The relative abundance of major insect pests and natural enemies were studied at three different growth stages in seven high yielding varieties, Ranjit, Ranjit Sub-1, Bahadur, Bahadur Sub-1, Gitesh, Swarna Sub-1 and Shraboni in kharif and rabi season at Titabar, Assam during 2020–2022. White stem borer (47.49%) was the most predominantly abundant species followed by yellow stem borer (41.97%) among seven major insect pest species e.g., leaf folder, gall midge, caseworm, whorl maggot and gundhi bug in kharif season. But, yellow stem borer (48.69%) was abundantly observed followed by white stem borer (40.63%) among all major insect pests during rabi season. Both the species of stem borer gradually increased and reached peak due to availability of enough food at reproductive stage and decreased in the ripening stage in both the season comparatively higher in rabi season Leaf folder was the most abundant leaf feeder and its population abundance was higher in Shraboni (9.89%) followed by Bahadur (9.67%), Bahadur Sub-1 (9.61%) and Swarna Sub-1 (8.47%) at vegetative stage which reached peak at reproductive stage during kharif season. The population of caseworm and whorl maggot was higher in vegetative stage and subsequently declined till the reproductive stage in kharif season. No caseworm population was observed during rabi season due to scanty rainfall. The gundhi bug population was higher in rabi season as compared to khairf season. Spider was the most prominent predator followed by coccinellid beetle in all the seven varieties during kharif and rabi season.
Research Article
Relative abundance of major insect pests and natural enemies in high yielding rice varieties of Assam
https://doi.org/10.21203/rs.3.rs-2756112/v1
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Relative abundance
Rice
Stem Borer
Leaf Folder
Spider
Being the staple food for almost two third of the world population, rice has been cultivated in Assam since time immemorial occupying approximately 25.03 lakh hectares with a production and productivity averaging 5.10 million tonnes and 2086 kg/ha (Anonymous 2022). The rice yield has been stagnant in Assam for the last three decades despite the improved varieties and technologies due to several factors. One of the major limiting factors of rice production is biotic stress. Rice crop is subjected to a considerable damage by nearly 300 species of insect pests, out of which only 23 species are serious pests of rice (Pasalu and Katti 2006) recorded yield loss estimated about 27.9 per cent (Mondal et al. 2017). The continuous rice cultivation has created favourable condition for the insect pests as well as the prevailing warm and humid climatic conditions have favoured rapid multiplication of insect pests and diseases.
Rice ecosystem is a unit of biodiversity in which a variety of flora and fauna including weeds and amphibians, bird and rodent live interacting with each other and forming a food web. According to Hurlbert et al. (1989) the numbers of species and individuals and their diversities and evenness were distinctly different according to habitat, type of field, environment, varieties, cropping patterns, cultivation practices and growth stage of the rice. Rice ecosystem plays an important role in managing insect pests by regulating the insect pest population naturally by their existing natural enemies (Conway and Pretty 1991). The important principle of integrated pest management is to maximize natural control and therefore the temporal changes in arthropod abundance, diversity and species richness are important considerations in designing pest management strategies.
Considering the yield loss due to major insect pests and the significant role of natural enemies in rice ecosystem, an attempt was made to assess the abundance and diversity of stem borers and natural enemies in rice both in kharif and rabi season. The present study was formulated in order to develop the components of an integrated management approach against major insect pests in the cropping system, characterized by the interaction between the pests and natural enemies.
The field experiment was laid out both in kharif and rabi season in the rice field of AAU-Assam Rice Research Institute, Titabar, Jorhat to study the relative abundance of insect pests and natural enemies in the seven high yielding rice varieties viz., Ranjit, Ranjit Sub-1, Gitesh, Bahadur, Bahadur, Sub-1, Swarna Sub-1, and Shraboni during 2020–2022. The thirty days old seedlings were transplanted following recommended agronomic practices in the plot size of 100 sqm (10 m x 10 m) each having a gap of 50 meter between each variety during kharif and rabi season. The design of experiment was laid out in randomized block design. No insecticide was applied in the experimental plots during the entire course of the study.
