Silicon Plays an Effective Role in Integrated Pest Management against Rice Leaffolder Cnaphalocrocis medinalis Guenée (Lepidoptera: Pyralidae)

Over the centuries, insect pests have remained a limitation for agriculture and pest outbreak incidences have been observed often with the transformation of pest complexities (Bajya et al., 2010). Over time, the status of insect pests has been changed. Some insects grew in significance whereas others have dropped in importance. There are conclusive pieces of evidence that ‘minor pest species’ have gained momentum by specific cropping culture and patterns or crop intensification (Singh et al., 2003). Intensification describes the modification in cultural practices such as cropping intensity, increased usage of chemicals (both fertilizers and pesticides), development of irrigation services and improvement of higher plant densities (Suresh and Venugopal, 1992).

Several biotic and abiotic factors are responsible to reduce the rice yield. The main reason of the reduction of the yield of the rice crop is the damage caused by different insect pests including stem borers (Yellow stem borer Scirpophaga incertulas Walker, White stem borer Scirpophaga innotata Walker and Pink stem borer Sesamia inferens Walker), rice leaffolder Cnaphalocrosis medinalis Guenée and rice planthoppers (Whitebacked planthopper Sogatella furcifera Horváth and Brown planthopper Nilaparvata Lugens Stål) (Hussain et al., 2018, 2019; Atta et al., 2019b; Bilal et al., 2019; Rizwan et al., 2019). The yield losses caused by these insect pests were estimated at up to 18% of the expected rice crop yield (Asghar et al., 2009; Ahmad et al., 2016b). Among all these insect pests, Cnaphalocrocis medinalis (Lepidoptera: Pyralidae), has become the key pest of rice crop (Bilal et al., 2019; Rizwan et al., 2019). Leaves are longitudinally stitched by the 2nd instar larvae of C. medinalis for accommodation and it feeds voraciously on the green matter of leaves which results in papery dry structures (Chatterjee, 1979). Due to the feeding activity, paddy leaves often result in twisting or yellowing of rice green foliage (Mishra et al., 1998). The plant may be damaged completely in case of severe infestation (Ramasubbiah et al., 1980). Infested leaves become susceptible to bacterial and fungal infections (Bashir et al., 2004). It is among the pests that posed severe damages to aromatic paddy areas in Asian countries (Salim et al., 1991; Han et al., 2015). It may cause 30-40% leaf infestation and 20-30% yield losses. In favorable conditions, 63-80% yield losses have been reported (Alvi et al., 2003). Peak activity of C. medinalis moth has been observed from the end of August to the end of September (Bilal et al., 2019). Control measures for C. medinalis primarily include the use of insecticides which disturb the beneficial insect fauna and bring environmental contamination (Heong, 2005).

Integrated pest management (IPM) approaches have facilitated the insect pest management and environmental safety (Farooq et al., 2019). IPM is the integrated use of all techniques in a compatible manner with the least dependence on synthetic insecticide. Among crop management techniques, one is the silicon (Si) fertilizer. Si is an important component of plant tissues and its scarcity may result in growth anomalies, development and reproduction abnormalities. Si fertilizer can improve the control of insect pests (Sidhu et al., 2013). It is not considered an essential part of the plant but its deficiency may result in abnormalities in growth, development and reproduction (Alvarez and Datnoff, 2001; Ma et al., 2001), moreover, the plant may become more susceptible to biotic and abiotic factors (Nakano et al., 1961; Ma, 2004). There is much work reported a positive association between Si high contents and variety resistance to herbivores insects in monocots and dicots (Ma, 2004; Laing and Adandonon, 2005; Li et al., 2013). Induced resistance against herbivory can be improved in plants through Si contents amendments (Savant et al., 1997; Keeping and Meyer, 2002; Hou and Han, 2010; Sidhu et al., 2013; Atta et al., 2019a).

Si contents can affect herbivory in several ways. It accumulates in the leaf sheath and blade’s epidermal layer to develop silica double layer (Massey and Hartley, 2009). It also accumulates in the vascular bundle and other tissues associated with protection (Sangster et al., 2001), thus provides physical obstruct to herbivory. Si addition in rice plants for rice borer, Chilo suppresalis (Walker) prevents larval boring, expanded larval growth and impaired weight gain (Hou and Han, 2010; Han et al., 2010). Immatures of African armyworm, Spodoptera exempta (Walker) and locust, Schistocera gregaria (Forskål), when raised on Si added plants, their ability of conversion ingested food and growth rate reduced (Massey et al., 2006).

Rice plant is a typical example of Si accumulating plant and takes through roots in bio-available form Silicic acid, Si(OH)4 (Epstein, 2009). Rice varieties resistant to C. medinalis have been found with higher Si contents. It has been reported that C. medinalis larvae nourished on Si-treated paddy plants obtained less mass (Ye et al., 2013).

