Seasonal incidence of major insect pests of rice in Bastar region

Introduction

Rice (Oryza sativa L.) is the staple food of more than 65 per cent of the world's population and contributes about 15 per cent of global calorie intake (Mathur et al., 1999) [13] . India accounts for nearly 24 per cent of world rice production, and Chhattisgarh, popularly known as the "Rice Bowl of India", is the sixth largest rice-producing state in the country. The rice crop is attacked by more than 100 species of insects, of which about 20 are of economic importance (Pathak and Khan, 1994) [17] . Among these, the yellow stem borer, Scirpophaga incertulas (Walker), is the most dominant and destructive pest, causing dead heart and white ear head symptoms and yield losses ranging from 25 to 70 per cent (Satpathi et al., 2012) [23] . The rice leaf folder, Cnaphalocrocis medinalis (Guenee), once considered a minor pest, has assumed major pest status over the last two decades (Nanda et al., 2000) [14] , while gall midge, Orseolia oryzae(Wood-Mason), and whorl maggot, Hydrellia sasakii, inflict significant additional losses, particularly during the vegetative and tillering stages of the crop. The extent of damage caused by these pests is closely governed by the prevailing weather conditions, which influence pest emergence, multiplication, and population build-up at different crop growth stages (Krishnaiah and Varma, 2011) [11] . Knowledge of the seasonal incidence pattern of major insect pests and their relationship with abiotic factors is, therefore, fundamental to developing reliable pest forecasting models and to planning timely, needbased insecticide application, thereby minimizing indiscriminate chemical use and associated problems such as resistance development and destruction of natural enemies (Dhurwey and Deole, 2021) [6] . Considering the economic importance of rice in the Bastar Plateau region of Chhattisgarh and the paucity of location-specific information on pest dynamics under this agro-climatic zone, the present investigation was undertaken to study the seasonal incidence of major insect pests of rice and their correlation with prevailing weather parameters.

Materials and Methods

The investigation was carried out at the Research-cum-Instructional Farm of Shaheed Gundadhoor College of Agriculture and Research Station (SGCARS), Jagdalpur, Bastar, Chhattisgarh (494001), during Kharif 2024. Bastar district lies in the sub-humid climatic zone of Chhattisgarh, situated between 17°45′ and 20°34′ N latitude and 80°15′ and 82°15′ E longitude, at an altitude of 550 to 850 m above mean sea level, within the Bastar Plateau agro-climatic zone, the southernmost zone of the state, characterized by heavy rainfall, comparatively lower temperature and higher relative humidity (average annual rainfall 1200-1400 mm). Weekly meteorological data recorded during the crop growth period (32 nd to 47 th SMW) at the Meteorological Observatory of SGCARS, Jagdalpur, are presented in Table 1. The crop received a total rainfall of 603.1 mm, with a weekly average of 37.69 mm. Rainfall was mainly distributed between late August and September, peaking at 157.3 mm during the 36 th SMW (3-9 September) and 75.2 mm during the 37 th SMW, and declined progressively from the 40 th SMW onward, ceasing completely during late October and November. The maximum and minimum temperatures during the period ranged from 28.1 °C to 32.9 °C and 10.8 °C to 22.6 °C, with seasonal means of 30.75 °C and 20.20 °C, respectively. Morning relative humidity (RH-I) ranged from 85.9 to 93.4%, and evening relative humidity (RH-II) ranged from 37.3 to 85.9%, with seasonal means of 90.98% and 61.78%, respectively. Mean weekly rainfall for weeks with rainfall during the observation period. SMW = Standard Meteorological Week; RH-I = morning relative humidity; RH-II = evening relative humidity.

The study was conducted on rice variety Swarna, transplanted on 24 July 2024 at a spacing of 20 × 15 cm in a plot of 700 m². Observations on major insect pests were recorded at weekly intervals (Standard Meteorological Week) from 10 randomly selected hills, starting from the third week after transplanting until crop harvest. Yellow stem borer incidence was recorded as per cent dead heart during the tillering stage and per cent white ear head during the panicle initiation stage, computed as the number of dead hearts/white ear heads divided by the total number of tillers, multiplied by 100. Gall midge incidence was expressed as per cent silver shoot, calculated as the number of silver shoots divided by the total number of tillers, multiplied by 100. Leaf folder and whorl maggot incidence were recorded as per cent leaf damage, based on the number of damaged leaves relative to the total number of leaves, multiplied by 100.

For yellow stem borer:

Percent dead heart = Number of dead heart or white ear Total number of tillers × 100

Percent silver shoot = Number of silver shoots Total number of tillers × 100

For leaf folder per cent:

Percent damage leave = Number of damaged leaves Total number of leaves × 100

For whorl maggot per cent

Percent damage leave = Number of damaged leaves Total number of leaves × 100

The data on seasonal incidence of insect pests were correlated with the corresponding weather parameters (maximum and minimum temperature, morning and evening relative humidity, and rainfall) using the simple correlation coefficient (r), computed following the procedure of Draper and Smith (1998) [7] .

Results and discussion

Studies on the seasonal incidence of major insect pests of rice were conducted at SGCARS, Jagdalpur, Bastar, during Kharif 2024 on the variety Swarna, from August till crop maturity, along with the prevailing weather conditions (Table 3). Yellow stem borer, Scirpophaga incertulas (Walker); gall midge, Orseolia oryzae (Wood-Mason); leaf folder, Cnaphalocrocis medinalis (Guenee); and whorl maggot, Hydrellia sasakii, were recorded as the major insect pests during the crop growth period (Table 2). [4] , who reported that stem borer incidence is strongly influenced by minimum night temperature, and Singh and Reddy (2020) [28] , who recorded peak dead heart incidence during mid-tillering under warm, moderately humid conditions. Patel et al. (2021) [16] similarly observed that white ear head formation is highly stage-specific, occurring mainly during panicle initiation and heading. [2] and Hussain and Prakash (2019) [8] , who reported peak gall midge damage during the reproductive stage owing to availability of tender tillers. The decline toward season's end corroborates Ramesh et al. (2020) [19] and Sathish et al. (2021) [24] , who attributed reduced late-season activity to declining humidity and increasingly tough plant tissue.

