Life cycle and description

• Adult stage: Adult moths have a wingspan of 40mm. Forewings are generally sand, grey, or brown in color with a distinctive white dot in the centre. Hindwings are pale brown with a slightly darker area on the posterior edge. Adults feed on flower nectar or decaying fruit. Adults are active at night and therefore rarely seen. Females lay eggs in rows or clusters of 50 to 150. Females may lay up to 2000 eggs in a lifetime. Adult lifespan can reach 17 days.
• Egg stage: Eggs are laid on leaf sheaths, leaves or around the base of the plant. Leaf blades are often folded or rolled around the egg masses. Eggs are white, beadlike and smaller than a pin head and hatch in 3 to 4 weeks.
• Larval stage: Larvae pass through 6 instars which can take up to 4 weeks. 1st instar larvae range from 2 to 4mm in length and move in a looping motion. Mature larvae range from 25 to 35mm in length. Larvae are smooth-bodied and can be dark green, brown or black with alternating light and dark stripes running along the body. Larvae feed nocturnally and hide under crop debris during the daytime. Larvae burrow 2 to 3cm below the soil surface to pupate.
• Pupal stage: Pupae are reddish-brown in color and range from 15 to 20mm in length. Pupae are found just below the soil surface.

Confirmation

Armyworms are predominantly nocturnal and therefore are often not detected until the crop is damaged. Moth traps can be used to detect presence of adult moths. Under severe infestation, larvae are often visible migrating between fields in large numbers.  Assessment of larvae should be used for identity confirmation. Armyworm larvae are very similar to fall armyworm larvae. The two can be distinguished by the presence of a white ‘Y’ mark on the on head of fall armyworm.

Problems with similar symptoms

Damage due to armyworm is similar to that caused by fall armyworm, grasshoppers, and cutworms.

Why and where it occurs

• Adult moths prefer dense vegetation for oviposition and heavy infestation is more common in grassy and weedy fields and those under reduced tillage.
• Mild winters and warm summers enable pupae to overwinter and larvae to develop quickly.

Host range

Most cereals including maize, wheat, oats, barley, rye and rice, in addition to legumes, forage grasses, and various vegetable crops.

Geographic distribution

Armyworm is widely distributed in the tropics and subtropics throughout the world.  Armyworm is also found in Europe, Russia and the United States.

Damage

• Mechanism of damage:Loss of photosynthentic area due to foliar feeding.  Under severe infestations entire young plants can be consumed.
• When damage is important: Damage is most intense when climatic conditions and agricultural factors favor armyworm development. Young plants are most vulnerable. If larvae are not controlled at early instars, extensive foliar damage is common. 6th instar larvae are responsible for over 80% of feeding.
• Economic importance: Severe damage due to armyworm is rare as most plants can recover from low levels of infestations. Under severe infestation entire plants can be consumed. Damage is often localized.

Management principles

Monitoring

• Monitoring for armyworm presence is critical to avoid crop damage.
• Moth traps can be used to detect the presence of adult moths.
• Fields should be regularly inspected for presence of eggs, frass, and young larvae. Larvae are predominantly nocturnal; therefore, inspections should include the soil around plants and under crop debris where larvae hide during the day.
• High bird presence in the field may reflect heavy armyworm infestation.
• Monitoring is particularly important following herbicide application as larvae are then forced to infest maize plants.
• Field edges should be monitored for migrating masses of larvae.
• Control should commence when 75% of the plants have one larva or 25 to 30% of the plants have two or more larvae.

Cultural control

• Plowing a deep ditch with steep edges or filled with water or insecticide can form a physical barrier to migrating larvae.

Chemical control

• Insecticides can be applied to the crop at early larval stages if infestation levels are high. However, application of insecticides is ineffective once larvae are mature as most crop damage has been completed. Insecticide application is most effective in the early evening or under overcast conditions when larvae are feeding and therefore exposed.
• Insecticides can be selectively applied to field margins to create a barrier to migrating larvae.
• Various pyrethroids, carbamates, and organophosphates are approved for armyworm control.

Biological control

• There are numerous natural predators and parasitoids of armyworms, including parasitic wasps, birds and toads.
• The egg parasite Telenomus minimus and species of braconid wasps such as Apanteles laeviceps, A. marginiventris, and A. militaris have been used to successfully manage armyworm infestation.
• Bt (Bacillus thuringiensis) toxins and products that mimic the molting hormone ecdysone have been used to manage larvae populations.

References

Catindig, J.L. and K.L. Heong. 2003. International Rice Research Institute Rice Doctor. http://www.knowledgebank.irri.org/riceDoctor_MX/default.htm (16 November 2006).

Davidson, R.H. and W.F. Lyon. 1987. Insect Pests of Farm, Garden and Orchard. Hoboken, NJ: John Wiley & Sons.

Glogoza, P. 2000. The Armyworm and the Army Cutworm. North Dakota State University Agriculture and University Extension. http://www.ag.ndsu.edu/pubs/plantsci/pests/e830w.htm (13 November 2006).

Hellman, L. and J.K. Mathias. n.d. IPM Control Options for Key Turfgrass Insect Pest: Armyworm and Fall Armyworm.  University of Maryland. http://iaa.umd.edu/umturf/Insects/Armyworm.html (13 November 2006).

Holt, J., W. Mushobozi, R.K. Day, J.D. Knight, M. Kimani, J. Njuki and R. Musebe. 2006.  A simple Bayesian network to interpret the accuracy of armyworm outbreak forecasts. Annals of Applied Biology 148: 141-6.

King, A.B. and J.L. Saunders. 1984. The Invertebrate Pests of Annual Food Crops in Central America. London, UK: Overseas Development Admin.

Kuhlman, D.E. and R.G. Dedert. 1987. Insect Pests of Maize in Zambia. Urbana – Champaign, IL:: University of Illinois.

Maund, P. 2002. Armyworm. Fredericton, NB: New Brunswick Department of Agriculture, Fisheries and Aquaculture. http://www.gnb.ca/0057/armyworm.pdf (13 November 2006).

Ortega, A. 1987. Insect pests of Maize: A Guide for Field Identification. Mexico, D.F.: CIMMYT.

Contributors: Gabrielle Turner, Biswanath Das