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Field damage (IRRI)
Rice leaffolder, rice leaf roller, grass leaf roller
Cnaphalocrocis medinalis (Guenee), Marasmia patnalis (Bradley), M. exigua (Butler)
Folded leaf with silk (IRRI)
The folded leaves enclosing the larvae are typical and can be observed easily. Likewise, the linear white feeding areas on the leaves are also visually seen.
The folded leaves are also symptoms similar to the ones caused by rice skippers and green semilooper. The whitish streaks are comparable to leaves damaged by rice whorl maggot and thrips.
The heavy use of fertilizer encourages rapid multiplication of the insect. High humidity and shady areas of the field also favor their development. The presence of grassy weeds from rice fields and surrounding borders support continuous development of the pest.
Expanded rice areas with irrigation systems, multiple rice cropping and insecticide induced resurgences are important factors in the insect’s abundance.
The rice leaffolders occur in all rice environments and are more abundant during the rainy seasons. They are commonly found in shady areas and areas where rice is heavily fertilized. In tropical rice areas, they are active year-round, whereas in temperate countries they are active from May to October. The adults are nocturnal and during the day, they stay under shade to escape predation. Moths fly short distances when disturbed.
There are three species of leaffolder and the adult moths can be distinguished by the markings on the wings. The adult is whitish yellow or golden yellow. It has three black bands on the forewings, either complete or incomplete. It has a wing span of 13 to 18 mm.
The pupa is light brown or bright brown. With age, it turns reddish brown. It is 6 to 12 mm long.
Neonate larvae are yellow. With age, they turn yellowish green with brown or black heads. They have distinct pinnaculae or one to two pairs of subdorsal spots on the mesonotum or metanotum. A third species lack spots on the notum. The apex of their pronotum is angulated, convex or always straight. They are from 8.5 to 25 mm long.
The newly laid egg is jelly-like and transparent. It is oblong with an irregular upper surface. The mature egg is ovoid and whitish yellow. It is ventrally flattened.
Aside from the rice plant, the rice leaffolder also feeds on Avena sp. (oats), Cocos nucifera L. (coconut), Echinochloa colona (L.) Link (jungle rice), Eleusine coracana (L.) Gaertn. (koracan), Hordeum sp. (barley), Musa sp. (banana), Nicotiana tabacum L. (tobacco), Panicum milliaceum L. (millet), Saccharum officinarum L. (sugarcane), Saccharum spontaneum L. (wild sugarcane), and Setaria italica (L.) P. Beauv. (German millet).
The larva forms a protective feeding chamber by folding a leaf blade together and glues it with silk strands and feed on leaf tissues. Longitudinal white and transparent streaks on leaf blades are created.
rice leaffolder is very common and can be found in all rice growth stages. The damage may be important when it affects more than half of the flag leaf and the next two youngest leaves in each tiller.
Feeding damage of the rice leaffolders during the vegetative stage may not cause significant yield losses. Crops generally recover from these damages. Leaffolder damage at the reproductive stage may be important. Feeding damage, if it is very high, on the flag leaves may cause yield loss.
The highly visible symptoms are often the cause of farmers’ early season insecticide use. Most of these sprays have little or no economic returns. Instead, they can cause ecological disruptions in natural biological control processes, thus enhancing the development of secondary pests, such as planthoppers. In some countries, about 40% of farmers’ sprays target leaffolders. Through participatory experiments, farmers who stopped early season sprays had no yield loss and saved up to 15-30% in pesticide costs. The spray reduction also decreases farmers’ exposure to health risk posed by pesticides.
The rice leaffolders may be managed by cultural practices, the use of biological agents and resistant varieties. In most cases, chemical control is not advisable.
In cultural control, it is advised not to use too much fertilizer. It was observed in a field experiment that highly fertilized plots attract females. Surrounding grass habitats should be maintained because these serve as temporary reservoirs of natural enemies like crickets, which are egg predators of leaffolders. Herbicide spraying and burning of these non-rice habitats might not be useful.
Among the biological control agents, there are small wasps and crickets that attack the eggs. The larval and pupal stages are parasitized by many species of wasps. Damselflies, ants, beetles, wasps, mermithids, granulosis virus, and nucleopolyhedrosis virus prefer the larval stages. Spiders and mermithids attack the adults.
There are many varieties from the Philippines, Korea, United States, Honduras, Taiwan, Vietnam, and the former USSR that show resistance to the rice leaffolders.
Control of the rice leaffolders using chemicals during the early crop stages is not advisable. A general rule-of-thumb is “spraying insecticides for leaffolder control in the first 30 days after transplanting (or 40 days after sowing) is not needed.” The rice crop can compensate from the damage when water and fertilizer are well managed. Pyrethroids and other broad-spectrum insecticides can kill the larvae but can put the crop at risk because of the development of secondary pests, such as the brown planthopper.
If infestations of the flag leaves are extremely high (>50%) during maximum tillering and maturity stage, insecticide sprays may be useful. Such applications may stop further defoliation and may avoid losses.
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