Host plant resistance against yellow stem borer and rice leaf folder through varietal screening of rice

Rice plant is attacked by many insect-pests of which yellow stem borer (dead heart at vegetative stage and white ear head at reproductive stage) and leaf folder are considered as prime devastator, responsible for major economic loss. To combat this problem, resistant varieties of rice are required for better production as well as better productivity. Host plant resistance is very much effective in integrated pest management (IPM) system, where negligible pesticidal hazard is present as well as environmental safety, low cost farming by without or minimum pesticide application and proper identification of resistant varieties for selection as parent in crossing programme to develop resistant varieties in future. The 78 rice varieties were screened out against Scirpophaga incertulas (yellow stem borer) and Cnaphalocrocis medinalis (rice leaf folder) of rice during Kharif, 2010-2012 at Rice Research Station, Chinsurah, Hooghly, under Government of West Bengal. The experiment was carried out to determine the level of resistance in early, mid-early, medium and late duration rice varieties, and shallow and semi-deep rice varieties. The pest infestation level was determined by counting dead heart, white ear head and damaged/folded leaves. The experimental results revealed that the early duration variety viz. Narendra 97 and IR 50, the mid-early duration variety i.e. IR 64 and IET 17904 proved resistant against both yellow stem borer (dead heart) and leaf folder. The medium duration variety, Ranjit was highly resistant against both yellow stem borer (dead heart) and leaf folder, and the variety, Pratiksha showed a fair degree of resistance against both yellow stem borer and leaf folder; while Jarava, the late duration variety exhibited high degree resistance against yellow stem borer (dead heart and white ear head). Besides, the varieties, Sashi and Giri recorded very low level of infestation of both lepidopteran pests while shallow and semi-deep variety such as Sashi was also considered resistant against the noxious pests.


INTRODUCTION
Globally, the country India ranks second in terms of area of cultivation (about 44.6 million hectares) and production (104.32 million tonnes) under rice (Oryza sativa L.). The potential to yield is dented due to a lack of inbuilt resistance to different biotic stresses as discernible in ∼1,000 rice cultivars across the country (Chatterjee et al., 2020). India, the second-largest rice growing country has a production of 104.32 million tonnes and cultivation area of about 44.6 million hectares with an average productivity of 2.34 tonnes per hectare (Rajasekar & Jeyakumar, 2014). The state West Bengal ranks first in the area of cultivation as well as production of rice in India. About 78% of the total area under rice in the state is concentrated under high and medium productivity groups, which accounts for nearly 84% of total production of rice in the state (Chatterjee et al., 2016). A critical analysis of the gap between the potential and actual yield of rice across the nation reveal that there are a several factors which act as yield constraints. Riceplant is subjected to attack by more than 100 species of insects, of which 20 species of economic importance cause 20-30% yield losses every year (Chatterjee et al., 2017). Yellow stem borer (YSB: Scirpophaga incertulas Walker) and leaf folder (LF: Cnaphalocrocis medinalis Guenée) of rice are considered as prime devastators responsible for major economic loss (Chatterjee & Mondal, 2014;Chatterjee et al., 2017).
The host plant resistance depends upon the relationship between the plant-feeding insects and their host plants (Painter, 1951) which enables plants to avoid, tolerate or recover from the effects of insectpest attack and this mechanism has been proved to be a successful tool to protect crops from insects attack (Felkl et al., 2005). Potential to yield is dented due to a lack of inbuilt resistance to different biotic stresses as discernible in ∼1,000 rice cultivars across the country (Chatterjee et al., 2020). Plant genotypes, either due to environmental stress or genetic makeup, possess physiological and biochemical differences which alter the nutritional value (primary metabolites) for plant feeding insects (Kaltenbach et al., 1994;Mısırlı et al., 2000;Siemens et al., 2002;Alvim et al., 2004). In some cases, the combined nutritional and allelochemical alterations either improve the quality of the host plant as a source of food and can therefore be considered favorable to herbivorous insects (Baur et al., 1996;Alvim et al., 2004) or make the quality of host plant as source of food unfavorable to phytophagous insects (Stadler et al., 2002). Islam & Karim (1997) studied the leaf-folding behavior of Cnaphalocrocis medinalis and reported that the rice leaf blade morphology i.e. width, length, and toughness might play a vital role in resistance against Cnaphalocrocis medinalis. They concluded that longer and tougher leaf blades might also contribute to resistance. Host plant selection is mainly a behavioral process that is primarily regulated by chemoreception (Jeremy & Szentesi, 2003). The identification of resistant/tolerant rice varieties will help breeders for future use in developing multiple resistant new breeding rice lines (Chatterjee et al., 2016). The introduction of novel germplasm in breeding programme is very much required to diversify the genetic base (Thompson et al., 1988). The genetic improvement depends upon the amount of genetic variability present in the population of any crop (Vivekanandan & Subramanian, 1993). Genotypes that have diverse genetic background are very much helpful to obtain high heterotic response and transgressive segregants for future breeding programme (Verma et al., 2004). The objective of the present research work is to assess the level of insect-pest tolerance as well as host plant resistance individually and multiple resistance against yellow stem borer and rice leaf folder under present climatic condition.