A fine nylon cloth sweep net (30 cm diameter) was used to collect the insect pests of rice and their natural enemies. In each variety, twenty numbers of complete sweep was made to collect the insect pests and natural enemies which were taken as one replication. Ten replications were taken to count the population of the major insect pests and natural enemies at an interval of ten days starting from 15 days after transplanting till the harvesting of each of the varieties both in kharif and rabi rice.
Population estimation of dominant natural enemies like coccinellid beetles and ground beetles (number per hill), spider (number per square meter) and damsel fly and dragon fly (number per square meter) were recorded early in the morning. The insect pest and natural enemies of each sample were sorted out in labelled containers containing 70% alcohol. The collected samples were counted carefully and identified. The relative abundance of major insect pests and natural enemies were calculated in vegetative, reproductive and ripening stage in each variety with the help of the formula as described by Baskaran et al. (2017).
Statistical Analysis
Data on damage incidence were transformed by square root transformation and analyzed by using IBM SPSS Statistics for ANOVA and means were separated by critical difference (CD) value.
During the entire course of study, all total twelve species of insect pests have been observed in all the rice varieties. Out of which, the major insect pests like white stem borer, yellow stem borer, leaf folder, gall midge, caseworm, whorl maggot and gundhi bug were considered in the present study.
Two stem borer species (Scirpophaga incertulas and S. innotata), rice leaf folder (Cnaphalocrosis medinalis), rice gall midge (Orseolia oryzae), caseworm (Paraponyx stagnalis), whorl maggot (Hydrellia philippina) and gundhi bug (Leptocorisa oratorious) were observed both in kharif as well as rabi season and considered as major as well as persistent in the rice field.
The results on the study of the relative abundance of major insect pests in rice are interpreted in three different growth stages, viz., vegetative, reproductive and ripening stage in kharif season and are depicted in Tables 1, 2 and 3 respectively. The population abundance of six persistent insect pests, viz.,white stem borer, yellow stem borer, rice leaf folder, gall midge, caseworm, whorl maggot and gundhi bug were found to be fluctuated in all the seven rice varieties viz., Ranjit, Ranjit Sub-1, Bahadur, Bahadur Sub-1, Gitesh, Swarna Sub-1 and Shraboni and is presented in Fig. 1.
| Variety | White stem borer | Yellow stem borer | Rice leaf folder | Gall midge | Caseworm | Whorl maggot |
|---|---|---|---|---|---|---|
| Ranjit | 31.45a (34.11) | 25.13b (30.09) | 6.46c (14.72) | 4.31d (11.98) | 5.34d (13.36) | 4.27d (11.93) |
| Ranjit Sub-1 | 32.26a (34.61) | 24.72b (29.81) | 6.30c (14.54) | 3.71d (11.11) | 4.86d (12.74) | 4.12d (11.71) |
| Gitesh | 30.26a (33.37) | 24.14b (29.43) | 6.61c (14.90) | 3.22d (10.34) | 4.29d (11.95) | 3.77d (11.20) |
| Bahadur | 38.26ac (38.21) | 34.41bc (35.92) | 9.67aa (18.12) | 7.67c (16.08) | 8.45cd (16.90) | 6.77c (15.08) |
| Bahadur Sub-1 | 39.57ac (38.98) | 33.63bc (35.44) | 9.61aa (18.06) | 7.45c (15.84) | 8.83cd (17.29) | 6.89c (15.22) |
| Swarna Sub-1 | 37.49c (37.76) | 29.86a (33.12) | 8.47aa (16.92) | 6.90c (15.23) | 8.29cd (16.73) | 7.67c (16.08) |
| Shraboni | 39.95ac (39.20) | 34.95bc (36.24) | 9.89aa (18.33) | 6.86c (15.18) | 8.94cd (17.40) | 7.27c (15.64) |
| CD (0.01) | 2.37 | 2.43 | 1.89 | 1.73 | 1.56 | 1.69 |
| CV (%) | 11.67 | 10.45 | 9.83 | 8.56 | 7.59 | 8.43 |
| Means within a column followed by same letters are not significantly different by critical difference (p ≥ 0.1). Figure in parentheses is arc sine transformed value. | ||||||
| No of sweeps per replication = 20 Nos. ; No of Replication = 10 | ||||||
