
52
|
2.96
|
0.207
|
||
Sub total
|
60
|
3.42
|
0.232
|
||
Adult
|
85
|
K
|
1.314
|
Number of female moths emerged:46
Number of eggs produced :2162
Trend index (N2/N1): 1.23
The mortality was caused primarily by the failure of the young larvae to bore into the tillers quickly. The dispersion of the newly hatched larvae and exposure to predation during their dispersal appeared to be the major mortality factors, although it could not be measured directly. The migration of the first instars immediately after hatching was by ballooning, mechanical touch by adjacent foliage, or by swimming of those larvae fall in water. While upward wind blows, the neonate larvae blown through the salivary threads. Possibly, the spiders of many species grab the opportunity of large scale hatching of neonate larvae. From second instar onwards the losses occurred due to disease and unknown reasons. In general, the mortality of late instars was more in the first season planted crop than in the other one.
The number of larvae that successfully pupated was 131 (2.27 % of cohort population) and 82 (2.25 %) in the years 2008 and 2009 respectively in TN 1 in post rainy season. Population density was further reduced during pupal stage. Nearly 145 (8.26%) and 76 ( 2.09%) of the entered pupal stage in the cv. Ramaboita respectively. Pupal mortality was caused by the failure of moths to emerge due to unknown reasons and parasitization. Only 4.98 and 4.85 % of the cohort population traced reached the adult stage in both the cvs during 2008. It was 1.56 and 0.88% reached adults during 2009 in both the cultivars. Laboratory reared moths from field collected pupae showed a sex ratio of 1:04 (male:female) in TN 1 and 1: 0.87 in cv Ramaboita. The expected density of eggs was calculated based on dissection of females and it revealed an increasing trend of 1.85 in cv. TN 1 and 1.23 in cv. Ramaboita of cohort population in post rainy season planted crops respectively during 2008. While it was 0.84 and 0.39 population trend during 2009 in both the cultivars respectively.
Table 3. Life tables of YSB on susceptible cv. T N 1 and resistant during Post Rainy 200.
Age
interval(x)
|
No living at beginning of x (ix)
|
Factors responsible for dx (dxf)
|
Number dying during x (dx)
|
Dx as a % of Ix (100 qx)
|
k
|
Egg 3
|
3647
|
Infertility
|
436
|
11.96
|
0.055
|
Parasitization
|
247
|
6.77
|
0.035
|
||
Arthropod predation
|
186
|
5.10
|
0.028
|
||
Blown by winds
|
412
|
11.30
|
0.070
|
||
Sub total
|
1281
|
35.13
|
0.188
|
||
Larva I
|
2366
|
Predation + inability to establish
|
435
|
11.93
|
0.088
|
Disease
|
182
|
4.99
|
0.043
|
||
Sub total
|
617
|
16.92
|
0.131
|
||
II instar
|
1749
|
unknown
|
303
|
8.31
|
0.083
|
Diseased
|
164
|
4.50
|
0.052
|
||
Sub total
|
467
|
12.81
|
0.135
|
||
III instar
|
1282
|
unknown
|
265
|
7.27
|
0.101
|
Diseased
|
234
|
6.42
|
0.114
|
||
Sub total
|
499
|
13.69
|
0.215
|
||
IV instar
|
783
|
Migration + unknown
|
218
|
5.98
|
0.142
|
Predation
|
106
|
2.91
|
0.090
|
||
Sub total
|
324
|
8.89
|
0.232
|
||
V instar
|
459
|
Migration + unknown
|
164
|
4.50
|
0.192
|
Predation
|
213
|
5.84
|
0.556
|
||
Sub total
|
377
|
10.34
|
0.748
|
||
Pupa
|
82
|
Parasitization/Predation/ Disease
|
13
|
0.36
|
0.075
|
Failure to emerge
|
12
|
0.33
|
0.083
|
||
Sub total
|
25
|
0.69
|
0.158
|
||
Adult
|
57
|
K
|
1.807
|
Number of female moths emerged:38
Number of eggs produced:3078
Trend index (N2/N1): 0.84
The life tables of YSB on susceptible cv. T N 1 and resistant cv. Ramaboita are presented in Tables 3 and 4 for the post rainy season of 2009. In both the varieties, 35 and 44% of the eggs laid did not hatch. Natural enemies including hymenopteran parasites and predators accounted for the maximum mortality, the other mortality factors being desiccation, infertility and loss due to wind blow.