Gall midge,

Leaf folder, Cnaphalocrocis medinalis (Guenee)

Leaf folder infestation was recorded throughout the crop growth period, commencing at 1.03% during the 32 nd SMW and rising progressively to a peak of 10.54% during the 37 th SMW, a period marked by high relative humidity (above 90% RH-I) and substantial rainfall. Damage subsequently declined to 4.35% by the 41 st SMW, showed a minor resurgence (6.93-8.05%) during the 42 nd -43 rd SMW, and gradually declined toward crop maturity to 4.42% by the 47 th SMW. The seasonal mean leaf damage was 5.80%. Padmanabhan and Mohan (2016) [15] , who reported that canopy closure and moisture retention under humid conditions favour larval survival. Chakraborty et al. (2017) [5] and Sontakke et al. (2018) [30] similarly noted that high humidity and moderate temperature favour leaf-folding activity, while the decline toward maturity aligns with Rahman et al. (2021) [18] , who attributed reduced larval folding ability to leaf hardening with advancing crop age. Sahu and Kujur (2019) [21] and Sahu et al. (2020) [22] , working under similar Bastar Plateau conditions in Jagdalpur, also reported that fluctuating humidity and intermittent rainfall favour leaf folder build-up.

Whorl maggot, Hydrellia sasakii

Whorl maggot infestation was comparatively higher at the start of the season, recording 9.84% damage during the 32 nd SMW and peaking at 15.32% during the 35 th SMW, before declining steadily through the vegetative and reproductive stages to negligible levels by the 47 th SMW. A mild resurgence noted during the 41 st SMW (6.34%) could be attributed to favourable humidity and light drizzling conditions supporting larval establishment. These observations agree with Roy and Dutta (2015) [20] , Thakur and Mishra (2017) [31] , who recorded peak whorl maggot activity 25-45 days after transplanting under cloudy, humid conditions, with a sharp decline thereafter due to leaf hardening. Behera et al. (2018) [1] and Kale et al. (2023) [9] similarly reported early crop infestation peaking within the first 30-40 days after transplanting, consistent with the present findings.

Correlation of major insect pests with weather parameters

The seasonal incidence data of the major insect pests were correlated with the prevailing weather parameters, and the correlation coefficients are presented in Table 4. SMW = Standard Meteorological Week; RH-I = morning relative humidity; RH-II = evening relative humidity. [10] and Singh and Reddy (2020) [28] , who reported that higher temperature and humidity favour larval survival and tunnelling activity. White ear head incidence, in contrast, showed a significant negative correlation with minimum temperature (r = -0.744) and evening relative humidity (r = -0.805), consistent with Singh and Reddy (2020) [28] , suggesting that relatively higher night temperature during the reproductive stage may interfere with adult emergence and oviposition. Silver shoot incidence showed a significant positive correlation with minimum temperature (r = 0.661), corroborating Sharma and Raghuraman (2023) [26] , who reported that moderate temperature and humidity promote gall midge larval survival and adult emergence. Leaf folder damage, however, showed no significant correlation with any individual weather parameter, indicating that factors such as crop growth stage, varietal susceptibility, and natural enemy activity, as suggested by Shyamrao and Raghuraman (2019) [27] , may exert a more decisive influence than weather alone. Whorl maggot incidence was significantly and positively correlated with minimum temperature (r = 0.787), evening relative humidity (r = 0.869), rainfall (r = 0.704), and morning relative humidity (r = 0.596), indicating a strong dependence on humid monsoon conditions. These findings are in close agreement with Lakshmi et al. (2020) [12] and Bhattacharya and Samalo (2018) [3] , who reported that high night temperature coupled with humid, intermittent-rainfall conditions favour adult activity and larval survival inside the leaf whorl. Similar associations between rainfall, humidity and whorl maggot population were also reported by Sinha et al. (2020) [29] and Sharma et al. (2019) [25] .

Conclusion

The present investigation revealed that yellow stem borer, gall midge, leaf folder and whorl maggot were the major insect pests infesting rice under Bastar Plateau conditions during Kharif 2024, with distinct peak periods of incidence: dead heart at the 40 th SMW, white ear head at the 44 th SMW, silver shoot at the 41 st SMW, leaf folder damage at the 37 th SMW, and whorl maggot damage at the 35 th SMW. Minimum temperature emerged as the most consistent weather parameter influencing pest incidence, showing significant correlation with dead heart, white ear head, silver shoot and whorl maggot populations, while evening relative humidity and rainfall additionally governed whorl maggot build-up. Leaf folder incidence appeared to be governed more by crop and biotic factors than by weather alone. These findings provide a sound basis for forecasting pest emergence and scheduling timely, need-based insecticidal interventions to minimize crop losses in the Bastar Plateau zone of Chhattisgarh.

Acknowledgement

AcknowledgmentsThe authors thankful to the Dean, Shaheed Gundadhur College of Agriculture and Research Station, Jagdalpur, and Director of Research, IGKV, Raipur, for extending the International Journal of Advanced Biochemistry Research https://www.biochemjournal.com necessary facilities, technical support. Sincere thanks are also due to the technical staff, and field assistants of the Research-cum-Instructional Farm, Jagdalpur, for their constant cooperation and assistance during the field observations and data collection. Corresponding authors' Email: [email protected]

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