| Variety | White stem borer | Yellow stem borer | Rice leaf folder | Gall midge | Caseworm | Whorl maggot | Gundhi bug |
|---|---|---|---|---|---|---|---|
| Ranjit | 38.89b (38.58) | 30.96ab (33.81) | 7.61a (16.01) | 5.42ac (13.46) | 1.24c (6.39) | 1.05c (5.88) | 7.39d (15.77) |
| Ranjit Sub-1 | 39.78b (39.10) | 31.73ab (34.28) | 7.52a (15.92) | 6.11ac (14.31) | 1.25c (6.42) | 1.45c (6.92) | 7.45d (15.84) |
| Gitesh | 38.23b (38.19) | 30.47ab (33.50) | 7.74a (16.15) | 6.21ac (14.43) | 1.21c (6.32) | 1.25c (6.42) | 7.00d (15.34) |
| Bahadur | 45.65cd (42.50) | 39.56bc (38.97) | 15.31aa (23.03) | 8.53ca (16.98) | 3.33c (10.51) | 2.54c (9.17) | 8.80ca (17.26) |
| Bahadur Sub-1 | 46.26cd (42.86) | 40.91bc (39.76) | 16.94aa (24.30) | 8.69ca (17.14) | 3.92c (11.42) | 3.19c (10.29) | 9.01ca (17.47) |
| Swarna Sub-1 | 45.23cd (42.26) | 38.78bc (38.52) | 15.40aa (23.11) | 8.97ca (17.43) | 3.54c (10.84) | 3.43c (10.67) | 9.15ca (17.61) |
| Shraboni | 47.49cd (43.56) | 41.97bc (40.38) | 15.47aa (23.16) | 9.14ca (17.60) | 3.09c (10.12) | 1.46c (6.94) | 9.26ca (17.72) |
| CD (0.01) | 2.27 | 2.16 | 1.96 | 1.89 | NS | NS | 1.35 |
| CV (%) | 11.89 | 10.56 | 9.32 | 8.29 | 0.67 | 0.45 | 6.37 |
| Means within a column followed by same letters are not significantly different by critical difference (p ≥ 0.1). Figure in parentheses is arc sine transformed value. | |||||||
| No of sweeps per replication = 20 Nos. ; No of Replication = 10 | |||||||
| NS = Non significant | |||||||
| Variety | White stem borer | Yellow stem borer | Gall midge | Gundhi bug |
|---|---|---|---|---|
| Ranjit | 31.79c (34.32) | 23.37a (28.91) | 1.35b (6.67) | 12.67ac (20.85) |
| Ranjit Sub-1 | 30.30c (33.40) | 22.26a (28.15) | 0.77b (5.03) | 10.99ac (19.36) |
| Gitesh | 29.47c (32.88) | 21.84a (27.86) | 0.67b (4.70) | 11.07ac (19.43) |
| Bahadur | 41.66ba (40.20) | 28.10cd (32.01) | 1.54b (7.13) | 16.47bc (23.94) |
| Bahadur Sub-1 | 40.29ba (39.40) | 29.02cd (32.60) | 1.85b (7.82) | 15.23bc (22.97) |
| Swarna Sub-1 | 41.78ba (40.27) | 28.55cd (32.30) | 1.54b (7.13) | 15.95bc (23.54) |
| Shraboni | 43.67ba (41.36) | 29.43cd (32.85) | 2.45b (9.01) | 16.52bc (23.98) |
| CD (0.01) | 2.34 | 2.28 | NS | 1.78 |
| CV (%) | 11.45 | 10.26 | 0.53 | 8.37 |
| Means within a column followed by same letters are not significantly different by critical difference (p ≥ 0.1). Figure in parentheses is arc sine transformed value. | ||||
| No of sweeps per replication = 20 Nos. ; No of Replication = 10 | ||||
| NS = Non significant | ||||
Kharif season
White stem borer was recorded as highly abundant among all the insect pests observed at vegetative stage during kharif season. The mean relative abundance of white stem borer ranged from 30.26 per cent to 39.95 per cent at vegetative stage in the seven varieties (Table 1). The population abundance of white stem borer was significantly higher in Shraboni variety followed by Bahadur Sub-1, Bahadur and Swarna Sub-1, whereas it was found to be significantly lower in Gitesh followed by Ranjit and Ranjit Sub-1.Yellow stem borer was the second highest abundant insect pest, of which population abundance varied from 24.14 per cent to 25.13 per cent in Gitesh, Ranjit Sub-1 and Ranjit that were significantly lower than that of Shraboni (34.95%), Bahadur (34.41%), Bahadur Sub-1 (33.63%) and Swarna Sub-1 (29.86%). Rice leaf folder was the most abundant leaf feeding insect pest, of which population abundance varied from 6.30 per cent to 6.61 per cent in Ranjit Sub − 1, Ranjit and Gitesh without significant difference with each other. Higher leaf folder population was recorded in Shraboni (9.89%) followed by Bahadur (9.67%), Bahadur Sub-1 (9.61%) and Swarna Sub-1 (8.47%). The population of one of the most important tiller damaging insect pest, i.e., gall midge was minimum in Gitesh (3.22%) which was not significantly different with that of Ranjit Sub − 1 (3.71%) and Ranjit (4.31%), whereas it was maximum in Bahadur (7.67%) without significant difference with Shraboni (6.86%), Bahadur Sub-1 (7.45%) and Swarna Sub-1 (6.90%). The relative abundance of caseworm was higher in Shraboni (8.94%) followed by Bahadur Sub-1, Bahadur and Swarna Sub-1 and it was lower in Gitesh (4.29%) as compared to Ranjit and Ranjit Sub-1. The population of whorl maggot was 7.67 per cent and 7.27 per cent in the variety Swarna Sub-1 and Shraboni respectively that was significantly higher with that of Gitesh, Ranjit and Ranjit Sub-1 at vegetative stage.
