All extension officials and farmers are advised to carry out regular monitoring of the armyworm particularly in paddy nurseries and maize fields to implement control measures on time.
Why is it a problem?
Armyworms can cause heavy losses to rice nurseries, with caterpillars destroying seedlings. They also attack rice and maize fields and, to a lesser extent, barley and buckwheat. Considerable losses in rice seed can result from larvae cutting of the panicles. They are readily managed provided infestations are detected early enough.
Where and when is it a problem?
The early arrival of warm dry weather followed by rain in spring is favorable for reproduction and multiplication of army worms. Local impacts can occur every year, but significant damage only occurs when there are severe outbreaks. Such outbreaks, when large numbers of larvae move from field to field voraciously feeding on foliage, can occur every several years but are typically quite localized. They are difficult to predict.
Identification
Eggs are off-white and are laid in groups low on the leaves of the plant, often between the sheaths or on the blades. Caterpillars (larvae) move with a lopping motion. The young caterpillar is green. Later instars are brownish with a thin pale dorsal line, and a dark lateral line on each side. The head has a light and dark brown pattern. Solitary individuals remain fairly pale in color. Crowded caterpillars, such as those feeding communally, develop a much darker shade. Pupa is brown, and form in surface litter or in the soil. Adult moths are pale and brick-red to pale brown with a very hairy body covered with dark specks and patches. The wingspan is 35–50 mm.
Symptoms
Caterpillars are large, external feeders so they and their feeding damage are generally easily spotted. They feed on the leaves, leaving only the midrib uneaten. Caterpillars can form swarms that move from one field to another, voraciously feeding on foliage.
During the vegetative stage of rice, damage is evident as massive leaf removal, often including leaf veins. They may also eat the lemma and palea of the developing grains as well as the anthers of flowers. Large angular notches can be cut away from young seedlings in a seedbed, giving an irregular appearance. Damage is often localized to one part of a field. During outbreaks many fields can be affected at the same time.
Biology
Life cycle. Adults feed, mate and migrates at night and rests during the day at the base of the plant. Larvae feed in the upper portion of the rice canopy on cloudy days or at night. Females live about a week during which time the lay about 800-1000 eggs. The moth flies from January to April depending on the location.
Dispersal. Adults are migratory, and can fly long distances. Larvae can also move through and between fields.
When can damage be expected? Outbreaks appear to be favored by periods of drought followed by heavy rain. The presences of alternative hosts also sustain populations. Adults survive better and produce more eggs when the temperature is at 15oC maximum.
Hosts. This species has a very wide host range. In Bhutan it has been reported off rice, maize, barley wheat and buck wheat.
Management Practices
This is a sporadic pest and often localized pest. The key to successful management is detecting infestations before serious damage has occurred.
On-Farm Monitoring
Scout weekly for eggs and larvae from soon after planting. Be extra vigilant if outbreaks are being reported in the neighborhood. Scouting should be done throughout the entire field as populations are often highly aggregated. Control decisions should be made before the larvae reach the last larval stage which is both more damaging and more difficult to control owing to its large body size.
Pheromone traps can be used to monitor adult moths, with the economic threshold for management being 25-50 moths per trap per day.
Non-chemical management
- Flooding the seed bed is the best defense against swarming caterpillars as it drowns them. Alternatively plough a ditch and fill it with water, especially when caterpillars are moving towards your field from adjacent fields.
- Another method is to dig a deep ditch with vertical sides to trap the caterpillars and prevent them from crawling out. Collect and properly dispose the trapped caterpillars.
- Grass should be cut regularly from bordering fields to reduce breeding sites and shelter for armyworm.
- Avoid killing natural enemies of armyworms such as wasps and spiders.
Chemical management
- If there is high infestation of army worm chemical spray may be necessary as a last resort
- Spray Cypermethrin at 1 ml/1 Litre water. Since the armyworm larvae usually feeds at night, the best time to spray is late in the day.
- To prevent the caterpillars from moving to another field, apply a 15 metre border spray around the non-infested field.
Click on link below for data sheet for monitoring the Armyworm:
If you detect armyworm outbreaks in your locality, please contact at National Plant Protection Centre at Phone: +975–2–351–016 , email: nppcsemtokha@gmail.com.
