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Larvae and pupae of armyworm (IRRI).
Rice armyworm, paddy armyworm, rice ear-cutting caterpillar
Mythimna separata (Walker), Spodoptera mauritia acronyctoides (Guenee), Spodoptera exempta (Walker)
Fed upon leaf blades (IRRI).
The plant can be checked for the feeding damage by visually locating the presence of the insect pest. The characteristic form of leaf removal can confirm its symptom damage.
The symptom damage can be confused with feeding damage caused by cutworms.
A maximum temperature of 15 °C favors adult longevity, oviposition period and egg output and hatching of armyworms. Periods of drought followed by heavy rains sustain the development of the insect pest. Naturally fertilized plants can produce more offsprings. Likewise, the presence of many alternate hosts can fully support the continuous development of the insect pest.
The insect is nocturnal. The adult feeds, mates, and migrates at night and rests in daytime at the base of the plant. The insect is highly attracted to light traps. The larvae feed in the upper parts of the plant on cloudy days and at nighttime. Pupation occurs in the soil or at the base of the rice plants in dryland fields. In wetlands, they pupate on the plants or grassy areas along the field borders.
The adult is either grayish black with black markings on its forewings or pale red-brown with fewer markings on the front wings or it has pale red-brown forewings with two pale round spots. Its hindwings have two colors, dark red-brown on top and white underneath or the hindwings are lighter than the forewings. The insect has a body length of more than 15.0 mm.
The pupa is 13.0 to 20.0 mm long. It is dark brown.
Young larvae have two pairs of prolegs. They have brown to orange head with an A-marking on the frons. They are grass green with gray dorsal stripes. The body of mature larva has shades of green, gray, brown, pink, or black with dorsal or subdorsal longitudinal light gray to black stripes or clear yellow stripes running along the entire length of the body. Two rows of C-shaped black spots are either present or absent along the back. They are 31.0 to 45.0 mm long.
The rounded eggs are either bare or covered with a thin layer of blackish felt and are laid in oblong clusters. They are subspherical and greenish white or pearly white. They turn yellow or dark brown with age.
The rice insect pest is polyphagous. Aside from the rice plant, it also feeds on bamboo, barley, cabbage, castor, cotton, cruciferous vegetables, flax, jute, maize, mungbean, oats, sorghum, sugarcane, sweet potato, tobacco, wheat, Cynodon dactylon (L.) Pers., Cyperus sp., Echinochloa sp., and Imperata sp., Poa sp.,
In a greenhouse study, the pest was found to feed on rice, Brachiaria distachya (L.) Stapf., Echinochloa glabrescens Munro ex Hook f., E. colona (L.) Link, E. crus-galli (L.) Beauv. subsp. hispidula (Retz.) Honda, Paspalum conjugatum Berg., and Leptochloa chinensis (L.) Nees. These hosts can support development of the armyworm from egg to egg.
The larvae feed on the crop by removing large portions of leaf epidermis using its mandibulate mouthparts.
The rice armyworm is present in all stages of the rice crop. It becomes very destructive when the population is high that it can totally devour the host plant. Mature panicles are cut off from the base of the plants.
This sporadic pest occasionally causes losses especially when an outbreak occurs. They become highly abundant and can move in large groups from field to field just so to feed and attack the crop.
Parasitoids such as tachinids, ichneumonids, eulophids, chalcids, and braconid wasps parasitize this pest. Meadow grasshoppers, ants, birds and toads feed on the pest. Fungal diseases and a nuclear polyhedrosis virus also infect the larvae.
Chemical control may be needed when populations are extremely high. Pyrethroids can often kill the larvae but can also cause development of secondary pests, such as the brown planthopper.
JLA Catindig and KL Heong