Both the species of stem borer was found to be relatively more abundant in reproductive stage than that of vegetative stage during kharif season (Table 2). The highly abundant insect pest was white stem borer, of which relative abundance varied from 38.23 per cent to 47.49 per cent in the seven varieties at reproductive stage. White stem borer population was higher in Shraboni (47.49%) that was not significantly different with Bahadur, Bahadur Sub-1 and Swarna Sub-1, whereas white stem borer population was lower in Gitesh without significant difference between Ranjit and Ranjit sub-1. Similar trend was followed in case of yellow stem borer with higher population in Shraboni and lower in Gitesh. The population of rice leaf folder was recorded comparatively higher in this stage than that of vegetative stages in all the varieties. Higher leaf folder population was observed in Bahadur Sub-1 (16.94%) followed by Shraboni (15.47%), Swarna Sub-1 (15.40%) and Bahadur (15.31%) without significant difference. On the other hand, lower leaf folder population was recorded in Ranjit Sub-1 (7.52%) followed by Ranjit and Gitesh. The gall midge population was relatively more abundant in the variety Shraboni (9.14%) than other varieties, whereas it was lower in Ranjit (5.42%) followed by Ranjit Sub-1 and Gitesh. Caseworm and whorl maggot population reduced drastically at reproductive stage of the crop. Gundhi bug population appeared at reproductive stage with lower population abundance in Gitesh (7.00%) and higher population in Shraboni (9.26%).
White stem borer and yellow stem borer were found to be relatively more abundant species in ripening stage also followed by gundhi bug in all the varieties during kharif season (Table 3). The population of white and yellow stem borer were maximum in Shraboni (43.67% and 29.43%) as compared to Gitesh (29.47% and 21.84%), respectively. The gundhi bug population was significantly higher in Shraboni (16.52%) than that of other varieties and lower population was observed in Ranjit Sub-1 (10.99%) at the ripening stage. The population of gall midge reduced drastically during ripening stage. No population of leaf folder, caseworm and whorl maggot was recorded in this stage.
Rabi season
On the contrary to kharif season, yellow stem borer was observed as the predominant over all the persistent insect pest species followed by white stem borer throughout all the growth stages of rice throughout rabi season. The population abundance of all the major insect pests were found to be fluctuated in all the seven rice varieties viz., Ranjit, Ranjit Sub-1, Bahadur, Bahadur Sub-1, Gitesh, Swarna Sub-1 and Shraboni and is presented in Fig. 2. Yellow stem borer was relatively more abundant in the variety Shraboni with mean abundance (43.52%) as against in Gitesh with lower relative abundance (32.26%) at vegetative stage (Table 4). The relative abundance of white stem borer varied from 26.14 to 38.53 per cent in the seven varieties at vegetative stage significantly higher abundance in Shraboni and lower abundance in Gitesh. Rice leaf folder population was relatively lower in Ranjit Sub-1 (4.46%) without significant difference with Ranjit (4.57%) and Gitesh (4.53%) and relatively higher in Shraboni followed by Bahadur Sub-1, Bahadur and Swarna Sub-1. No gall midge and caseworm population was observed in this stage. The whorl maggot population was higher in Bahadur (5.86%) as compared to other varieties being lower in Gitesh (3.18%).