Almost the similar proportion of cohorts entered the first instar stage in both varieties. Significant difference was observed in the population that successfully completed the first instar stage. The suspected arthropod predation and dispersal loss of cohorts before boring into the tillers was (11.3%) in the cv. TN 1, while it was 15.59 % in the resistant Ramaboita. Only 2.25% of the cohorts reached the pupal stage on TN 1. compared to 2.08 % on Ramaboita. Apart from suspected predation and dispersal loss during the first instar, significant change in the population density occurred due to migration and unknown loss in the later instars. Loss of cohorts due to this factor is more in the fifth instar compared to earlier instars in both the varieties.
Table 4. Life tables of YSB on Resistant cv. Ramabiota during Post Rainy 2009.
Age
interval(x)
|
No living at beginning of x (ix)
|
Factors responsible for dx (dxf)
|
Number dying during x (dx)
|
Dx as a % of Ix (100 qx)
|
k
|
Egg 4
|
3656
|
Infertility
|
214
|
5.85
|
0.026
|
Parasitization
|
917
|
25.08
|
0.135
|
||
Arthropod predation
|
345
|
9.44
|
0.064
|
||
Blown by wind
|
137
|
3.75
|
0.028
|
||
Sub total
|
1613
|
44.12
|
0.253
|
||
Larva
|
2043
|
Predation + inability to establish
|
570
|
15.59
|
0.142
|
diseases
|
326
|
8.92
|
0.109
|
||
Sub total
|
896
|
24.51
|
0.251
|
||
II instar
|
1147
|
unknown
|
331
|
9.05
|
0.148
|
Disease
|
120
|
3.28
|
0.069
|
||
Sub total
|
451
|
12.33
|
0.217
|
||
III instar
|
696
|
unknown
|
219
|
5.99
|
0.164
|
Disease
|
216
|
5.91
|
0.262
|
||
Sub total
|
435
|
11.9
|
0.426
|
||
IV instar
|
261
|
Migration + unknown
|
74
|
2.02
|
0.145
|
Diseased
|
45
|
1.23
|
0.120
|
||
Sub total
|
119
|
3.25
|
0.265
|
||
V instar
|
142
|
Migration + unknown
|
36
|
0.98
|
0.127
|
Diseased
|
30
|
0.82
|
0.144
|
||
Sub total
|
66
|
1.8
|
0.271
|
||
Pupa
|
76
|
Failure to emerge
|
44
|
1.20
|
0.376
|
Sub total
|
44
|
1.2
|
0.376
|
||
Adult
|
32
|
K
|
2.059
|
Number of female moths emerged :18
Number of eggs produced (N2) :1426
Trend index ( N2/ N1) :0.39
The proportion of adults failed to emerge from pupae in TN 1 was 49% and it was and 45.9% on resistant cv. Ramaboita which could not be explained. But only 2.54 % and 2.62% of the cohorts reached the adult stage on the resistant cvs TN1 and .Ramaboita respectively during 2008. It was 1.04% and 0.88% in cv. TN 1 a known susceptible cultivar.
Survivor curve
|
Fig.1. Survivorship curve for YSB on susceptible and resistant rice during 2008
|
|
Eggs I II III IV V Pupae Adults
|
|
lx
|
|
Stage of insect (x)
|
|
Eggs I II III IV V Pupae Adults
|
|
Fig.2. Survivorship curve for YSB on susceptible and resistant rice during 2009
|
|
lx
|
|
Stage of insect (x)
|
Differences in population trend due to season of planting and varieties under cultivation are best shown through survivorship curves (Figures 1 and 2). It is indicated in Figure 1 that the larval mortality is more rapid in the early instars in late main season crop compared to early main season crop, though there was not much difference in the ultimate survival to pupal and adult stages. In 2009, the survivorship curves (Figure-2) showed that egg mortality was apparently unaffected by the varietal resistance. But there was a high mortality of first instar larvae on the resistant cv.Ramaboita compared to susceptible TN1 which ultimately determines the population density on these two varieties. All the curves fitted to type IV of Slobodkin indicating the high mortality occurred in the early stages of the insect life cycle.
K-factor analysis
Contribution of each factor’s to ultimate survivorship of population in the cohorts, their k values were summed over the development period and expressed as a percentage of the total (Table 5). On an average 75% of cohort population died during the larval stage in TN1 and 63 % of the larval population died in 2009 due to various reasons. In the susceptible rice cultivar, 48% of the total loss in the cohorts was caused by migration and unknown causes followed by 15.97% loss due to inability of the first instar larvae to bore into the tillers and suspected predation and dispersal losses during this period. On the other hand in the resistant cv. Ramaboita, maximum loss of 7.76% was caused by suspected arthropod predation and dispersal loss during the first instar stage. Of course this is followed by loss due to migration and unknown causes in the later instars. These are the two key mortality factors causing significant change in the population density of YSB. When all the mortality factors in the larval stage were considered together, they contributed to about 63% loss in the cohorts in variety Ramaboita.