MATERIALS AND METHODS
The different categories of rice varieties viz. early duration, mid-early duration, medium duration, late duration and shallow and semi-deep rice (Table 1) were screened out against yellow stem borer and leaf folder during Kharif, 2010-2012 at Rice Research Station, Chinsurah, Hooghly, West Bengal situated at 88º 24′ E longitude and 22º 52′ N latitude with an altitude of 8.62 m amsl in the alluvial zone of West Bengal, with sub-humid and sub-tropical climate. The experiment was conducted to find out the reaction of rice varieties (Table 1) against yellow stem borer and leaf folder in Kharif season. The five categories of rice varieties were transplanted in two rows of ten hills each with one skip row between entries with spacing of 20 × 15 cm 2 (row to row and plant to plant) @ single seedling per hill. The state recommended inorganic fertilizers were applied as N:P 2 O 5 :K 2 O @ 80:40:40. All the recommended agronomic package of practices were adopted during the experimentation in field. No plant protection measure i.e. no insecticidal treatment was done to maintain an optimum population pressure of target insect-pest on the field crop. The level of target insectpest infestation was determined by counting the proportion of affected hills showing dead heart, white ear head and folded leaves from ten numbers of hills of each variety. The observations were noticed on total tillers and dead heart by yellow stem borer and total leaves and folded leaves by rice leaf folder at the active tillering stage of the crop while, at pre-harvest stage the total panicle bearing tiller and white ear head by yellow stem borer from randomly selected ten hills of each variety.

RESULTS AND DISCUSSION
Several scientists have screened out many rice lines/entries/genotypes/varieties against different insect-pests of rice and they have identified many resistant/tolerant lines/entries/genotypes/varieties. In this experiment, the authors recorded the multiple resistant of different categories rice varieties against yellow stem borer (for per cent dead heart and white ear head) and rice leaf folder (for per cent folded leaves by rice leaf folder).

Screening of shallow and semi-deep rice
The shallow and semi-deep rice, Sabita (4.80%) followed by Pooja (4.82%) and Biraj (5.36%) showed lower infestation of dead heart in 2010 (Table 6). In 2011, the minimum dead heart incidence was observed in Biraj (4.09%), Pooja (5.11%) and Sashi (6.36%), accordingly. Whereas in 2012, the lowest dead heart was observed in Niraja (6.25%) followed by Pooja (6.54%) and Sashi (6.80%). The three years consecutive mean on dead heart incidence indicated that the lowest infestation was found in Pooja (5.49%) followed by Biraj (5.57%) and Sashi (6.22%). The lowest white ear head incidence was noticed in Sashi (0.77%) followed by Jalamagna (1.50%) and Samba Mashuri Sub-1 (1.51%) in 2010 and the lowest infestationwas recorded in Samba Mashuri Sub-1 (1.33 and 0.70%, respectively) during 2011 and 2012 followed by Sashi (1.87%), Purnendu (1.96%) in 2011 and Sashi (0.90%), Giri (1.00%) in 2012. The three years mean data indicated that the lowest white heads was discernible in both Samba Mashuri Sub-1 and Sashi (1.18%) followed by Giri (1.72%), Jalamagna (2.09%). In 2010, the minimum leaf folder incidence was recorded in Sashi (1.50%) Mandira (1.61%) and Sunil (1.72%). The lowest folded leaf was observed in Bhudeb (1.31%) followed by Sashi (1.42%) and Niraja (1.67%) in 2011. Whereas in 2012, the minimum incidence was noticed in Purnendu (1.37%), Sashi (1.60%), Bhudeb and Biraj (1.61%). The three years mean data indicated that the lowest folded leaves was discernible in Sashi (1.51%) followed by Bhudeb (1.74%) and Sunil (1.94%). It may be concluded that among the shallow and semi-deep rice varieties, Sashi was highly resistant against both yellow stem borer and leaf folder.  Balasubramanian et al. (2000) screened 178 advanced yield trial genotypes of rice for their reaction to insect pests under natural conditions as per Standard Evaluation System (SES) of IRRI, Philippines. The experiment results showed the rice genotypes, IET 15742 and IET 15072 against YSB and IET 16120 against rice leaf folder were found moderately resistant from out of 178 total genotypes. Singh et al. (2006) screened 53 rice cultivars against Scirpophaga incertulas under natural condition. Observations based on per cent damage incidence of YSB were recorded as the eighteen rice varieties were totally free from stem borer damage in terms of DH and WE. Thirty-one rice entries were screened against Scirpophaga incertulas, under field and greenhouse conditions by Dhuyo & Soomro (2007). Field studies were conducted in West Bengal, India, during the 2010 Kharif season, to screen different rice cultivars against rice leaf folder (Cnaphalocrocis medinalis) and stem borer (Scirpophaga incertulas) and find out the tolerant and susceptible strains (Chatterjee et al., 2011).

CONCLUSION
Authors have tried to find out the multiple resistant rice varieties against yellow stem borer (dead heart and white ear head) and leaf folder which may be considered as host plant resistance to insect-pests of rice. The host plant resistance mechanism may be used as an effective, eco-friendly and economical method of pest control where no pesticide is required in the field. The early duration rice varieties viz. Narendra 97 and IR 50 mid-early duration variety viz. IR 64 and IET 17904 medium duration variety viz. Ranjit and Pratiksha late duration varieties viz. Jarava and Giri the and shallow and semi-deep variety Sashi showed different levels of resistance against yellow stem borer and leaf folder and can be recommended for cultivation where regular infestation of these pests was recorded.