| Variety | Yellow stem borer | White stem borer | Rice leaf folder | Whorl maggot |
|---|---|---|---|---|
| Ranjit | 35.51a (36.58) | 27.71b (31.76) | 4.57c (12.34) | 3.54ab (10.84) |
| Ranjit Sub-1 | 34.67a (36.07) | 27.67b (31.74) | 4.46c (12.19) | 3.67ab (11.04) |
| Gitesh | 32.26a (34.61) | 26.14b (30.75) | 4.53c (12.29) | 3.18ab (10.27) |
| Bahadur | 42.53ac (40.7) | 36.61bc (37.23) | 6.74aa (15.05) | 5.86aa (14.01) |
| Bahadur Sub-1 | 42.47ac (40.67) | 36.82bc (37.36) | 6.83aa (15.15) | 5.78aa (13.91) |
| Swarna Sub-1 | 40.51ab (39.53) | 32.78cd (34.93) | 6.53aa (14.81) | 5.45aa (13.50) |
| Shraboni | 43.52ac (41.28) | 38.53bc (38.37) | 7.23aa (15.60) | 5.57aa (13.65) |
| CD (0.01) | 2.45 | 2.19 | 1.54 | 1.43 |
| CV (%) | 10.37 | 10.23 | 8.97 | 6.78 |
| Means within a column followed by same letters are not significantly different by critical difference (p ≥ 0.1). Figure in parentheses is arc sine transformed value. | ||||
| No of sweeps per replication = 20 Nos. ; No of Replication = 10 | ||||
During reproductive stage in rabi season, two stem borer species were recorded as the predominant over all the major insect pest species, yellow stem borer being relatively more abundant species (Table 5). The population abundance of yellow stem borer ranged from 42.52–48.69 per cent in the seven varieties, of which it was significantly abundant in Shraboni variety and least abundant in Gitesh at reproductive stage. White stem borer succeeded over white stem borer, of which the population abundance varied from 35.39 per cent to 40.63 per cent in the seven selected varieties being lowest in Gitesh followed by Ranjit and Ranjit Sub-1 and higher in Shraboni followed by Bahadur Sub-1, Bahadur and Swarna Sub-1. The population abundance of rice leaf folder was found to be higher in reproductive stage as compared with vegetative stage. The population abundance of leaf folder ranged from 5.54 per cent to 8.67 per cent, of which leaf folder was found to be highly abundant in Bahadur Sub-1 without significant difference with that of Shraboni, Bahadur and Swarna Sub-1 and least abundant in Gitesh without significant difference with Ranjit and Ranjit Sub-1. The gall midge population varied from 3.53 per cent to 7.35 per cent in the seven varieties with higher abundance in Shraboni and lower abundance in Ranjit without significant difference with Gitesh (4.43%) and Ranjit Sub-1 (4.34%). No caseworm population was recorded during rabi season. Whorl maggot was found to be relatively less abundant than that of leaf folder and its relative abundance was 6.38 per cent in the variety Shraboni that was significantly different with that of Bahadur Sub-1, Bahadur and Swarna Sub-1 and the lower in Ranjit Sub-1 without significant difference with Ranjit and Gitesh. The gundhi bug population ranged from 11.46 per cent − 14.36 per cent throughout the reproductive stage recording higher abundance in Shraboni followed by Swarna Sub-1, Bahadur Sub-1 and Bahadur and lower abundance in Ranjit Sub-1 which was at par with that of Gitesh and Ranjit.
