Cnaphalocrocis medinalis (rice leaf folder)

Before feeding, C. medinalis larvae fold the leaves longitudinally by stitching the leaf margins. Individual spun threads fuse to form a band: the desiccation of the band facilitates contraction of silk stitches, hence, the leaf rolls. Once protected, the larvae scrape and feed on the green tissues (the mesophyll layer) of the rice leaves, resulting in the appearance of linear, pale-white stripe damage. In severe infestations, damaged plants appear sickly and scorched.

Host-Plant ResistanceAt IRRI, 115 out of 17,914 screened varieties were found resistant or moderately resistant to C. medinalis (Heinrichs et al., 1985). Ten wild rices demonstrated potential sources of resistance (Heinrichs et al., 1985; Medina and Tryon, 1986). In India, IET 8675, 8698, and 8710 rice entries are very promising against the rice leaffolder, whereas IET 7776 is promising in the Punjab. Varieties with narrow leaves such as TKM6 were found to be more resistant to leaffolders than varieties with wider leaves (Islam and Karim, 1996, 1997).Several rice lines transformed with toxin genes from Bacillus thuringiensis (Bt) have been shown to be highly resistant to C. medinalis (Wunn et al., 1996; Maqbool et al., 1998; Shu Qing Yao et al., 1998).Cultural ControlWider spacing (22.5 x 20 cm and 30 x 20 cm) and low usage of nitrogenous fertilizer decrease the percentage of leaf folder infestation. Early planting may enable plants to escape a high degree of defoliation.Biological ControlC. medinalis has a rich community of natural enemies in insecticide-free ricefields. In China, Trichogramma japonicum and Apanteles cypris are the most important egg and early larval instar parasitoids, respectively. Spraying rice plants with a kairomone derived from C. medinalis larva frass, haemolymph and labial gland secretion increased parasitization from 15 to 25% (Hu and Chen, 1987). A granulosis virus caused 30-40% larval mortality in Guandong province, China (Pang et al., 1981). In Sri Lanka, larval parasitization ranges from 38 to 70%. Five parasitoids (four hymenopterans and a tachinid) are quite common.In Japan, an ichneumonid Trathala flavoorbitalis kills 34-54% of leaf folder larvae from late June to early July, whereas only 12% are killed in the normal season. In late August to mid-October, the bethylid causes larval mortality varying from 1 to 35%. On the other hand, Itoplectis naranyae and Brachymeria excarinata parasitize the pupae: in late October, parasitism is 31 and 11%, respectively.In the Philippines, 83 predators, 55 parasites, and 6 pathogens attack the leaf folder at all stages. The most important predators, however, are the gryllid genera of Metioche and Anaxipha on the eggs and Ophionea spp. on the larvae. Effective parasitoids are Copidosomopsis nacoleiae, Cotesia angustibasis, Cardiochiles philippinensis and Macrocentrus cnaphalocrocis. During wet seasons with frequent moderate rainfall, the pathogen Zoophthora radicans can wipe out the entire larval population (Barrion et al., 1991).Chemical ControlMany different kinds of insecticides including neem seed oil (Saxena et al., 1980) have been used for leaffolder control. Please refer to Valencia and Heinrichs (1979, 1982), Endo and Masuda (1981), Hirao (1982), Saroja and Raju (1982), and Endo et al. (1987) for more information. At present, however, modern plant varieties can compensate for defoliation, making insecticides no longer necessary for leaf folder control.

C. medinalis increased in importance with the widespread use of high-yielding varieties and changes in cultural practice. Moreover, injudicious use of insecticides, coupled with excessive use of nitrogenous fertilizer, contributed to dramatic increases in population. Outbreaks were reported in India, Sri Lanka, Bangladesh, Nepal, China, Korea, Japan, Vietnam, Malaysia, Philippines, and Fiji (Pathak and Khan, 1994). In all cases, yield losses were reported. Recently, however, studies at the International Rice Research Institute (IRRI), Los Baños, Philippines, and in China and Japan report no yield loss despite substantial leaf removals. The rice plants are able to compensate for the high degree of larval feeding or defoliation (Heong, 1992; Heong and Escalada, 1998). In the Mekong Delta of Vietnam, thousands of farmers have decreased applications of early-season (within 40 days of crop establishment) insecticides, which were formally directed principally at leaffolders (Heong et al., 1998). This decrease resulted from a media campaign in which farmers were encouraged to compare yields in control plots (receiving no early-season insecticide) and treated plots.Compensation occurs most readily at the vegetative stage and when plants have adequate water and fertilizer. More research is needed to evaluate the effects of damage to the flag leaf at the reproductive stage. Samalo et al. (1996) found that high levels of flag leaf damage (45-70% of the area removed) resulted in yield losses.