Case worm (Nymphula depunctalis - Parapoynx stagnalis)
The case worm is a common pest on wetland rice. Moths are small (1 to 1.2 cm in wingspan) with white markings and black specks on the wings. Females lay eggs in small batches (about 20) on the lower side of leaves that are floating on the water surface. Upon hatching caterpillars are yellow to green with light brown heads. They climb onto a leaf and begin feeding by scrapping the leaf surface causing linear grazing of leaves giving the leaf tissue a ladder-like appearance. Later caterpillars cut a piece of rice leaf, roll it up into a case and seal the edges with silk material leaving the interior end open. The cut near the tip of a leaf is characteristic.
At all times the caterpillar is likely to be partly or wholly enclosed in its portable leaf case. The caterpillar attacks the food plant only in the vegetative stage, during the first 4 weeks after transplanting. Caterpillars of the case worm are semi-aquatic, ascending the plants at night to feed. Heavy infestation on small seedlings may completely destroy a rice crop. Damaged plants may recover but crop maturation may be delayed about a week. Yield loss may occur when the caseworm occurs in combination with other non-defoliating insects such as whorl maggots and stemborers. Damaged plants are stunted and produce fewer tillers.
- Practise field sanitation (burning debris or feeding of debris to livestock after harvest).
- Practise early and synchronised planting.
- Proper plant density. A study in West Africa showed that defoliation due to the caseworm ranged from 16% in seedlings transplanted at a wide spacing (40 X40 cm) to 68% at a spacing of 10 X 10 cm (WARDA).
- Practise proper water management. Ensure good drainage for 3 days, since larvae cannot survive without water.
- Hand pick and destroy rolled leaves in the nursery.