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On Rice Seedlings
Severely attacked seedlings do not grow. They are stunted, wilt, and eventually die (Dale, 1994).
On Older Rice Plants
The nymphs and adults suck cell sap at the base of the rice plant and the leaf surface. The attacked plants turn yellow and later acquire a rust-red appearance, spreading from the leaf tips to the rest of the plants (Atwal et al., 1967; Dale, 1994). S. furcifera can become sufficiently numerous to kill the plants by hopperburn: the tillers dry up and turn brown as a result of excessive removal of plant sap (Dhaliwal et al., 1983; Khan and Saxena, 1985; Reissig et al., 1986). Gravid females cause additional damage by making oviposition punctures in leaf sheaths. The honeydew produced by the hoppers serves as a medium for mould growth (Dale, 1994), which imparts a smoky hue to the paddy field (Atwal et al., 1967).
On Rice Grains
The number of grains and the panicle length decrease when rice is infested at the panicle initiation stage (Dale, 1994). During the heading stage, damaged glumes become brown and some remain unfilled (Noda, 1986). Grains do not fill fully and ripening is delayed when plants are attacked at the maturation period (Dale, 1994).
Integration of varietal resistance and biological control provides effective control of populations of S. furcifera under field conditions (Salim and Heinrichs, 1986). An IPM programme was successfully implemented in China, using 63 selected resistant cultivars, lower plant densities, and selected insecticides (Hu and Chen, 1986).
Growing no more than two crops per year and using early maturing varieties may help control this planthopper (Reissig et al., 1986). S. furcifera is unable to complete a third generation on early-maturing varieties. Judicious use of fertilizer by splitting nitrogen applications can also reduce planthopper outbreaks (Litsinger, 1994).
Synchronous planting, including planting neighbouring fields within 3 weeks of each other and maintaining a rice-free period may be effective (Reissig et al., 1986), but this approach is controversial (Way and Heong, 1994).
The field should be drained for 3 or 4 days when heavy infestations occur (Reissig et al., 1986). In China, draining of fields at the 8- to 9-leaf stage of the first crop and at the 12- to 13-leaf stage of the second crop improved rice growth and increased populations of natural enemies while inhibiting population growth of S. furcifera (Zhang, 1991).
S. furcifera population is generally maintained at low levels by naturally occurring biological control agents (Reissig et al., 1986). Most egg parasitoids can parasitize alternate delphacid hosts present in non-rice habitats, which plays an important role in their conservation. These parasitoids require one or two generations for host adaptation (Yu et al., 1998). Migration of S. furcifera along with some of its natural enemies, viz. the hemipteran predator Cyrtorhinus lividipennis and the fungal pathogen Erynia delphacis, appears to play a significant role in the redistribution of the pest and its natural enemies (Reynolds et al., 1999; Matsui et al., 1998).
In northern Japan egg mortality varies between 30 and 70% due to predation, parasitism and inactivity after hatching (Iitomi, 1999). In Peninsular Malaysia in 1989, mortality in the egg stage of S. furcifera varied from 11 to 90% in direct-seeded ricefields. Anagrus flaveolus, A. perforator and A. frequens were the major mortality factors which accounted for a maximum of 69% egg mortality (Watanabe et al., 1992).
In Thailand, A. optabilis parasitized 14 to 100% egg masses of S. furcifera in the field. Other common natural enemies attacking the pest were stylopids, ants, pipunculids and dryinids (Hirashima et al., 1979).
The egg predation potential of female C. lividipennis was estimated at 6.36 per day and that of the first-instar nymph at 1.47 per day (Geetha and Gopalan, 1999). The predation rates of adult males and females, and fifth-instar nymphs on first-instar S. furcifera were 3.71, 4.84 and 3.46 per day, respectively. Of five predatory spiders (Lycosa pseudoannulata, Tetragnatha javana, Eucta javana, Thomisus cherapunjeus and Oxyopes javanus), L. pseudoannulata prefers S. furcifera to green leafhoppers (Kumar and Velusamy, 1997).
Diverse sources of resistance to S. furcifera, both in cultivated and wild species of rice, have been identified by several workers. At IRRI (Philippines), evaluation for resistance began in 1970. About 50,000 Oryza sativa accessions have been screened, and numerous resistance sources have been identified (Heinrichs et al., 1985; Romena et al., 1986). About half of the 437 wild rice accessions evaluated were also resistant to S. furcifera. These resistant wild rice accessions were primarily O. minuta, O. nivara and O. officinalis. In India, O. officinalis, O. punctata, and O. latifolia showed high levels of resistance to the pest (Velusamy et al., 1994).