Infertility was not much as it was only 2.53 and 1.32% in both the cvs. Natural enemies played significant role causing egg mortality. The mortality in the pupal stage was also considerably high to an extent of more than 9.4 and 20.2 % respectively in both the years in post rainy seasons.
Table 5. Summary of k-values in cohorts on susceptible and resistant varieties
Cause of loss/ mortality in life stage
|
TN 1
|
Mean
|
%
|
Ramaboita
|
Mean
|
%
|
|||||
2008
|
2009
|
2008
|
2009
|
||||||||
k-
value
|
k-
value
|
k-
value
|
k-
value
|
||||||||
Egg
|
|||||||||||
Infertility
|
0.014
|
0.055
|
0.035
|
2.53
|
0.014
|
0.026
|
0.02
|
1.32
|
|||
Parasitization
|
0.026
|
0.035
|
0.031
|
2.24
|
0.023
|
0.135
|
0.08
|
5.20
|
|||
Arthropod predation
|
0.064
|
0.028
|
0.046
|
3.32
|
0.080
|
0.064
|
0.072
|
4.74
|
|||
Blown by winds
|
0.135
|
0.070
|
0.103
|
7.44
|
0.147
|
0.028
|
0.088
|
5.75
|
|||
Sub-total
|
0.239
|
0.188
|
0.215
|
15.53
|
0.264
|
0.253
|
0.26
|
17.01
|
|||
Larvae
|
|||||||||||
Arthropod predation + inability to establish
|
0.354
|
0.088
|
0.221
|
15.97
|
0.094
|
0.142
|
0.118
|
7.76
|
|||
Diseased
|
0.098
|
0.209
|
0.154
|
11.13
|
0.214
|
0.704
|
0.459
|
30.20
|
|||
Migration & unknown
|
0.513
|
0.815
|
0.664
|
47.98
|
0.177
|
0.584
|
0.381
|
25.03
|
|||
Sub-total
|
0.965
|
1.112
|
1.039
|
75.08
|
0.485
|
1.43
|
0.958
|
62.99
|
|||
Pupae
|
|||||||||||
Parasitization Predation/ Disease
|
0.057
|
0.075
|
0.066
|
4.77
|
0.025
|
0.00
|
0.013
|
0.82
|
|||
Failure to emerge
|
0.044
|
0.083
|
0.064
|
4.62
|
0.207
|
0.376
|
0.292
|
19.18
|
|||
Sub-total
|
0.101
|
0.158
|
0.13
|
9.39
|
0.232
|
0.376
|
0.305
|
20.00
|
|||
Total
|
1.384
|
100
|
1.520
|
100
|
|||||||
Natural enemies
There were 11148 eggs observed all together in both the years in post rainy seasons in both the varieties. Among the eggs 1370 eggs were found to be parasitized by Trichogramma japonicum and Telenomus dignoides jointly which worked out to be 12.3%. Since the eggs were covered by the annul hairs of the females after oviposition, the activities of the egg parasites were very much restricted owing to the physical barrier of hairs.
Arthropods predated 10 and 11% of the eggs and possibly they may play a much more important role in the predation of the neonate larvae before entering the tillers. Natural enemies recorded/ observed as suspected predators of the YSB are given in table 6.
Table 6. Natural enemies recorded/ observed as suspected predators of the YSB
Parasitoids
|
Predators
|
Pathogens
|
Hymenoptera: Trichogrammatidae
Trichogramma japonicum (Ashm.)
|
Odonata: Libellulidae
Urothemis signata (Ramb.)