| Variety | Yellow stem borer | White stem borer | Rice leaf folder | Gall midge | Whorl maggot | Gundhi bug |
|---|---|---|---|---|---|---|
| Ranjit | 42.67b (40.79) | 35.46ab (36.55) | 5.65a (13.75) | 3.53ac (10.83) | 3.56ac (10.88) | 12.69bd (20.87) |
| Ranjit Sub-1 | 42.81b (40.87) | 35.61ab (36.64) | 5.78a (13.91) | 4.34ac (12.02) | 3.49ac (10.77) | 11.46bd (19.79) |
| Gitesh | 42.52b (40.70) | 35.39ab (36.51) | 5.54a (13.61) | 4.43ac (12.15) | 3.67ac (11.04) | 12.00bd (20.27) |
| Bahadur | 48.00cd (43.85) | 40.48bc (39.51) | 8.45aa (16.90) | 6.72ca (15.02) | 5.63a (13.73) | 13.47ab (21.53) |
| Bahadur Sub-1 | 48.47cd (44.12) | 40.59bc (39.58) | 8.67aa (17.12) | 6.83ca (15.15) | 5.87a (14.02) | 13.51ab (21.57) |
| Swarna Sub-1 | 45.43cd (42.38) | 40.45bc (39.49) | 8.53aa (16.98) | 6.87ca (15.20) | 4.48ac (12.22) | 14.15ab (22.10) |
| Shraboni | 48.69cd (44.25) | 40.63bc (39.60) | 8.48aa (16.93) | 7.35ca (15.73) | 6.38ca (14.63) | 14.36ab (22.27) |
| CD (0.01) | 2.54 | 2.27 | 2.11 | 1.89 | 1.85 | 2.30 |
| CV (%) | 12.49 | 11.67 | 9.45 | 8.51 | 8.36 | 10.34 |
| Means within a column followed by same letters are not significantly different by critical difference (p ≥ 0.1). Figure in parentheses is arc sine transformed value. | ||||||
| No of sweeps per replication = 20 Nos. ; No of Replication = 10 | ||||||
Yellow stem borer, white stem borer and gundhi bug were recorded as the major insect pest species, of which yellow stem borer was found highly abundant followed by white stem borer and gundhi bug at ripening stage during rabi season (Table 6). The relative abundance of yellow stem borer ranged from 33.56 per cent to 42.45 per cent in the seven varieties, of which higher population abundance was recorded in Shraboni and lower in Gitesh. Similar trend was followed in case of white stem borer as it was relatively more abundant in Shraboni (31.63%) and less abundant in Gitesh (25.34%). Higher abundance of gundhi bug was recorded in Shraboni (19.52%) without significant difference among Bahadur Sub-1, Bahadur and Swarna Sub-1 and lower abundance in Gitesh (14.87%) followed by Ranjit and Ranjit Sub-1.
| Variety | Yellow stem borer | White stem borer | Gundhi bug |
|---|---|---|---|
| Ranjit | 33.56c (35.40) | 26.53a (31.00) | 15.75ac (23.38) |
| Ranjit Sub-1 | 33.78c (35.54) | 26.67a (31.09) | 15.78ac (23.41) |
| Gitesh | 33.91c (35.61) | 25.34a (30.22) | 14.87ac (22.68) |
| Bahadur | 41.83ba (40.30) | 31.62cd (34.22) | 18.51bc (25.48) |
| Bahadur Sub-1 | 41.67ba (40.20) | 30.52cd (33.54) | 18.75bc (25.66) |
| Swarna Sub-1 | 41.87ba (40.32) | 29.61cd (32.97) | 18.45bc (25.44) |
| Shraboni | 42.45ba (40.66) | 31.63cd (34.22) | 19.52bc (26.22) |
| CD (0.01) | 2.39 | 2.31 | 2.19 |
| CV (%) | 11.48 | 10.29 | 10.76 |
| Means within a column followed by same letters are not significantly different by critical difference (p ≥ 0.1). Figure in parentheses is arc sine transformed value. | |||
| No of sweeps per replication = 20 Nos. ; No of Replication = 10 | |||
The present findings on phytrophagous insects observed in both the seasons of the rice fields corroborate the findings of Bhattacharyya et al. (2006) and Khan (2013). The population of the five persistent insect pests fluctuated variedly in all the seven rice varieties viz., Ranjit, Ranjit Sub-1, Bahadur, Bahadur Sub-1, Gitesh, Swarna Sub-1 and Shraboni during the crop period that was in close conformity with Kalita et al. (2020).