In this study, effects of Si doses/addition in rice variety (originally susceptible to the C. medinalis infestation) through a comprehensive assessment of development, food consumption and weight gain/loss parameters were assessed. This study may help the researchers in establishing evidence of Si-mediated resistance in susceptible rice lines/varieties to the C. medinalis and advance management tactics of this insect pest in rice production.

The present study was conducted at Rice Research Institute, Kala Shah Kaku (31.7213° N, 74.2700° E), Ministry of Agriculture, Government of Punjab, Pakistan under controlled conditions during the year 2019.

Seeds of Basmati 515, susceptible to C. medinalis (Ahmad et al., 2016a) were soaked at room temperature for 24 h and germinated for 72 h in a growth chamber at 30°C before seeding. Seeds were sown in the soil for raising plants. The soil of the research institute is calcareous with impervious subsoil. The pH of the soil was 7.5, organic matter C = 0.3-0.6%, N = 0.35 gkg-1, available P = 1 gkg-1, available Si = 0.11 gkg-1 and available K = 67.12 mgkg-1. The seedlings were transplanted to plastic pots (22 cm diameter × 15 cm height). Two seedlings were transplanted in a single pot. Each pot contained 3.8 kg dry soil, amended with Si source, potassium silicate (soluble Si ≥ 12%) at different rates before 3 days of transplanting. Si concentrations were used at 1.25 and 2.50 g Sikg-1 soil along with an untreated check (control).

All the pots were treated with urea (N ≥ 46.4%), diammonium phosphate (N = 16.0%; P2O5  = 44.0%) and potassium chloride (K2O ≥ 60.0%) at the rate of 0.37 gkg-1 soil, 0.25 gkg-1 soil and 0.35 gkg-1 soil, respectively. Urea was applied into soil 3 days before transplanting or top dressing at tillering, heading and milking stages at the ratio of 4:3:2:1(Hou and Han, 2010). Pots then placed in cages and water level of 2 cm maintained in the pots. The pots were arranged randomly in the greenhouse. Watering was administered as necessary and water level in the pots was always below the upper edge. No pesticide was used throughout the experiment.

Larvae of C. medinalis were collected during early July 2018 from the nursery of the experimental area. The larvae were fed on Basmati 515 cultivar in cages for rearing. The stock culture was maintained following the method of Rizwan et al. (2019). The ovipositional cages were kept under observation till the hatching of 3rd instar larvae and were used in this experiment (< 24 h).

The single leaf segment method was used to check the effects of Si doses on the development and survival of the C. medinalis. A segment of half feet (9 cm long) of the fresh leaf was cut down from a rice plant of 40 days after transplanting (40-DAT) and spread in a petri dish on a moistened filter paper. Five leaf segments were used in a dish and moistened wool was used at the ends of segments. Thirty newly transformed 3rd instar larvae (slightly dark green in color and a brownish patch on either side of pronotum) starved for 3 h were transferred to each petri dish with the help of a camel hairbrush. The larvae were developed in a growth chamber at 28 ± 1°C, 75 ± 5% R.H. and a constant photoperiod of 16:8 (L:D) h. Petri dishes were observed twice a day to check the developmental stages. Dead and survived larvae were counted daily and dead were removed. Leaf segment was replaced daily after 24 h until the pupation period. Data regarding larval duration, pupal duration and survival rate was recorded. Larvae and pupae were weighed individually using an electronic balance (NAPCO, JP-410) to the nearest of 0.001 g. The study was replicated thrice.

Newly molted 3rd instar C. medinalis larvae starved for 3 h were placed in a growth chamber for 72 h to feed on the leaf segment inside the petri dish. Then larvae were weighed and dried till constant weight achieved and then weighed again. Remained leaf segments were dried and weighed. Feces were also dried and weighed. Dry weight for before feeding was calculated Ten pieces (9 cm long) of the reciprocal 4th leaves from a 40-DAT of treatment were cut. The segment was dried to a constant weight and then weighed. Based on the difference between fresh and dry weight water contents of the leaf segment were calculated. Food consumption per day was recorded for comparison. The study was replicated thrice.

Data was subjected to Analysis of variance (ANOVA) using statistical package Statistix® (version 8.1). Treatment means were separated using Tucky’s HSD test at α = 0.05.

