Pseudaletia separata (or Mythimna separata)
Crops: Rice, maize, wheat and buckwheat
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 bitter buckwheat. Although not quantified in Bhutan, considerable losses in rice seed can result from larvae cutting of the panicles. Armyworms are readily managed, provided infestations are detected early enough.
Where and when is it a problem? 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 localised. They are difficult to predict. Outbreaks have been reported from all altitudes, although it may not all be the one species.
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. Older larvae 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 colour. Crowded caterpillars, such as those feeding communally, develop a much darker shade. Pupa are 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.
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. This can result in massive leaf removal, although it is often localised within a field. A characteristic of Chinese armyworm (Pseudaletia separata) observed elsewhere during feeding at the grain filling stage is that mature larvae cut off panicles at their base causing some to bend while others are completely severed and fall to the ground.
Confusion with other pests: There are several armyworm species in Bhutan, and their identities and relative importance is not yet fully understood. Another armyworm that periodically affects rice is Mythimna venalba but it is not normally the problem species. Management is the same for all species. Armyworm damage can be mistaken for cutworm feeding. The characteristic form of armyworm damage is leaf removal. To confirm, check for the presence of larvae and egg masses.
Life cycle: Adults feed, mate and migrate at night. They rest during the day at the base of plants. Larvae feed in the upper portion of the rice canopy on cloudy days or at night. Females live about a week during which time they 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 favoured by periods of drought followed by heavy rain. The presence of alternative hosts also sustain populations. Adults survive better, and produce more eggs, when maximum temperatures are around 15oC.
Hosts: This species has a very wide host range. In Bhutan it has been reported off rice, maize, barley, wheat and bitter buckwheat. Elsewhere it has been recorded off 33 plant species in 8 families. It is considered one of the most serious pests of cereal in Asia.
This is a sporadic and often localised pest. The key to successful management is detecting infestations before serious damage has occurred.
Scout weekly for eggs and larvae from soon after planting through to flowering. Be extra vigilant if outbreaks are being reported in the neighbourhood. 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. This stage is both more damaging and more difficult to control because of its size.
Pheromone traps can be used to monitor adult moths, with the economic threshold for management being 25-50 moths per trap per day. Contact NPPC on how to procure pheromone traps.
Effect of variety
Not expected given its wide host range.
- Flooding the seed bed is the best defence against swarming caterpillars as it drowns them. Alternatively plough a ditch around the field 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 of 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.
- Classical biological control has been found to be effective in other parts of the world.
- Chemical spray may be necessary as a last resort during an outbreak.
- Spray Cypermethrin at 1 ml/1 Litre water.
- Since the armyworm 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.
Image acknowledgements: NPPC.