Five genes for resistance to S. furcifera have been identified. These are Wbph 1 from cultivar N22; Wbph 2 from ARC 10239; Wbph 3 from ADR52; wbph 4 from Podiwi A8; and Wbph 5 from N'diang Marie (Sidhu et al., 1979; Angeles et al., 1981; Hernandez and Khush, 1981; Nair et al., 1982; Saini et al., 1982; Wu and Khush, 1983). However, resistance of IR2035-117-3 is governed by two dominant genes (Wbph 1 and Wbph 2) which segregate independently of each other (Angeles et al., 1981). A single dominant gene controls resistance in Shiyazhan and a single recessive gene controls resistance in Baigannuo and Nabeshi in China (Li et al., 1996). IR2035-117-3 has been used in breeding programmes for S. furcifera resistance (Kim et al., 1989; Le Thi Sen, 1994). Quantitative trait loci (QTL) analysis of parents and double haploid lines from the cross/Azucena indicated the presence of a major QTL on chromosome 11 for phenotypic values of PDLOSS (Kadirvel et al., 1999).
S. furcifera resistance factors are attributed to low chlorophyll, low sugar, low amino acid and high phenol content in the plant (Rath and Mishra, 1998). Mechanisms of resistance to S. furcifera include antixenosis for oviposition and antibiotic effects on feeding, survival, and development (Xiao et al., 1989). Insects that fed on resistant varieties had lower fecundity, smaller body size, significantly lower nymphal survival with longer nymphal duration, slower population increase, and produced less honeydew, indicating antibiosis (Zhou, 1987; Liu et al., 1989; Gunathilagaraj and Chelliah, 1991; Lal et al., 1992; Nalini and Gunathilagaraj, 1992). Reduction in feeding on the resistant cultivar Rathu Heenati could be attributed to the presence of certain water-soluble inhibitors in the plant (Liu et al., 1990). Zhang et al. (1987) reported that resistance of hybrid rice to S. furcifera was mainly attributable to tolerance. Some varieties show tolerance as young seedlings and antibiosis as they grow older (Yu et al., 1990).
It was observed recently that rice plants form watery lesions at oviposition sites in response to egg laying by S. furcifera (Seino et al., 1996). Benzyl benzoate, which has ovicidal activity against S. furcifera eggs, was extracted and identified from these watery lesions. Egg mortality due to watery lesions is known as physiological egg mortality (Suzuki et al., 1996). Sogawa et al. (1999) considered that, as in variety CJ06 in China, the ovicidal reaction of rice plants is one of the critical components of resistance to S. furcifera.
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S. furcifera is an important pest of rice in the tropics and subtropics in Asia (Heinrichs, 1994). It attained major pest status after the intensive cultivation of short-statured, nitrogen-responsive, high-yielding varieties, where both Nilaparvata lugens and S. furcifera generally have the same ecological niche (Heinrichs and Rapusas, 1983). Both nymphs and adults directly suck the phloem sap of rice plants (Auclair and Baldos, 1982), resulting in slow growth, stunting, yellowing, reduced tillering and reduction in panicle formation. Insect feeding until the booting stage suppresses internode elongation and reduces grain number per panicle (Naba, 1992). The dry weight of planthopper adults, leaf area and shoot dry weight of rice plants decreases with increasing nymphal density (Singh et al., 1998). The main effect of S. furcifera on yield components are a reduction of 1000 grain weight and an increase in the percentage of unfilled grains per panicle as insect number increases (Li et al., 1999). Under favourable conditions, S. furcifera produces several generations that can eventually cause 'hopperburn', killing the plant. Unlike Nilaparvata lugens and Nephotettix virescens, S. furcifera is not known to transmit any virus disease (Romena et al., 1986).
A serious outbreak of S. furcifera was reported in Pakistan in 1978, in the north-west of West Malaysia in May 1979, and in India in 1982 (Majid et al., 1980; Ooi et al., 1980; Khan and Kushwaha, 1991). In May-June 1985, it severely damaged rice for the first time in Assam, India, where heavily infested fields were hopperburned and a range of 800-1400 hoppers per sweep were sampled. Large populations were accompanied by heavy rainfall in early April, followed by a prolonged dry period with high temperature and humidity in May (Saha, 1986).
Factors that contributed to the outbreak were excessive use of nitrogen fertilizer, close spacing, continuous submerged conditions in the fields and low populations of natural enemies due to indiscriminate use of insecticides (Yein and Das, 1988; Bhathal and Dhaliwal, 1991). Similarly, in China, yield loss was positively correlated with the level of nitrogen fertilizer applied, because the planthoppers prefer (and multiply rapidly) in closed canopied, succulent rice plants (Hu et al., 1986). In addition, the population of S. furcifera was affected by temperature in China. The population was low when the weather was cold and overcast, intermediate when it was warm and rainy, and high when it was warm and dry (Zhu et al., 1990). Ghauri (1979) reported that in 1952 crop losses in Pakistan amounting to 60% were attributed to S. furcifera infestation, which indicates that there are factors other than the introduction of new varieties that increase the importance of the pest.
Economic thresholds have been estimated at 12-16 nymphs per hill (Ye et al., 1993) and 10-20 delphacids per hill (Naba, 1992). An economic threshold of 180-190 hoppers per 100 hills of Japonica rice in China was estimated (Zhang et al., 1999a). A control threshold of 150 insects per 100 hills was established in areas north of the Huai river in the Jiangsu province of China (Wang et al., 2000), while threshold levels of 150-250 insects per 100 hills at culm extension and 250-300 insects per 100 hills at differentiation are recommended for the Taihu district of China.