Larval development duration of C. medinalis differed significantly between the treatments (P < 0.01, F2, 8 = 19). Si used at low and high rates enhanced the larval development period as compared to control. Larval development duration was enhanced by 17.00±0.47% and 18.33±1.09% at low and high Si dose applications, respectively (Fig. 1A). The larval survival rate of C. medinalis was insignificant between the treatments (P > 0.05, F2, 8 = 4.20) which was reduced as much as 26.67±1.92% and 23.33% from control to Si application at a low and high Si dose application, respectively (Fig. 1B). However, the 3rd instar larval weight was significantly different among treatments (P < 0.01, F2, 14 = 17.3) which was reduced by 32.71±0.27% and 22.22±0.57% as compared to control at low and high Si application rate, respectively (Fig. 1C).

Pupal duration of C. medinalis differed insignificantly among treatments (P > 0.05, F2,8 = 1.00) which was reduced by 5.11±0.03 and 5.00±0.05 as compared to control at low and high Si dose rate, respectively (Fig. 1D). Similarly, pupation rate differed insignificant among treatments (P > 0.05, F2,8 = 5.03) which was reduced by 40.00±4.34% and 33.33±2.11% at low and high Si dose applications, respectively (Fig. 1E). However, the pupal weight differed significantly among treatments (P < 0.01, F2,8 = 33.8) which was reduced by 19.60±0.30% and 17.68±0.44% as compared to control at low and high rate of Si application, respectively (Fig. 1F).

Food consumption rate per day of C. medinalis among treatments was highly significant (P < 0.01, F2,29 = 401) which was increased by 16.01±1.95% and 25.02±2.47% as compared to control at the low and high dose rate of Si, respectively. Higher the Si dose, the higher the consumption rate. While a less green matter of rice leaf was consumed by C. medinalis in the control treatment (Fig. 1G).

Plants are vulnerable to biotic and abiotic stresses. Insects are among biotic stressors that cause damage to economic plants and produce quantitatively and qualitatively (Kogan and Lattin, 1999; Farooq et al., 2020). Plants respond to these factors directly and indirectly. Morphological characteristic is among direct responses and indirect responses include metabolites. Both are responsible for induced resistance against biotic factors. Induced resistance is the key element of IPM. Si amendments are an effective way to produce induced resistance (Atta et al., 2019a).

Rice plant is a typical Si accumulating economic plant (Ma et al., 2006). It is absorbed by roots in bioavailable form silicic acid, Si(OH)4 (Epstein, 2009). Si confers resistance to the C. medinalis as indicated by less larval survival rate. Enhanced Si assists in plant defense against C. medinalis (Ye et al., 2013; Han et al., 2015) and also reported for other pests such as desert locust, S. gregaria (Massey et al., 2006) and the Asiatic rice borer, Chilo suppressalis Walker (Hou and Han, 2010). In addition, rice varieties resistant to C. medinalis have closer silica chains and high epidermal silica deposition in contrast to susceptible varieties (Hao et al., 2008).

Results of the present study indicated that Si amendments enhanced larval developmental duration, reduced larval survival rate as well as bodyweight of 3rd instar larvae, also reduced pupal duration, pupation rate and pupal weight of C. medinalis. Han et al. (2015) concluded that Si amendments reduced net reproductive rate, intrinsic rate of increase and finite rate of increase which indicates the decreasing population trend. However, Si reduced the quality of food and efficiency/ability of the larvae to convert ingested food into body mass. Reduced larval bodyweight of S. exempta, S. gregaria (Hou and Han, 2010) and C. medinalis (Han et al., 2015) were also reported previously.

Results of the present study also indicated that Si addition led to the high consumption rate of green matter by 3rd instar larvae. This increased rate of consumption has also been reported by Peterson et al. (1988) for Spodoptera eridania (Cramer), Massey et al. (2006) for S. gregaria and Han et al. (2015) for C. medinalis. This is in contrast to C. suppressalis, where Si amendments reduced the boring activity of studied insect. This may be due to the different feeding behavior of the two pests. Increased feeding by the larvae represents more damages to the rice plant in terms of green matter consumption.

Si addition leads to activities of plant enzymes (Cai et al., 2008; Ye et al., 2013). This mechanism paves the way for the increased accumulation of defensive metabolites such as phytoalexins, phenolics and momilactones (Fawe et al., 1998; Rodrigues et al., 2004), and is also associated with volatiles which is part of the plant’s defense mechanism to herbivory and attracts natural enemies of herbivore insects (Kvedaras et al., 2010). Si addition improves the soil for paddy production in Si deficient soils. Better growth and development paddy for sustainable production can be expected (Savant et al., 1997; Ma and Takahashi, 2002). This addition could be helpful in pest management against herbivory.

Si applications reduced larval survival, extended growth period and reduced the efficiency of C. medinalis larvae to convert ingested green matter into bodyweight. It also reduced the pupal duration, pupation rate and pupal weight. Hence, it came to know that Si fertilizer can reduce the susceptibility of rice varieties to herbivore and could be effective for IPM programs.

The authors have declared no conflict of interest.