Seasonal incidence of earhead bug, Leptocorisa acuta thunberg (Coreidae: Hemiptera) infesting rice

Introduction

Rice (Oryza sativa L.) is the staple food for more than 60% of the world's population and serves as a major source of dietary energy, particularly in Asia. Rice grains contain approximately 78.2 g carbohydrates, 6.8 g protein, 0.5 g fat, 0.2 g crude fibre, 0.6 g mineral matter, 10 mg calcium, and 160 mg phosphorus per 100 g of edible portion (Mangalarai and Mauria, 1999). Owing to its high nutritional value and adaptability to diverse agro-climatic conditions, rice plays a crucial role in ensuring global food and nutritional security. Globally, rice is cultivated over approximately 167.1 million hectares with an annual production of about 527.7 million metric tonnes (milled basis) during 2024-25. India and China are the two largest rice-producing countries, followed by Bangladesh, Indonesia, and Vietnam. The average global rice productivity is about 4.71 t ha⁻¹ (USDA, 2024) [12] . India has the largest area under rice cultivation, covering approximately 51.42 million hectares, with an estimated production of 150.0 million metric tonnes (milled basis) during 2024-25, resulting in an average productivity of 4.38 t ha⁻¹ (USDA-FAS, 2025) [13] . Rice is cultivated in almost all states of the country. Among the states, West Bengal continues to occupy the largest area under rice cultivation, whereas Punjab records the highest productivity due to the adoption of improved production technologies, assured irrigation, and intensive crop management practices. In Gujarat, rice is cultivated mainly during the kharif season in the southern and central agro-climatic regions of the state. According to the latest available statistics, rice occupies about 0.90 million hectares with an annual production of approximately 2.14 million metric tonnes, recording an average productivity of 2.36 t ha⁻¹. Major rice-growing districts of Gujarat include Navsari, Surat, Anand, Kheda, Bharuch, and Vadodara, where favourable rainfall and irrigation facilities support successful rice cultivation (Directorate of Agriculture, Government of Gujarat, 2022) [3] . More than seventy species of insects have been recorded as rice pests in India. Among them, fifteen insect species occurring regularly are of major significance and frequently found to cause severe damage thereby reducing the yield. These key pests are rice yellow stem borer (Scirpophaga incertulas Walker), rice brown plant hopper (Nilaparvata lugens Stal), rice leaf folder (Cnaphalocrocis medinalis Guenee), rice hispa (Dicladispa armigera Olivier), rice gall midge (Orseolia oryzae Wood-mason), rice horned caterpillar (Melanitis leda ismene Cran), rice skipper (Pelopidas mathias Fabricius), rice caseworm (Nymphula depunctalis Guenee), rice white backed plant hopper [2] . The rice earhead bug (Leptocorisa acuta) is an emerging pest of concern nationwide, primarily targeting rice crops during the milky stage of the panicle. It feeds by piercing and sucking the milky content from developing grains, resulting in discoloured and chaffy kernels, which can lead to significant yield loss. Although numerous studies have explored the seasonal incidence and population dynamics of the rice earhead bug globally, the existing research remains limited. A significant gap persists in understanding how weather parameters influence population density and behavior across various seasons and climatic conditions on the earhead bug. Therefore, a comprehensive investigation was essential to gain deeper insights into this pest and to formulate effective management strategies. In light of this, the present study was undertaken.

Material and Methods

The present study was conducted at the Main Rice Research Centre, Navsari Agricultural University, Navsari, in the Kharif season 2023-2024. The "Gujarat Rice-11" rice variety was sown on 3 rd July 2023 and transplanted into main plots on 26 th July 2023. The crop was planted with a spacing of 20 x 15 cm in a large block of 40 x 15 m (600 m 2 ) and all the standard agronomic practices were followed.