Orthetrum sabina (Drury)
|
Unidentified bacterium is recorded and yet to be identified- may be saprophytic
|
Hymenoptera: Scelionidae:
Telenomus dignoides (Nixon)
|
Odonata: Brachydiplactinae
Potamarcha obscura (Ramb.) |
|
Odonata: Sympetrinae
Brachythemis contaminate (Fabr.)
|
||
Odonata: Rhyotheminae
Trithemis pallidinervis (Kirby)
|
||
Pentatomidae
Amyotea malabarica (Fabr.)
|
||
Orthoptera:Gryllidae
Metioche vittaticollis (Stal)
|
||
Araneae:Tetragnathidae
|
||
Araneae: Lycosidae
Lycosa pseudoannulata,
|
||
Araneae: Thomisidae Thomisus cherapunjeus Tikader
|
||
Discussion
The mortality of the early stages in YSB confined mostly to eggs and Ist and IInd instars. The causes for egg mortality could be estimated only for egg parasitoids such as Trichogramma japonicum (Ashm.) and Telenomus dignoides (Nixon). Beyond this assessment, further mortality at the egg stages could be due to wind blow and a small quantity by the predations. According to Senapati and Panda (1999) the young larvae immediately after hatching crawl around egg mass and spread upwards on the lamina and using their silkan threads made from saliva float in the air and lands adjacent plants. After landing they move downwards and enter the stem just above water level by cutting the leaf sheath. Under Orissa conditions, on an average 27% grain yield loss has been estimated in semi-deep water rice (Tripathy and Senapati, 1995).
The larval dispersion which appeared to be the highest loss during the early larval stages. Movements of insects at the early stage have both advantage and disadvantages. The advantage being spreading to larger area for their survival with out much competition for food and space, while the disadvantage being exposure to predation and death due to starvation when landing on non-hosts (Manson, 1976). The dispersion (migration) during late instars (IVth and Vth instars) accounts for considerable mortality. The full grown larvae comes out of stem / tunnel and after moulting to pre-pupa, forms silken cocoons and pupates inside the leaf sheeth or in stubbles.
Egg parasitoids of yellow stem borer such as T. dignoides, T. rowani, T. dignus Gahan, Tetrastichus schoenobii (Ferr.) and T. japonicum are quite effective in controlling the pest population. Among the three parasitoids contribution of individual parasitoids was 52.8% due to Telenomus dignoides, 40.5% due to T. schoenobii and 6.6% due to T. japonicum (Behera et al. 2003). In our study also there were 5.6 and 5.1 % parasitization by combined action of egg parasites in Post Rainy 2008 in both cvs. TN 1 and in Ramaboita. During Rainy 2009 season, it was 6.8 % in TN1 and exceptionally 25.1% in Ramaboita.
T. dignoides is the most frequent, widely distributed and effective parasitoid of all the egg parasitoids available. The extent of egg mass parasitism in Orissa ranged from 7.1 to 35.2%. The cumulative egg mass parasitism due to all the egg parasitoids reported from different parts of India ranged from 4 to 97.2% (Senapati and Panda, 1999). Behera and Prakash (2004) revived the Indian Insect Predators on Insect Pests of Rice, wherein they have dicussed on the role of Odonates and Penatomids. Among the spiders Oxyopes sp. was more abundant. In all the larval instars, Species from the genera Cheiracanthium, Lycosa, Plexippus, Tetragnatha and Thomisus were found in al the rice ecologies (Ansari and Pawar, 1992). Twenty species of spiders recorded in the rice ecosystem from Jammu and Kashmir increased in number gradually with the growth of rice plants and almost doubled from August to September (Thakur et.al., 1995).
The role of predatory insects belonging to Odonates include Ictinogomphus rapax (Ramb.), Macrodiplactidae, Urothemis signata (Ramb.), Libellulidae: Orthetrum sabina (Drury), Libellulidae: Potamarcha obscura (Ramb.), Sympetrinae : Acisoma panorpoides (Ramb.), Diplacodes trivialis (Ramb.) etc. (Israel and Padmanabhan, 1976). The Pentatomidae, Amyotea malabarica (Fabr.) (Pati and Mathur,1986) and Andrallus spinidens (Fab.) (Rao, 1965;Rao and Rao,1979) do cause mortality of the larvae of the YSB. Since predators usually consumes their prey and leave no scope for direct estimation.
The life tables of both susceptible and resistant were, when compared, a high level of population loss occurred in resistant variety Ramaboita compared to the susceptible TN 1. The neonate larvae are less to enter the tillers in Resistant Ramaboita, probably, due to thicker leaf sheath having stem hardness due to silica and wax content (Chandramani et al., 2009). Out of 86.89% survival of the larvae after egg stage 61.42% accounted for mortality due to various factors such as predation, migration, disease etc. in TN 1 a susceptible and it was 65.78% mortality in resistant Ramaboita rice.
To conclude, predation in eggs and first instar larvae and migration and unknown causes in late instar stages are the important factors operating the mortality of the YSB in rice during post rainy season.
References
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