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 off 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 localized. They are difficult to predict. Outbreaks have been reported from all altitudes, although it may not all be the one species.

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. 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 color. Crowded caterpillars, such as those feeding communally, develop a much darker shade. Pupa are brown, and form in surface litter or 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

Rice seedlings damaged by armyworm (Luni gewog, Paro, 2024)

Rice seedlings damaged by armyworm (Damji,Gasa, 2024)

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 localized within a field. A characteristic of the 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 are 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.

BIOLOGY

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 favored by periods of drought followed by heavy rain. The combination of factors—reduced natural enemy pressure during drought, followed by an abundance of host plants and favorable conditions after rain creates an ideal environment for the rapid increase of populations, leading to outbreaks. 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.

MANAGEMENT PRACTICES

This is a sporadic and often localized pest. The key to successful management is detecting infestations before serious damage has occurred.

Monitoring

Scout weekly for eggs and larvae from soon after planting through to flowering. 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. 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 for pheromone traps.

Effect of variety

Not expected given its wide host range.

Non-chemical management

• Flooding the seedbed is the best defense against swarming caterpillars as it drowns them. Alternatively, plow 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 armyworms.
• Avoid killing natural enemies of armyworms such as wasps and spiders.

Chemical management

• Chemical spray may be necessary as a last resort during an outbreak.
• Spray Chlorantraniliprole 18.5% SC (0.3 ml per liter of water) or Emamectin benzoate 5% SG (0.2 gram per liter of 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.

Version: NPPC 2017. Armyworm V1.0. Bhutan Pest Factsheet. www.PestsofBhutan.nppc.gov.bt. Date produced: 14 April 2017. Last date modified: 18th June 2024. Contact: nppcsemtokha@gmail.com

Image acknowledgements: NPPC.