Method of observation

The entire field was divided into six equal quadrants and twenty hills were randomly selected from each section. Weekly observation of Nymphs and adult insects was taken from these hills throughout the crop season. No insecticidal sprays were applied to the plots during this period to ensure natural pest incidence. Weekly environmental data including maximum and minimum temperatures, morning and evening relative humidity, wind speed, bright sunshine hours, rainfall, and the number of rainy days was collected from the Meteorological Observatory at the College Farm of N.M. College of Agriculture, Navsari Agricultural University, Navsari, during the year 2023. This environmental data was then used to calculate the correlation coefficient with the incidence of the rice earhead bug. The result of the present investigation was closely associated with Kalita et al. (2015) [5] , who reported that the percentage of damaged grain caused by the rice earhead bug reached its peak during the second fortnight of October, with a recorded maximum of 14.80 to 16.40 gundhi bugs per 10 hills. Sulagitti et al. (2017) [10] reported that the gundhi bug was initially detected during the second week of September (37 th SMW), with a count of 1 bug per 5 sweep nets. Its activity increased during the third week of September (38 th SMW) and peaked during the fourth week of October (43 rd SMW), with 16 bugs per 5 sweep nets. Gupta (2018) [4] reported that the initial incidence of the rice gundhi bug was observed during the 38 th SMW (the third week of September) with an infestation of an average of 0.40 gundhi bug per hill and reached its peak level of 3.80 gundhi bug per hill during the 43 rd SMW (the fourth week of October). Tigga et al. (2018) [11] observed that the occurrence of gundhi bug commenced from the 37 th SMW (2 nd week of September) with an average of 0.57 gundhi bug per hill. The gundhi bug infestation increased and gradually reached a peak level of 3.02 gundhi bugs per hill at the 41 st SMW (2 nd week of October). The reports were more or less in accordance with the present findings, but some differences might be due to changes in geographical location and climatic conditions.

Results and Discussion

Effect of Weather Parameters on Incidence of Rice Earhead Bug:

The correlation between the nymphal and adult population of rice earhead bug, L. acuta was worked out with weather parameters viz., maximum temperature, minimum temperature, morning relative humidity, evening relative humidity, wind speed, sunshine hours, rainfall and rainy days. The results were presented in Table 2. The results on the correlation between the incidence of rice earhead bug and different weather parameters revealed that the maximum temperature (r=0.533 * ) showed a significant positive correlation (Fig. 1) and wind speed (r=-0.629*) showed a significant negative correlation (Fig. 2), whereas morning relative humidity (r=0.402) and bright sunshine hours (r=0.426) had a non-significant positive correlation.

Minimum temperature (r=-0.180), evening relative humidity (r=-0.222), rainfall (r=-0.165) and rainy days (r=-0.335) showed a non-significant negative correlation with the incidence of rice earhead bug (Table 2). The findings of the present investigation are in close agreement with those reported by Singh and Sharma (2018) [9, 8] , who observed that sunshine hours exhibited a significant positive correlation with the incidence of rice earhead bug (Leptocorisa spp.) (r = 0.740). In contrast, rainfall, morning relative humidity, evening relative humidity, wind velocity, morning vapour pressure, evening vapour pressure, and the number of rainy days showed significant negative correlations with pest incidence, with correlation coefficients of -0.460, -0.551, -0.782, -0.657, -0.839, -0.868, and -0.704, respectively. Similarly, Sharma et al. (2019) [8] reported a significant negative correlation between rice earhead bug population and rainfall (r = -0.497). However, the pest population exhibited positive but non-significant correlations with maximum temperature (r = 0.152) and morning relative humidity (r = 0.018). In contrast, non-significant negative correlations were observed with minimum temperature (r = -0.397), average temperature (r = -0.255), evening relative humidity (r = -0.040), and average relative humidity (r = -0.086). Comparable observations were also made by Das et al. (2021) [1] , who reported that the incidence of Leptocorisa oratorius was significantly and negatively associated with rainfall (r = -0.606). The authors further recorded positive but non-significant correlations with maximum temperature (r = 0.047) and bright sunshine hours (r = 0.399), whereas minimum temperature (r = -0.200), morning relative humidity (r = -0.433), and evening relative humidity (r = -0.354) exhibited non-significant negative correlations with pest incidence. Likewise, Paikra et al.

(2021) [6] found that the population of gundhi bug was significantly and positively correlated with sunshine hours (r = 0.767). Conversely, significant negative correlations were observed with minimum temperature (r = -0.668), average temperature (r = -0.547), morning relative humidity (r = -0.671), evening relative humidity (r = -0.853), average relative humidity (r = -0.859), and rainfall (r = -0.616).

Overall, the findings of the present study corroborate the results of earlier investigations, indicating that weather parameters, particularly sunshine hours and rainfall, play a crucial role in influencing the population dynamics of rice earhead bug. Minor variations among studies may be attributed to differences in geographical location, agroclimatic conditions, rice cultivars, crop management practices, and seasonal weather variability.

References

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