White stem borer was the most dominant insect pest followed by yellow stem borer in three stages of rice in kharif season which was also supported by Pujari and his co-workers (2008) and Baruah and Dutta (2020). The aberration in weather variables might be one of the causes of reduction of the population of the previously dominant yellow stem borer. The observation of weather parameters have clearly indicated that annual average rainfall has been showed a declining trend over the last decade. Yellow stem borer larvae cannot able to enter through the internodes of the rice plant in low water table prevailing in the rice field thereby their population have been decreased and at the same time white stem borer larval population become higher in scanty rainfall (Bora 1993).On the other hand, yellow stem borer was found to be more relatively abundant species followed by white stem borer throughout the three stages in all the varieties in rabi season which was in close conformity with the results of Rahaman et al. (2014) and Baskaran et al. (2017). Pasalu et al. (2002) also stated that yellow stem borer was the number one pest, which attacked the crop both at vegetative and reproductive stages. The findings of the present study are in agreement with the findings of Rahman et al. (2004) who reported that the abundance of insect pest was influenced by different growth stages of rice plant and the higher abundance was observed in reproductive stage. The abundance of six major insect pests was relatively higher in the rice variety Shraboni which was followed by Bahadur Sub-1 and Bahadur among all the seven selected varieties. Both the stem borer population increased significantly and reached peak at reproductive stage that corroborates with the finding of Bonaventure et al. (2018). The present finding is in conformity with Khan (2013) who reported that stem borer population was more in vegetative stage than ripening stage. The population of white and yellow stem borer differed significantly among different rice growth stages and higher number of white stem borer and yellow stem borer was recorded in reproductive stage and lower in vegetative stage which was in close conformity with Ragini et al. (2000). That variation might be due to availability of enough food for borers to feed that favoured fast growth and population build up during reproductive stage and less during vegetative stage. The population of both the stem borer species was found to be relatively higher in all the growth stages in rabi season as compared to kharif season as because rice was cultivated in some selected areas due to scanty rainfall and availability of food become limited to stem borer during rabi season and at the same time one peak of stem borer population also occurred in the month of February-March.
Rice leaf folder was found maximum in reproductive stage in all the varieties which was in consensus with the finding of Alvi et al. (2003). The population of caseworm and whorl maggot were found maximum in vegetative stage and subsequently declined till the reproductive stage in all the seven rice varieties as the leaves became matured. Gundhi bug was observed on the crop from tillering stage to harvest of the crop, but the maximum incidence of rice bug population occurred during the ripening stage of the rice crop that was supported by Hosamani et al. (2009) and Kalita et al. (2020). The higher abundance of insect pests was observed at reproductive stage and the lowest in ripening stage except gundhi bug that was in consonance with Khan and Alam (2007).
From the present investigation, it may be concluded that white stem borer has acquired the status of the most dominant insect pest replacing yellow stem borer in rice ecosystem in the recent years, because all the earlier management practices were targeted against yellow stem borer thereby decreased the population of latter one. The aberration in weather variables might be one of the reasons of the change in pest scenario in last few decades. As rice is grown in few pockets of Assam during rabi season due to scanty rainfall, higher incidence of stem borer followed by gundhi bug was observed, because availability of food was confined to a particular area thereby limited food supply encouraged the stem borer incidence during this season. The varieties selected in the present investigation are suitable climate resilient technologies as they can cope up with diversified situations in the rice field. The information gathered from the present study will surely help in survey and surveillance of rice insect pests to know the status of changing pest scenario with regard to climate change in these varieties.
Acknowledgements
The authors are very grateful to Department of Entomology and Central Laboratory of AAU-DBT Centre, Assam Agricultural University, Jorhat, Assam, India and AAU-Assam Rice Research Institute, Titabar, Jorhat, Assam to provide necessary facilities to conduct the field experiment.
Author contribution
All the authors contributed to the study conception and design, material preparation, data collection and analysis were performed. Mayuri Baruah created the first draft of the manuscript and reviewed by Bijon Chandra Dutta and Inee Gogoi.
Declarations
We confirm that this work is original and has not been published elsewhere and is not currently under consideration for publication elsewhere. We confirm that the results presented clearly, honestly, and without fabrication, falsification or inappropriate data manipulation.
Consent for publication
All authors declare that there are no issuesrelating to journal policies.
Competing interests
The authors declare that they have no conflicts of interest.
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