Armyworm (Mythimna separate) Outbreak Alert, 2016

Armyworm larvae in Maize
Armyworm larvae in Maize

With weather conditions becoming more favorable for the armyworm outbreaks (prolonged dry periods followed by heavy rainfall) , all extension officials are instructed to carry out regular monitoring of the pests particularly in maize fields and paddy nurseries to assist farmers to implement control measures on time.

Factors favoring the outbreak

  1. Periods of dry spells followed by heavy rains
  2. Presence of many alternate hosts such as grassy weeds belonging to Gramineae family


Armyworms are caterpillars that crops belonging to rice and maize (particularly Gramineae). There are three armyworm species which are the rice swarming caterpillar, common cutworm, and the rice ear-cutting caterpillar. A single armyworm egg mass contains hundreds of eggs and each female lays 800−1000 eggs during its lifetime of about one week. The name armyworm is applied to those species that migrate in masse to neighboring fields after severely defoliating a crop.

What it does?

Armyworm feeds on paddy/maize leaves and young seedlings at the plant’s base. They also cut off rice and maize panicles from base.

Why and where it occurs?

Adult armyworms survive better and produce more eggs when the temperature is at 15 °C maximum, and when plants are naturally fertilized. Periods of dry spells followed by heavy rains (spring and summer), and the presence of alternate hosts also sustain the development of armyworms.

Armyworms are nocturnal. The larvae usually feed on the upper portion of the rice and maize canopy on cloudy days or at night; while the adult feeds, mates, and migrates at night and during daytime take shelter at the base of the plant/under the soil clods. In dryland fields, armyworm pupa can be found in the soil or at the base of the rice/maize plants. In wetlands, they pupate on the plants or on grassy areas along the field borders.

How to identify?

  • Check for feeding damage.
  • Armyworms feed on leaf tips and along leaf margins. When they eat whole leaves, they can remove them completely or leave only the midribs.
  • Armyworm damage can be mistaken for cutworm feeding. The characteristic form of armyworm damage is leaf removal.

Why is it important?

The armyworm can be a problem in rice and maize crop.Yield losses can occur when the population is high. It becomes so destructive that it can totally damage the host plant in a short period of time. During an outbreak, armyworms become highly abundant and can move in large groups from field to field to feed and attack the crops

How to prevent? Grass weeds from bordering fields should be cut and weeds removed regularly to reduce breeding sites and shelter for armyworm.

How to control?

  • Flooding seedbed is the best defence against armyworms when the population is in the larvae stage. Flooding drowns the swarming larvae
  • To prevent the caterpillars from moving to another field, apply a 40 foot border spray with Cypermethrin around the non-infested field
  • Plough a deep ditch and filled it with water. This method is helpful when caterpillars are found to be moving towards field from the adjacent fields.
  • Another method is to dig a deep ditch with vertical sides to trap the larvae and prevent them from crawling out. Collect and properly dispose the trapped larvae.
  • If there is high infestation of army worm chemical spray is necessary. It is recommended to spray Cypermethrin@1 ml/1 L water. Since the armyworm usually feeds at night, the best time to spray is towards the later part of the day.


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