True Armyworm

Mythimna unipuncta

1. Identification

• Common name: True armyworm
• Scientific name: Mythimna unipuncta Haworth
• Order: Lepidoptera
• Family: Noctuidae

2. Pest description

• Adults: Moths with a wingspan of 30–40 mm, light brown forewings with a characteristic white spot; whitish hindwings.
• Larvae: Greenish-brown caterpillars, with light longitudinal lines; highly mobile and gregarious in the early stages.
• Pupae: Formed in the soil, in earthen chambers.
• Eggs: Laid in masses on low vegetation, generally on grasses.

3. Main hosts

• Forage and wild grasses, including Lolium spp., Festuca spp., Poa spp. and Dactylis spp..
• Cereals such as maize, wheat, barley and oats.
• Permanent pastures and temporary meadows.

4. Symptoms and damage

• Intense defoliation of grasses, leaving only the central veins.
• “Shaved” appearance of the pastures due to rapid and collective consumption.
• More severe damage on young shoots and newly established pastures.
• In cereals, reduced vigour, delayed development and yield losses.
• In massive attacks, total destruction of plots.

5. Biological cycle

• Several generations per year, with greater intensity in spring and summer.
• Migratory adults, which may give rise to sudden populations.
• Eggs laid in masses; caterpillars develop over 3–5 weeks.
• Pupation in the soil; adults emerge and restart the cycle.
• Development favoured by mild temperatures and high availability of grasses.

6. Monitoring

• Direct observation of caterpillars on young shoots and pastures.
• Assessment of the degree of defoliation and the presence of “shaved” patches.
• Light traps to detect migratory adults.
• Reinforced monitoring after periods of rain followed by heat.

7. Management measures

• Cultural: Early cutting of pastures; surface tillage of the soil to expose pupae; adequate fertilization management; crop rotation with non-host species.
• Preventive: Frequent monitoring during risk periods; avoiding excess accumulated plant matter; maintenance of dense and balanced pastures; avoiding the establishment of young pastures during periods of high pest pressure.
• Biological: Use of authorized entomopathogenic microorganisms; application of microbial bioinsecticides; release of available beneficials; conservation of natural enemies through adequate cultural practices.
• Integrated protection: Regular monitoring; intervention only when necessary; practices that reduce the survival of pupae in the soil and the availability of food for young caterpillars.

Bibliographic references

• EPPO Global Database – Mythimna unipuncta.
• CABI Invasive Species Compendium – Mythimna unipuncta.
• Capinera, J. L. (2001). Handbook of Vegetable Pests. Academic Press.
• Hill, D. S. (1987). Agricultural Insect Pests of Temperate Regions and Their Control. Cambridge University Press.
• Showers, W. B. (1997). Migratory ecology of the armyworm, Pseudaletia unipuncta. Annual Review of Entomology, 42: 393–425.
• Pogue, M. G. (2002). A world revision of the genus Mythimna. Memoirs of the American Entomological Society.