Parapoynx stagnalis

Crop: Rice

Why is it a problem? This leaf-feeding moth causes most damage within 2-3 weeks of transplanting. They cut off leaf tips, thereby reducing plant growth rates.

Where and when is it a problem? It is more serious in poorly drained fields, but damage is rarely serious enough to require management. It is mostly reported from eastern Bhutan.

IDENTIFICATION

 

 

 

 

 

 

 

Adults are small (about 1.5 cm in length) and delicate. They are snowy white, and the white wings are marked with a few light brown to black specks. Eggs are laid on the underside of the leaves. Larvae have a transparent green body and a light brown head.

SYMPTOMS

Young larvae cause linear grazing of leaves. Later in development larvae cut off the tips of leaves to make the portable, tubular case in which it lives. The cut near the tip of the leaf is characteristic. It is made at right angles, as if cut with scissors. The cylindrical tubes can be seen attached to the plant or floating on the water surface.

Confusion with other pests: It is the only moth that makes cases from rice leaves in Bhutan.

BIOLOGY

Lifecycle: The life cycle is completed in about 32 days under optimal conditions. This suggests that multiple generations a year are likely in Bhutan. The first instar is free-living.  Subsequent larval instars construct and inhabit leaf cases. Larvae hide in their case then float on the water surface during the day. They crawl, still in their case, onto rice plants to feed. Adults are nocturnal.

Dispersal: Adults fly. Larvae can float around rice fields in their cases.

When can damage be expected? Larvae mainly occur in fields of standing water, with greatest damage observed within the first few weeks of seedling transplantation.

Hosts: This pest can utilise a range of grasses and sedges. In Bhutan it has only been reported in rice.

MANAGEMENT PRACTICES

This pest is commonly seen in rice paddies, but is only rarely sufficiently common to require control.

Monitoring

Careful monitoring is needed during the weeks following seedling transplantation. At least 25% of leaves need to be injured before management action is economically justified.

Effect of variety

IRRI report that dwarf, compact, heavy tillering and high yielding varieties may be particularly susceptible.

Non-chemical management

• Larval cases floating on the water can be collected and destroyed.
• Draining the field for at least three days will kill most of the larvae because they are highly dependent on water for oxygen.
• Use of older seedlings reduces the duration of the susceptible stage of the crop.
• Elsewhere trichogrammatid wasps, ants and spiders have been reported as important natural enemies.

Chemical management

• Use of insecticides is only justified if infestations are high (> 25 % of leaves injured in a paddy)
• Caseworms are highly sensitive to insecticides. Spray Cypermethrin (1 ml per 1 litre of water) or Chlorpyrifos (4 ml per 1 litre of water).

 

Version: NPPC 2017. Rice caseworm V1.0. Bhutan Pest Factsheet. www.PestsofBhutan.nppc.gov.bt. Date produced: 14 April 2017. Contact; nppcsemtokha@gmail.com

Image acknowledgements: Rice knowledge bank, IRRI