Why is it a problem? The rice leaf folder attacks tillering and booting stages. High densities can reduce seed production, but this is rare and management is normally not needed.
Where and when is it a problem? It is a commonly encountered pest, in part because of the obvious damage, but it rarely if ever reaches damaging densities. Most farmer complaints are received from eastern Bhutan.
Adult moths are medium-sized (8-10 mm long and 16-20 mm wing expanse). They are light brown with shiny, brownish yellow wings adorned with dark, broad margins and 2-3 dark vertical stripes. Eggs are oval, creamy white. Newly emerged larvae are dull white or light yellow with a brown head, but soon turn green once feeding starts. Fully-grown larvae are slender and 20-25 mm long.
It is a leaf folding and leaf feeding caterpillar which attacks tillering and booting stages. Caterpillars fold a rice leaf around them and attach the leaf margins together with silk strands. They feed inside the folded leaf creating longitudinal white and transparent streaks on the blade. A heavily infested crop has streaks on the leaves and appears whitish from a distance. Feeding may also make plants prone to fungal and bacterial infection.
Confusion with other pests: There are several leaf-feeding pests of rice, but the rice leaf-folder is the only one that commonly folds leaves. Look for the larvae to confirm identification.
Lifecycle: Eggs are laid singly or in pairs on the leaves and leaf sheaths. Young larvae feed on tender leaves without folding them. Older larvae construct leaf folds. Single larvae may damage multiple leaves. Larvae pupate in loose silken webs in between the leaves or in the leaf sheaths.
Dispersal: Adults fly.
When can damage be expected? It is a commonly encountered pest in rice fields, but outbreaks are rare.
Hosts: It has a wide host range but in Bhutan it has only been recorded from rice.
Although common and conspicuous, specific management is rarely needed. Management is only needed at high leaf-roller densities as modern varieties are generally capable of compensating for feeding damage that occurs during the vegetative stage. IRRI recommend that management may only be justified if the leaf-folder affects more than half of the flag (highest) leaf and the next two youngest leaves in each tiller.
Monitoring should be done at the same time as general scouting for rice pests and diseases. Estimate the average number of flag, and next two youngest, leaves that are damaged.
Effect of variety
Studies by IRRI have found difference in resistance between cultivars.
- Use recommended fertiliser rates.
- Maintain recommending spacings (22.5 x 20 cm and 30 x 20 cm).
- Removing grassy weeds from rice fields and surrounding areas may prevent the build-up of rice leaf rollers on alternate hosts.
- A wide range of natural enemies normally keep this pest in check. Biological control agent are available elsewhere, including in Nepal.
The use of pesticides is not recommended. It would rarely if ever be economically justified in Bhutan. Furthermore, early-season spraying may make rice pest problems worse by killing beneficial insects. Contact NPPC for advice if serious outbreaks occur.
Image acknowledgements: NPPC.